{"1": {"fulltext": "y*6fl\\nmm\u00c2\u00ae\\n^L\\nHB\\nW5B6^^n5aWWlWWW\u00c2\u00bbK", "height": "3753", "width": "2385", "jp2-path": "elementaryastron00hold_0001.jp2"}, "2": {"fulltext": "m.\\nLIBRARY OF CONGRESS\\nQB45\\nChap. Copyright No\\nShelf__^iL7\\nUNITED STATES OF AMERICA\\n^MMMmlmm\\n1\\nVM*k\\n$k\\n|l\\ni%\\nllfpg|i\\nIp", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0002.jp2"}, "3": {"fulltext": "", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0003.jp2"}, "4": {"fulltext": "", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0004.jp2"}, "5": {"fulltext": "", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0005.jp2"}, "6": {"fulltext": "", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0006.jp2"}, "7": {"fulltext": "AMERICAN SCIENCE SERIES, ELEMENTARY COURSE\\nELEMENTARY ASTRONOMY\\ntA BEGINNER S TEXT-BOOK\\nEDWARD S. HOLDER M.A., Sc.D., LL.D.\\nSometime Director of the Lick Observatory\\nNEW YORK\\nHENRY HOLT AND COMPANY\\n1899", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0009.jp2"}, "8": {"fulltext": "TWO COPIES RECEIVED,\\nOffice of ths\\nRegister of Coevrl.k..\\nCopyright, 1899,\\nBY\\nHENRY HOLT CO.\\nSECOND COPY,\\nROBERT DRUMMOND. PRINTER, NFW YORK.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0010.jp2"}, "9": {"fulltext": "INTRODUCTION.\\nThe first notions of Astronomy are acquired in the\\nstudy of Geography. Geography lays special stress on the\\nfact that the surface of the Earth is in a state of constant\\nchange. Its oceans and its atmosphere are subject to\\ntides its surface is leveled for the sites of cities and\\ntowns its mines and quarries are explored for substances\\nuseful to mankind. Men navigate its seas and use its soils\\nto produce the food that supports them. Its ceaseless\\nchanges, natural and artificial, give to it a kind of life\\nfor the sign of life is change.\\nGeography teaches, also, that the Earth is one of the\\nplanets, but in this larger relation says little or nothing of\\nchanges taking place in the solar system. The 7 oung\\nstudent is very apt to conclude that the other planets of\\nwhose existence he knows Venus and Jupiter for ex-\\nample are changeless, immutable; that they are bright\\npoints of light without a history. This was the view of\\nthe ancients.\\nThe special business of Astronomy is to develop the ideas\\nof the student so that he may understand that all the\\nbodies of the Solar system the Sun and all the planets\\nare themselves subject to ceaseless changes and are thus\\nendowed with a kind of life. Not only this; the bodies\\nthroughout the whole universe Sun and stars alike are\\nperpetually altering both their places and the arrangement\\nof their separate parts. Our life on the Earth, for instance,\\nwould quickly cease were it not for changes in the Sun.\\nThere are many stellar systems in which such changes have", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0011.jp2"}, "10": {"fulltext": "iv INTRODUCTION.\\nalready ceased and which are themselves now dead as the\\nMoon is dead. Others again are in their prime of youth,\\nand still others are in their ripe maturity. The Cosmos is,\\nas it were, alive; and it is still in a state of uncompleted\\ndevelopment.\\nThe study of Astronomy should lead the student to com-\\nprehensive ideas of the universe at large. He will gradually\\nbecome possessed of at least a part of the vast body of re-\\nsults that has been slowly amassed, and, what is even more\\nimportant, of the methods that have been invented by the\\ngreat men of past times for the discovery of results. A\\npart of the lesson of the science will have been missed if it\\ndoes not teach a sympathetic admiration for great names\\nlike those of Galileo, Kepler, and Newton. Its history is\\nintimately connected with the history of the intellectual\\ndevelopment of mankind.\\nAs Astronomy is one of the oldest of the sciences its\\nmethods have been perfected to a very high degree, and\\nhave served as models for the methods of the other sciences.\\nIt is chiefly for this reason that it is so well fitted to be the\\nscience first studied by the young student.\\nIn teaching Astronomy every endeavor should be made\\nto have the student realize what he learns. What is al-\\nready known about the Earth will serve as a stepping-stone\\nto a knowledge of the planets. When something is learned\\nof the planets, the knowledge will throw light upon the\\npast (or the future) condition of the earth. Jupiter rep-\\nresents, in many respects, the past condition of the Earth,\\njust as the Moon, in all likelihood, represents its state in a\\nvery remote future. The Sun is like the bright stars\\nstrewn by thousands over the celestial vault not unlike\\nthem. Everything that can be learned regarding the Sun\\nhelps us to comprehend physical conditions in the stars,\\ntherefore; and the converse is true.\\nThe nebulas are not exceptional bodies of unique nature,", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0012.jp2"}, "11": {"fulltext": "INTRODUCTION. V\\nbut they are examples of what our own solar system\\nwas in ages long past. Though we cannot see any indi-\\nvidual nebula pass through all the stages of its life from its\\nbirth to its maturity, we can select from the vast numbers\\nof such bodies particular nebulae in each especial stage. As\\nSir William Herschel wrote in 1789, This method of\\nviewing the heavens seems to throw them into a new kind\\nof light. They are now seen to resemble a luxuriant gar-\\nden which contains the greatest variety of productions in\\ndifferent flourishing beds; and we can, as it were, extend the\\nrange of our experience to an immense duration. For is it\\nnot the same thing whether we live to witness successively\\nthe germination, blooming, foliage, fecundity, fading,\\nwithering, and corruption of a plant, or whether a vast\\nnumber of specimens selected from every stage through\\nwhich the plant passes in the course of its existence be\\nbrought at once to our view\\nIt should be the aim of the text-book and of the teacher\\nto so marshal the most significant of the results of observa-\\ntion that the student may acquire such wide and general\\nviews. If he at the same time gains a luminous idea of\\nthe most important of the methods by which such results\\nare reached, his teaching has been successful. It is neces-\\nsary to recollect, on the other hand, that it is not the\\nprovince of an elementary text-book to present all the\\nlatest interpretations of observation, or to give more than\\nthe principles of the methods employed. Details of the\\nsort cannot be thoroughly understood by the beginner.\\nQuestions that are still in debate, like the nature of the\\nplanet Mars or the constitution of comets, cannot be pre-\\nsented with fulness because the student is not yet sufficiently\\nequipped to judge the points at issue. At the same time\\nthe materials for such a judgment should be, so far as\\npossible, laid before him in such a way as to stimulate his\\nthought and his imagination.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0013.jp2"}, "12": {"fulltext": "vi INTRODUCTION.\\nIn all the natural sciences one of the very first matters\\nis to make an orderly inventory of the visible universe.\\nThings must then be grouped into classes, in order that\\nthe relations of the various classes may afterwards be\\nstudied. In Astronomy the classes are few; there are the\\nSun and the stars, the planets, the comets, the nebulae.\\nThe next step is to study typical members of each class\\nwith the telescope. All that the text-book can do is to\\ngive descriptions of the appearances presented by tele-\\nscopes. These must, in most cases, be taken on faith.\\nThe Moon can be studied to advantage by opera-glasses\\nor by such small telescopes as are available for use in\\nschools. Something can be learned, by like means, of the\\nspots on the Sun, etc. The existence of the brighter\\nsatellites of Jupiter and of Saturn can be verified. But for\\nall the more significant facts the pupil must accept the\\nverbal descriptions of the book. The apparent motions of\\nthe stars and planets can perfectly well be observed, out of\\ndoors, by the student who has time and opportunity. But\\nhere again there are difficulties. Dwellers in city streets,\\nseldom have an uninterrupted view of the sky; and even\\nthose who live in the country rarely have time enough to\\ngive to actual observation. It is entirely impossible in a\\nfew weeks to even verify what it has taken centuries to\\ndisclose.\\nAll the actual observing of the heavens that can be ar-\\nranged for should be done. Its chief use will be to illus-\\ntrate by actual examples the methods laid down in the\\ntext- book. Conviction will come to the pupil because he\\nhas learned hoio to prove or to disprove its theorems; not\\nbecause he has actually made the proofs for himself. He\\nknows that if he has sufficient time they can be proved or\\ndisproved by following a certain method. He thoroughly\\nunderstands the method and he has applied it in a few\\ncases. He is satisfied that the method itself is adequate", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0014.jp2"}, "13": {"fulltext": "INTRODUCTION. vil\\nand he accepts the conclusions even those that he has not\\nhimself tested. If the student will take the time and the\\npains to actually make the observations suggested, he will\\nlearn much. Enough is here given to start him on his\\nway and to make it easy for him to go on by himself.\\nThe present book endeavors to place the pupil in this\\nindependent position by suggesting tests that he can him-\\nself apply. Quite as much stress is laid on the spirit of the\\nmethods of the science as on the results to which those\\nmethods have led. And the separate results of observation\\nare prized mainly because each one bears on an explanation\\nof the whole universe.\\nThis book is condensed from two volumes previously\\nwritten by Professor Simon Newcomb and myself for the\\nAmerican Science Series. I have to express my sincere\\nthanks to him for permission to print the condensation in\\nits present form, and to the Astronomical Society of the\\nPacific, to Professor Charles A. Young, and to Dr. J. E.\\nKeeler, Director of the Lick Observatory, for permission\\nto use some of the cuts here printed.\\nThe book is addressed especially to pupils who are study-\\ning Astronomy for the first time. The chief difficulties of\\nsuch students are not due to the intrinsic complexity of the\\nseparate problems that they meet, but rather to their appar-\\nent want of connection one with another, and above all to the\\nunfamiliarity of the student with the methods of reasoning\\nemployed. It is therefore necessary to treat each new topic\\nwith great clearness, and not to dismiss it until its relation\\nto other topics has been at least partially apprehended.\\nThe important point is to present the subject in a way to\\nconvince and to enlighten the pupil, and this object can\\nonly be attained in a text-book by some repetitions and by\\navoiding undue brevity. This volume contains more\\npages than one of its predecessors in the American Science\\nSeries. The increased space is given to very full explana-", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0015.jp2"}, "14": {"fulltext": "viii INTRODUCTION.\\ntions of difficult points, to lists of test-questions, and to\\npictures and diagrams. Where the mathematical equip-\\nment of the pupil is not yet adequate as in the case of\\nNewtok s discoveries in Celestial Mechanics, for example\\nan historical treatment must be adopted,\\nIt is probable that most of the students who will read\\nthis book will not pursue the subject further in the way of\\nformal studies. Their ideas of the measurement of time,\\nof the apparent and real motions of the planets, of\\nthe cause of the seasons, and of other fundamental and\\npractical matters of the sort, will be derived from this one\\ncourse of study. Especial stress is therefore laid on such\\ntopics, and many interesting subjects of less importance are\\npassed by with a mere mention, or are omitted altogether.\\nThe prescribed limits of space do not permit a treatment\\nof all the parts of a vast science like Astronomy.\\nIt may sometimes be useful to the teacher, and it will\\nalways be so to the student, to refer to the questions printed\\nin Part I, which will suggest new ways of testing the\\nknowledge gained by the reading of each lesson. It is not\\nhere attempted to set down all, or any great part, of the\\nquestions which each topic may suggest, but only to give\\nsuch as are most essential and important.\\nIf the student finds that he has an answer in clear and\\ndefinite English for each of the questions given here, he\\nmay be sure that he has comprehended the explanations of\\nthe text. And he should not finally leave any topic until\\nhe does so.\\nThe second part of the book is mainly devoted to a de-\\nscription of the bodies of the solar system, one by one, and\\nto some account of nebulas, stars, and comets. It is to be\\nexpected that the formal studies of the pupil will have\\ncreated a living interest in such information, and that he\\nwill, for his own pleasure, read some of the many admira-\\nble popular works on Astronomy that we owe to Mr. Proc-", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0016.jp2"}, "15": {"fulltext": "INTRODUCTION. ix\\ntor, Sir Eobert Ball, and others. The text-book will\\nhave performed its part if such an interest has been awak-\\nened, and if at the same time a solid foundation for the\\nstudent s future reading has been laid. For this reason\\nParts II and III of this book have been somewhat ab-\\nbreviated.\\nIf the class has sufficient time it is desirable that the\\nteacher should supplement his instruction by reading, with\\nthe students, certain chapters from the books of the\\nschool library named in Chapter XXIX. Chapters bearing\\non a certain subject can be selected by the teacher from\\nthe books referred to, after the students have studied the\\ncorresponding chapter in the present volume. If such\\nbooks cannot be had articles from encyclopaedias will serve\\nin their stead.\\nIt will not be out of place to give a few practical hints\\nbased on experience. Excellent training in observation\\ncan be had from tracing the areas and the boundaries of\\nthe constellations. The positions of the brighter stars of\\neach constellation should first be fixed in the memory.\\nThere are ten stars of the first magnitude and about thirty\\nof the second magnitude in the northern sky. After these,\\nor most of them, have been identified, the constellation\\nfigures may be taken up one by one and their boundaries\\ntraced. The six small star-maps of this book can be\\nused for this purpose in connection with the Map of the\\nEquatorial Stars. A celestial globe is even more con-\\nvenient and satisfactory, and every school should own one\\nif it is practicable. It should be constantly used to illus-\\ntrate or to prove the theorems of the text-book.\\nThe globe will be a material aid in planning any\\nseries of observations, and it should be always at hand to\\nexplain the results of observations already made.\\nThe course of one of the bright planets among the stars", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0017.jp2"}, "16": {"fulltext": "x INTRODUCTION.\\nshould be mapped from night to night. The path of the\\nMoon, also, should be followed whenever it is practicable.\\nThe place of a planet can be fixed with considerable pre-\\ncision by noting its allineations with two or more stars. In\\nthese observations it will be found useful to employ a\\nstraight ruler three or four feet long. The phases of the\\nMoon can be studied with the eye, or better, with a com-\\nmon opera-glass. A watch regulated to sidereal time\\nshould form a part of the equipment of the school.\\nIf a small telescope on a firm stand is available much\\nmay be done by its aid. Many of the surface-features of\\nthe bright planets {Mars, Jupiter) can be made out. The\\nexistence of the larger satellites of Jupiter and Saturn can\\nbe proved. The ring of Saturn can be seen. Some of the\\ndouble stars can be separated. The brighter nebulae can\\nbe shown. Some of the principal star groups or clusters\\ncan be studied. The changes in brightness of a short-\\nperiod variable star can be observed. The spots on the\\nSun can be shown by projecting the Sun s image on a\\nscreen.\\nIn these observations it is important to do the work\\nthoroughly and systematically. If the satellites of Jupiter\\nare in the field every student in the class should see all of\\nthe bright satellites that are then visible. If a double star\\nis viewed it should be looked at until both its components\\nare plainly seen, and so with other cases. No one should\\nleave the telescope unconvinced. The object of such\\nobservations is to make an ocular demonstration of facts\\nthat have heretofore been received on faith, not to make\\nadditions to science. For this reason the instructor should\\nselect the objects to be examined, with care. They should\\nbe typical, but not difficult to make out. Each student\\nshould be required to keep neat, accurate, and concise\\nnotes of his own observations, and whenever a drawing or\\na diagram will explain the observation he should be", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0018.jp2"}, "17": {"fulltext": "INTRODUCTION. xi\\nrequired to make it. All observations should be dated and\\nauthenticated with the pupil s signature. He should be\\ntaught to feel a responsibility for the records that he\\nmakes.\\nThe student should be practised in pointing out in the\\nsky the principal lines and points of the celestial sphere\\nthe meridian, the equator, the ecliptic, the vernal equinox,\\nthe poles of the two last-named circles, and so forth. There\\nis no mystery in these plain geometric figures. A little\\npractice will serve to make them quite familiar.\\nThe school should own a small collection of works on\\npopular and descriptive astronomy, which can be loaned\\nto the students for reading at home. These can be selected\\nby the teacher and added to the equipment of the school\\nfrom time to time, as fast as circumstances permit.\\nSimple models to illustrate the motions of the different\\ninstruments of astronomy are easy to make, and they are of\\ngreat practical utility in the class-room. Most of them\\ncan be made by the pupils. If practicable, models of the\\nsextant, the transit instrument, the meridian circle and the\\nequatorial should be provided. Directions for making\\nsuch models are given in the text.\\nFinally it is of the first importance that difficulties\\nshould not be shirked. To be useful, the student s work\\nshould be thorough so far as it goes. An instructor (or a\\nwriter of text-books) is often tempted to smooth away ob-\\nstacles, forgetting that one great use of the study of\\nscience is to train the mind to resolutely meet and to con-\\nquer difficulties. The advantage of scientific problems is\\nthat they are capable of a definite solution, and that the\\nstudent himself cannot fail to know whether he has or has\\nnot accomplished that which he set out to do. If our\\nnation is to take and hold a foremost place in the world,\\nit will do so through the predominance of certain qualities", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0019.jp2"}, "18": {"fulltext": "xii INTRODUCTION.\\nin its citizens that scientific education can foster to a very\\nimportant degree. We cannot afford to neglect any means\\nof developing thoroughness and faithfulness in the per-\\nformance of duty in those who will soon be the responsible\\ngovernors of our country. E. tS. H.\\nNew York, June 17, 1899.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0020.jp2"}, "19": {"fulltext": "TABLE OF CONTENTS.\\n(Consult the index at the end of the book also.)\\nPART I.\u00e2\u0080\u0094 INTRODUCTION.\\nCHAPTER PAGE\\nI. Introductory Historical. 1\\nII. Space\u00e2\u0080\u0094 The Celestial Sphere\u00e2\u0080\u0094 Definitions 15\\nIII. Diurnal Motion of the Sun, Moon, and Stars. 41\\nIV. The Diurnal Motion to Observers in Differ-\\nent Latitudes, etc 59\\nV. Co-ordinates Sidereal and Solar Time 77\\nVI. Time Longitude 94\\nVII. Astronomical Instruments 112\\nVIII. Apparent Motion of the Sun to an Observer\\non the Earth The Seasons 154\\nIX. The Apparent and Real Motions of the Plan-\\nets Kepler s Laws 1 79\\nX. Universal Gravitation 203\\nXL The Motions and Phases of the Moon 216\\nXII. Eclipses of the Sun and Moon 222\\nXIII. The Earth 232\\nXIV. Celestial Measurements of Mass and Distance. 260\\nPART II.\u00e2\u0080\u0094 THE SOLAR SYSTEM.\\nXV. The Solar System 269\\nXVI. The Sun 280\\nXVII. The Planets Mercury, Venus, Mars 299\\nxiii", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0021.jp2"}, "20": {"fulltext": "xiv TABLE OF CONTENTS.\\nCHAPTER PAGE\\nXVI11. The Moon\u00e2\u0080\u0094 The Minor Planets 315\\nXIX. The Planets Jupiter, Saturn, Uranus, and\\nNeptune 325\\nXX. Meteors 347\\nXXI. Comets 357\\nPART III.\u00e2\u0080\u0094 THE UNIVERSE AT LARGE.\\nXXII. Introduction 369\\nXXIII. Motions and Distances of the Stars 379\\nXXIV. Variable and Temporary Stars 386\\nXXV. Double, Multiple, and Binary Stars 390\\nXXVI. Nebulae and Clusters 393\\nXXVII. Spectra of Fixed Stars 400\\nXXVIII. Cosmogony 407\\nXXIX. Practical Hints on Making Observations\u00e2\u0080\u0094 Lists\\nof Interesting Celestial Objects\u00e2\u0080\u0094 Maps of\\nthe Stars 414\\nAppendix\u00e2\u0080\u0094 Spectrum Analysis 433\\nIndex 441", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0022.jp2"}, "21": {"fulltext": "SYMBOLS AND ABBREVIATIONS.\\nSIGNS OF THE PLANETS, ETC.\\no\\nThe Sun.\\n3 Mars.\\nThe Moon.\\nU Jupiter.\\nMercury.\\nSaturn.\\n9\\nVenus.\\n5 Uranus.\\nor\\nThe Earth.\\nf Neptune.\\nThe asteroids are distinguished by a circle enclosing a number,\\nwhich number indicates the order of discovery, or by their names, or\\nby both, as (TOO) Hecate.\\nThe Greek alphabet is here inserted to aid those who are not\\nalready familiar with it in reading the parts of the text in which its\\nletters occur\\nLetters.\\nNames.\\nLetters.\\nNames.\\nA a\\nAlpha\\nN v\\nNu\\nB (3\\nBeta\\nE\\nXi\\nr r\\nGamma\\no\\nOmicron\\nJ d\\nDelta\\nn\\nPi\\nE e\\nEpsilon\\np P\\nRho\\nz c\\nZeta\\na s\\nSigma\\nHr,\\nEta\\nT v\\nTau\\ne\\nTheta\\nT v\\nUpsilon\\nI i\\nIota\\n(p\\nPhi\\nK K\\nKappa\\nX X\\nChi\\nA X\\nLambda\\nW tft\\nPsi\\nM ju\\nMu\\n\u00c2\u00a31 GO\\nOmega\\nTHE METRIC SYSTEM.\\nMEASURES OF LENGTH.\\n1 kilometre iOOO metres 0.62137 mile.\\n1 metre the unit 39.370 inches.\\n1 millimetre j^ of a metre 03937 inch.\\nMEASURES OF WEIGHT.\\n1 kilogramme 1000 grammes 2.2046 pounds.\\n1 gramme the unit\\n15.432 grains.\\nThe following rough approximations may be memorized\\nof a mile, but less than ^Tof\\nThe kilometre is a little more than\\na mile. The mile is 1 T kilometres.\\nThe kilogramme is 2i pounds. The pound is less than half a kilo-\\ngramme.\\nOne metre is 3.3 feet. One metre is 39.4 inches.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0023.jp2"}, "22": {"fulltext": "", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0024.jp2"}, "23": {"fulltext": "ASTRONOMY.\\nCHAPTER I.\\nINTRODUCTORY\u00e2\u0080\u0094 HISTORICAL.\\n1. Astronomy denned. Astronomy (from the Greek\\nacrrr/p, a star, and vojaos, a law) is the science that is con-\\ncerned abont the laws that the heavenly bodies obey, and\\nwith a description of the bodies themselves both as they ap-\\npear to be and as they really are. For instance the Snn ap-\\npears to ns very different from a bright star; bnt astron-\\nomy shows that the Sun is itself a star like thousands of\\nothers that we see in the sky at night. The Sun appears to\\nmove across the sky from east to west, from rising to set-\\nting, every day. Astronomy explains that this motion is\\nonly apparent and that, in fact, it is caused by the\\nEarth s turning on its axis, daily.\\nThe Sun appears to move among the stars so as to go\\ncompletely around the sky from one star back to the same\\nstar again every year. Astronomy proves that, in fact,\\nthe San does not move, but that its apparent course is\\nnothing but the result of the Earth s real motion around\\nan orbit a path with the Sun near its centre. The\\nplanets, like the stars, appear to shine by their own light.\\n1", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0025.jp2"}, "24": {"fulltext": "2 ASTRONOMY.\\nAstronomy shows that the planets shine by reflected sun-\\nlight, while the light of the stars is native to them. As-\\ntronomy is the science that seeks the trne explanation of\\nthe appearances presented by the stars. Astronomy is the\\nscience of the stars; or more particularly, it is the science\\nthat explains what the stars really are, how they really\\nmove, and why they appear to move as they do. The\\nword star is used so as to include planets, comets, the\\nSan, the Moon, etc. It is used as the Greeks used it, to\\nmean any heavenly body.\\n2. How we get our notions of the Universe of Stars. We\\nknow things on the Earth through our senses, by touching\\nthem, tasting, smelling, hearing, or seeing them. A piece\\nof iron can be felt and weighed as well as seen. If one\\nof our senses makes a mistake, another sense often comes\\nin to correct the error. A piece of cork might be painted\\nso as to look precisely like a piece of iron of the same size.\\nThe sight alone could not distinguish between them. But\\nif we take the two things in our hands, the sense of touch\\nor of weight detects a difference at once.\\nStars in the sky are known to us only through the sense\\nof sight. If that sense is deceived, there is no other one\\nto correct it. A blind person can know much about things\\non the Earth, but he can know very little indeed about\\nthe stars that he cannot see. All our first-hand notions of\\nthe universe of stars come to us through our sense of sight.\\nOur eyes tell us how things appear to be, and we do not\\nknow how they really are until we have reasoned about the\\nappearances and sifted out the truth. A bright rainbow\\nlooks almost like a solid arch in the sky, while it is, in\\nfact, not in the sky at all. It is in our own eyes. When\\nwe travel in a railway train, parts of the landscape seem\\nto be moving about other parts yet nothing is more cer-\\ntain than that the landscape is really unchanged. It re-\\nquires reasoning to interpret such appearances.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0026.jp2"}, "25": {"fulltext": "INTROD UGTOB Y\u00e2\u0080\u0094 HISTORICAL. 3\\nWhat senses can you use to learn about things on the Earth\\nGive an example of a thing that you can touch of a thing that you\\ncan taste of a thing that you can hear From all these separate\\nsenses you gain notions of things on the Earth. How do you get your\\nideas about a star It is by the sense of sight alone, is it not If\\nyou have only one sense to help you, you have to be extremely care-\\nful not to be deceived by it. Give some examples of how the sense\\nof sight deceives in appearances on the Earth.\\n3. The Heavens were carefully observed by the An-\\ncients. The very first man could not fail to notice the\\nrising and setting of the Sun. The coming of Night\\noften a time of terror and danger to him was a mystery\\nand the advent of successive days was a perpetual miracle.\\nThe Sun brought cheerfulness, safety, warmth, comfort.\\nHis rays made plants grow and provided food. He was\\nworshipped as a God by the men of early times. When\\nthe Suu was darkened by an eclipse and the day itself grew\\nblack the people were filled with dread. Special men\\npriests were appointed to observe such occurrences and\\nto foretell them. It was by the diligent watching of these\\npriests that the different appearances in the sky were first\\ncarefully noted, and the first lists of the stars made.\\nBy and by, as men in general had more leisure, the\\nscience of the stars, like other sciences, was studied for its\\nown sake. Men were curious to understand lohy the Sun\\nrose and set; why it was sometimes eclipsed, and so forth.\\nMoreover, their knowledge of astronomy was put to prac-\\ntical uses. In the earliest times navigators did not dare to\\nventure out of sight of land, or to make voyages at night.\\nThey sailed from headland to headland during the day, and\\ntied their little vessels to the shore at night. They steered\\ntheir course by landmarks. But wise men had noticed\\nthat while the stars in general rose and set, there were\\nsome stars that were always visible the North Star, for\\nexample. They could use the North Star for a steering-\\nmark by night, then and so they did.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0027.jp2"}, "26": {"fulltext": "4 ASTRONOMY.\\nNearly three thousand years ago (1012 B.C.) Solomon\\nbuilt the Temple at Jerusalem and ornamented it with gold\\nbrought by ships from South Africa. The Phoenicians\\n(who lived on the north shores of Africa) brought tin\\nfrom England about the same time. These long voyages\\nmust have been made by using the stars as guides by night.\\nFig. 1.\u00e2\u0080\u0094 The Stars of the Northern Sky.\\nThe Pole-star is at the centre of the cut. The Great Bear (the Dipper) is\\nat the left-hand side. The arrows show the direction in which the stars\\nmove round the pole.\\nThe mariner s compass, which is our guide nowadays, was\\nnot known in Europe before a.d. 1300, though the Chi-\\nnese sailors used it long before that time.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0028.jp2"}, "27": {"fulltext": "IN TROD UCTOR Y-HISTORICAL. 5\\nWhat was tlie earliest observation of astronomy? Who were\\nthe first astronomers? After the priests, who studied the stars?\\nHow did astronomy make itself useful to navigation What star is\\nalways visible on clear nights in our part of the world Does the\\nNorth Star rise and set Mention some long voyages made by the\\nancients.\\n4. Some of the great astronomers of ancient times.\\nWe do not know the names of the ancient priests and\\nwise men of Assyria, Babylon, China, and Egypt who first\\nstudied the stars. A few of their records that have come\\ndown to ns date back to 2200 B.C. in Chaldsea and to 2900\\nB.C. in China. Our history of astronomy begins long\\nafter their time, with the Greeks, about six centuries be-\\nfore Christ, about 2500 years ago. The Greeks of that time\\nwere a very intelligent, clever, hardy, adventurous people,\\neager to learn and to practice what they learned. They\\nwere good sailors and good soldiers.\\nThales (pronounced tha lez), one of the seven wise men\\nof Greece, was born about 640 B.C. He showed his coun-\\ntrymen how to divide the year into seasons. At midsum-\\nmer the Sun at noon was higher in the heavens than at any\\nother time of the year (about June 21). At midwinter\\nthe Sun, at noon, was lowest (about December 22). These\\nwere the two solstices (pronounced soTsti-ces). About\\nMarch 20 and September 22 the days and nights were of\\nequal length (at the two equinoxes). March 20 was the\\nvernal, or spring, equinox; September 22 was the autum-\\nnal equinox. Each and every year could be divided into\\nseasons in this way because the Sun was always highest in\\nthe heavens (nearest to the point overhead) in June, al-\\nways lowest in December because the days and nights\\nwere always of equal length in March and in September.\\nThales did not know why this was so. But he knew the\\nfacts. And he first showed the Greeks how to divide their\\nyear into parts. Before his time the Greek sailors had", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0029.jp2"}, "28": {"fulltext": "6 ASTRONOMY.\\nsteered their ships by the stars of the Great Bear (see Fig.\\n1). He showed them, it is said, that the stars of the\\nLittle Bear, which were nearer the pole, would serve the\\npurpose better.\\nAnax i^andee, a friend of Thales (610 B.C.) invented\\nthe sun-dial. The shadow of an upright column made by\\nthe Sun moved during the day, and the motion of the\\nshadow marked the passing of the hours. Sun-dials were\\nthe first clocks. He explained why it is that the Moon\\nchanges every month from a crescent (new Moon) to a\\nfull Moon and other matters.\\nPythag oras (born 582 B.C.) travelled in Egypt and\\nlearned much of the science of the Egyptian priests. Sev-\\neral of the Egyptian pyramids were built at least a thou-\\nsand years before Christ, and many of them were built by\\nastronomical rules, so as to face the North Star and so\\nforth. Pythagoras brought much foreign science home\\nto the Greeks. It was he who first taught his countrymen\\nthat the morning and the evening star Venus) was the\\nsame body. It had formerly been thought that Phos-\\nphorus (the name for the morning star) and Hesperus (the\\nevening star) were two different planets. It was a great\\ndiscovery to learn that there was only one planet, some-\\ntimes seen in the west at sunset, sometimes in the east\\nabout sunrise. You know the fact, just as Pythagoras\\ndid twenty-four centuries ago. He did not thoroughly\\nunderstand why it was so, but the reason will be plain to\\nyou before you have finished this little book.\\nAnaxag oras (born 500 B.C.) knew all the bright plan-\\nets Mercury j Venus, Mars, Jnpiter, and Saturn and\\nunderstood how they moved in the heavens, among the\\nstars. He knew that the stars did not move among each\\nother. A group of stars like the Great Dipper (see Fig.\\n1) keeps the same shape during centuries. Planets (wan-\\ndering stars) move among the fixed stars. Anaxagoras", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0030.jp2"}, "29": {"fulltext": "INTROD UCTORY\u00e2\u0080\u0094 HISTORICAL. 7\\nexplained eclipses too. He said that the dark body of the\\nMoon came in between the Sun and the Earth and shut off\\nthe Sun s light, just as your hand held in front of a candle\\nshuts off its light.\\nAr istotle (born 384 B.C.) was the first Greek to prove\\nthat the Earth is a globe. He was the friend of Alexan-\\nder the Great and the pupil of Plato, who was the pupil\\nof Socrates. He studied every kind of science and wrote\\nmany books. (Books in his day were manuscripts of\\ncourse printing was not invented in Europe till about\\na.d. 1450, though the Chinese had practiced the art long\\nbefore.) Alexander the Great founded a splendid city\\nAlexandria, in Egypt and endowed it with schools, col-\\nleges, libraries, museums, observatories. It was full of\\nlearned men of all sorts physicians, geographers, gram-\\nmarians, mathematicians, and among them were many\\nfamous astronomers. Euclid, the geometer, was born\\nthere about 300 B.C. Archime des, the famous mathe-\\nmatician (born 287- B.C.), studied there Eratos thenes\\n(born 276 B.C.) was the keeper of the Royal Library, and\\nthere he made a great map of the world and tried to\\nmeasure the circumference of the earth.\\nHipparchus (born 160 B.C.) was an Alexandrian, too,\\nand he is the Father of Astronomy. He collected all the\\nobservations of the men who had gone before him and\\nmade great discoveries of his own, of which we shall hear\\nmore. Most of his writings are lost, but his discoveries\\nare described by Ptolemy of Alexandria (who lived in the\\nfirst century after Christ) in his great work the Almagest.\\nThis book, which sums up all the astronomical knowledge\\nof the ancients, was the greatest scientific work of the Old\\nWorld. Its doctrines were believed and taught in all the\\nschools and universities of Europe from Ptolemy s time\\nup to the time of Galileo (died 1642) that is, for fifteen\\ncenturies.", "height": "3582", "width": "2230", "jp2-path": "elementaryastron00hold_0031.jp2"}, "30": {"fulltext": "8 ASTRONOMY.\\nPtolemy s book declared that the Earth was the centre\\nof the Universe, and that the Sun and all the planets moved\\nround it. Another great book was written by Copernicus\\n(died 1543) to prove that the Snn and not the Earth was\\nthe centre of the system; and that the Earth was only one\\nof the planets, all of which moved round the Snn. This\\nwas and is the truth but it was not established until the\\ndiscoveries made by Galileo (1610) with the telescope\\nthat he constructed.\\nStill another great book, the Principia, was written by\\nSir Isaac Newton in 1687, to prove that all the motions\\nof all the planets and all the stars are the results of one\\nsingle force the force of gravitation or attraction exerted\\nby every heavy body on every other such body. New-\\nton is the Father of Modern Astronomy, just as Hip-\\nparchus was the Father of the Astronomy of the Ancients.\\nThe books of Ptolemy, of Copernicus, and of Newton\\nare landmarks in the history of Astronomy. Their dates\\nshould be remembered: A.D. 140, a.d. 1543, A.D. 1687.\\nWhen does the history of astronomy begin? To what nation\\ndid the first learned astronomers belong? What sort of a people\\nwere the Greeks Who showed the Greeks how to divide the year\\ninto seasons When did Thales live Who invented the sun-\\ndial Who first explained the changes in the Moon s shape\\nWhen did Anaximander live What Greek brought home the\\nlearning of the Egyptians When did Pythagoras live Who\\nfirst taught how the planets moved among the stars and that the\\nstars were fixed? and explained eclipses? About what time did\\nAnaxagoras live When did Aristotle live? He was the friend\\nof what great King? Who founded Alexandria in Egypt? What\\nsorts of learning were encouraged there Name some of the famous\\nmen who studied and taught there. Who is called the Father of\\nAstronomy What is the name of Ptolemy s great book How\\nlong was the Ahnagest the greatest authority on astronomy Where\\nwas the centre of the Universe acording to Ptolemy Where did\\nCopernicus (1543) place it Whose discoveries proved Coperni-\\ncus to be right Who constructed the first telescope When did", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0032.jp2"}, "31": {"fulltext": "INTROD UCTOR Y\u00e2\u0080\u0094 HISTORICAL. 9\\nGalileo live Who is the Father of Modern Astronomy When\\nwas the Principia of Sir Isaac Newton written?\\n5. How Astronomy might be studied.\\nIn the paragraphs just preceding, a few of the names of the great\\nmen of past times have been mentioned and something has been said\\nof their discoveries. If there were time enough there could be no\\nbetter way to study Astronomy than to follow its history step by step\\nfrom the most ancient times until now. Six centuries before Christ,\\n2500 years ago, the earliest Greek philosophers began to study\\nscience for its own sake. They were curious about the world around\\nthem and about all the appearances they saw in the sky. They\\nwished to understand the motions of the planets, their distances, the\\nshape and size of the Earth, the cause of eclipses, and so on. One\\nafter another of their great men accurately described or fully ex-\\nplained motions or appearances in the sky.\\nEach philosopher taught what he had learned to his favorite\\npupils by word of mouth. They in their turn taught others in the\\nsame way. Finally in the time of Alexander the Great (332 B.C.)\\nthe city of Alexandria in Egypt was founded, and splendidly en-\\ndowed with colleges, schools, museums, observatories, and so forth.\\nLearned men were invited thither from every other city. For sev-\\neral centuries it was the centre of learning for the whole world. Its\\nlibraries contained 700,000 manuscripts. Here a succession of great\\nastronomers and mathematicians laid the foundations of the science.\\nThe work of Hipparchus was gathered together in a systematic\\ntreatise (the Almagest) by Ptolemy, and this book held its place of\\nauthority for fifteen centuries.\\nAlexandria was conquered by Rome in 30 B.C. When the Roman\\nEmpire was ruined in the IV century the fortunes of the city de-\\nclined, and it was itself captured and sacked by the Saracens in 641\\nA.D. Learning, especially scientific learning, was at a very low ebb\\nin Europe during the Dark Ages (a.d. 400 to 1400). It was not\\nuntil the time of Columbus (1492) and Copernicus (1543) that ad-\\nvances were made, except by the Moors in Spain (709 a.d. to 1492).\\nThe universities and schools established in these centuries still\\ntaught the astronomy of Aristotle and of Ptolemy.\\nThe great book of Copernicus (De Orbium ccelestium revolutioni-\\nbus on the revolutions of the celestial bodies) was printed in 1543.\\nIt announced and proved the great discovery that the Sun and not\\nthe Earth was the centre of the celestial motions. The doctrine of\\nPtolemy declared that the Sun and planets moved around the\\nEarth. Galileo constructed the first astronomical telescope in 1609,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0033.jp2"}, "32": {"fulltext": "10 ASTRONOMY.\\nand in 1610 he made such discoveries by its aid that the Copernican\\ndoctrine was fully established in the minds of all competent judges.\\nBut there were few such judges in his day.\\nKepler discovered the laws according to which the planets move\\nin their orbits in the years 1609-1618. But it was not until the ap-\\npearance of Sir Isaac Newton s Principia in 1687 a little more\\nthan two centuries ago that modern astronomy was born. He an-\\nnounced and proved that all the motions and all the appearances in\\nthe Universe were mere consequences of a single law of gravitation,\\nof attraction. Since his time immense advances have been made, but\\nmost of them are but consequences of his law, and are explained by it.\\nAstronomical instruments have been wonderfully improved also, and\\ngreat discoveries have been made. The system of Copernicus, as\\nexplained by Newton, has been firmly established by all these ad-\\nvances.\\nIf there were leisure to follow out in detail all these discoveries\\nand advances, the pupil could be taken through the experience of\\nthe race and could successively master each great problem just as it\\nwas mastered by Thales, Hipparchus, Copernicus, and Newton.\\nThere is no more satisfactory and thorough method of study than\\nthis. Unfortunately it requires far more time than is available. It\\nis impossible, here, to explain the system of the world according to\\nPtolemy and to take the time to prove it to be wrong. All that can\\nbe done is to explain the system of Copernicus and to prove it to be\\nright. It is necessary, therefore, to study Astronomy in our High\\nSchools in a different order. Each subject must be so treated as to\\nprepare the way for other topics, and everything must be presented\\nin the briefest manner. The science of Astronomy is so vast, and so\\nmany brilliant discoveries have been made by so many able men, that\\nthe limits of this little book do not permit an historical treatment.\\nHow long ago were the beginnings of the Astronomy of the\\nGreeks? How did the ancient Greek philosophers teach their pupils?\\nWhen was the city of Alexandria founded How many manuscripts\\nwere contained in its libraries? (The National Library at Washing-\\nton has not even now so many books.) Ask your teacher to tell you\\nabout the Dark Ages in Europe, or else read about them in an en-\\ncyclopaedia. What system of Astronomy was taught in European\\nuniversities in those times Where did Ptolemy say the centre of\\nthe universe was? Copernicus (1543) taught that the centre of the\\nworld was where Which is right It will be abundantly shown\\nin this book that Copernicus was right.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0034.jp2"}, "33": {"fulltext": "INTROD UCTOR Y\u00e2\u0080\u0094 HISTORICAL. 1 1\\n6. General notions of Astronomy.\\nEven before beginning the study of Astronomy every\\none of us has a certain knowledge of it. All of us have\\nstudied Geography and know what is there said of the Sun\\nand planets; all of us have heard certain facts of Astron-\\nomy talked about; and every one of us has observed a few\\nthings for himself. Let us set down something like an in-\\nventory of this general knowledge here. It is well to find\\nout just what we know now before going on to new things.\\nThe Earth is a huge globe, so large that any small part of it, where\\nwe live, looks flat and not round. Its diameter is nearly 8000 miles.\\nWe know that the Earth is a globe because it was circumnavigated\\nby Magellan s ships in the years 1519-1522, and by hundreds of\\nvessels since that time and because nearly every part of it has been\\nvisited by travellers and finally because surveys have been made of\\nmost civilized countries. The Earth is certainly a globe and its size\\nis enormous compared to our houses, cities, etc. It is not large com-\\npared to the Sun and to some of the other planets. It is isolated in\\nspace. It does not touch any other planet or star. Even the Moon\\nis very distant from it r and the Sun is much further away. The stars\\nare further off still.\\nThe Earth turns round on its axis once in every day, and its turn-\\ning makes the Sun and the Moon and the stars appear to rise and set.\\nMoreover the Earth, like the other planets, moves round the sun in\\nan orbit a path once in a year, and this revolution of the Earth\\nhas something to do with the seasons Spring, Summer, Autumn,\\nWinter which recur regularly every year.\\nThe Sun looks to us like a large flat disc, but it is, in fact, a huge\\nglobe, much larger than the Earth. It is the source from which\\ncomes all our light and all our heat. Our seasons Spring, Summer,\\nAutumn, and Winter depend upon the amount of heat received from\\nthe Sun at different times. Just what makes the light and heat we\\ndo not learn from Geography, and that is one of the important things\\ntaught in our text-books of Physics. All the planets the Earth,\\nVenus, Jupiter, etc., move round the Sun in orbits paths and to-\\ngether make up the Solar System the Family of the Sun.\\nThe Moon also looks to us like a flat disc, but it is, in fact, a globe,\\nsmaller than the Earth. It revolves about the Earth, not about the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0035.jp2"}, "34": {"fulltext": "12 ASTRONOMY.\\nSun, in an orbit path, and it is very far away from us. Sometimes\\nits shape is that of a crescent sometimes it is circular. It regularly\\ngoes through all its shapes and comes Back to the same shape again\\nonce in every month, and so on forever. The Moon sometimes comes\\nbetween the Earth and the Sun and shuts off some or all of the Sun s\\nlight in the daytime and makes an eclipse, just as a book held in\\nfront of a candle will cut off the candlelight and eclipse it. The\\nMoon is usually bright, but it is itself sometimes eclipsed. The face\\nof the Moon seen by the naked eye at night (or seen through an\\nopera-glass or a telescope) is not everywhere the same. Parts of it\\nare much brighter than other parts and there are mountains on the\\nMoon.\\nThe Planets look to us like bright stars. Venus is often seen to-\\nwards the west at sunset, and is called the Evening Star and some-\\ntimes we may have seen it towards the east about sunrise, when it is\\ncalled the Morning Star. It is brighter than most of the stars.\\nJupiter, Mars, and Saturn are planets that look like stars, too.\\nThe Comets are sometimes very bright, we have heard. They\\nmove about in space, and people say that if one should hit the Earth\\nthere would be a great disaster.*\\nThe Stars lie all around us, and they are visible by hundreds at\\nnight. Most of them rise and set, but there are some near the North\\nPole that are always visible whenever it is night. They are\\ndivided into constellations, or groups and one of the groups is called\\nthe Great Bear or the Dipper. Some of the stars are quite bright,\\nothers much fainter. If a telescope is used, many thousands of stars\\ncan be seen that are invisible to the naked eye. The stars are ex-\\nceedingly far away from us.\\nIf some one who had not yet studied Astronomy were\\nasked to give his notions about the heavenly bodies he\\nwould probably say something like what has been printed\\nin the preceding paragraphs. The information is generally\\ncorrect so far as it goes, but there are many things lacking\\nto make it complete. We ought to know why it is that\\nthe seasons come back to us year after year in order; why\\nIt may as well be said here that in the first place such a collision\\nis very unlikely to happen and that if it did happen it is probable\\nthat the Earth would not suffer.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0036.jp2"}, "35": {"fulltext": "INTROD UCTOR Y\u00e2\u0080\u0094 HISTORICAL. 1 3\\nthe Sun gives us light and heat why the Sun is eclipsed\\nby the Moon sometimes, and why it is not eclipsed every\\nmonth; why the Moon itself is sometimes eclipsed, and so\\nforth. Perhaps the most important thing that is left out\\nof this account is the fact that the Sun shines by its own\\nlight (just as an electric light does), while the Moon and\\nall the planets do not shine by their own light, but by re-\\nflected sunlight. They appear bright just as a mirror that\\nis shined upon appears bright. If you shut off the light,\\nthe mirror is no longer bright. If the Sun were to be an-\\nnihilated, all the planets and the Moon would instantly be\\ndark. They are only bright because the sunlight shines\\nupon them and because they reflect the sunlight back to the\\nEarth much as a mirror might do. All the stars are suns,\\nand each and every star shines by its own light, just as the\\nSun does.\\nWith these additional facts about the Sun, which shines\\nby its own light; about the planets, which shine by reflect-\\ned light; and about the stars, which shine, like the Sun, by\\nnative light, we can go on to study Astronomy in detail.\\nWe have a general idea of it to begin with, and we know\\nsome, at least, of the lacks in our present knowledge.\\nThis book does not take such general knowledge to be\\nproved. On the other hand the facts above set down will\\nbe explained and proved. But as every one has some\\nknowledge of astronomical facts, the book is not written as\\nif no one had any information of the kind.\\nHow do we know that the Earth is a globe Who first circum-\\nnavigated it? How long ago? How do we know that the Earth is\\nisolated in space? Two of the Earth s motions make the day and the\\nyear which two? What heavenly body that you know of goes\\nthrough a series of changes every month Does the Sun shine by his\\nown light? Does the Moon shine by her own light? Do the planets\\nso shine If you could stand a long way off from the Earth, would", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0037.jp2"}, "36": {"fulltext": "14: ASTRONOMY.\\nthe Earth be dark or would it shine like the other planets? Do the\\nstars shine by the light of the Sun Suppose the Sun suddenly be-\\ncame dark so that it gave out no more light, what changes would\\nthis make in the appearance of the sky at night What bodies\\nwould no longer be visible What others would continue to shine\\nunchanged", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0038.jp2"}, "37": {"fulltext": "CHAPTER II.\\nSPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS.\\n7. Space. The Sun, the planets, and all the stars are\\nmoving in Space. It is often called empty Space be-\\ncause it contains no large masses except the Snn and plan-\\nets, the stars, the comets, and so forth. It is necessary to\\nhave some idea of its vastness, for it contains all these\\nbodies and every other thing that exists. It is infinite\\nwithout any limits or boundaries. For suppose it had a\\nboundary, what would lie beyond that Only more and\\nfurther extensions of space. We cannot realize exactly or\\neven imagine what Space is; but we can obtain a few cor-\\nrect ideas about it.\\nSuppose that on a clear night you look up at the full\\nMoon in the heavens. It seems to be, and it is, extremely\\ndistant. It is 240,000 miles away. It is 240 times as dis-\\ntant from us as New York is from Chicago. Now think of\\nthe Sun, which is 870,000 miles in diameter. The diam-\\neter of the globe of the Sun is about 3^ times the distance\\nof the Moon from the Earth. The Sun is one of the stars,\\naud there are hundreds and hundreds of bright stars visi-\\nble to the naked eye. There are millions and millions of\\nstars visible in a great telescope. All these stars are scat-\\ntered about in space somewhat as pictured on page 16.\\nSpace contains millions of stars, and each star (as a, b,\\nc, etc.) is at least as far from every other star (as/, g, h, i,\\nk, I, m, etc.) as the nearest stars (Z c, g, h, I, m) are\\nfrom the Sun.\\n15", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0039.jp2"}, "38": {"fulltext": "16 ASTRONOMY.\\nTry to conceive this arrangement of stars clearly. They\\nare scattered everywhere in Space. There are millions\\nupon millions of them. Each one of them is as distant\\nfrom its nearest neighbor as the stars nearest to the Sun are\\ndistant from the Sun. Now how far is the nearest star\\nfrom the Sim? We shall see by and by that it is at least\\n20,000,000,000,000 miles; that is, twenty millions of mil-\\nlions of miles. Every other star in the sky is as distant\\nfrom its nearest neighbor as this. And there are millions\\nof such stars in succession one to another as we go out-\\na\\nb\\nc\\nd\\ne\\nK\\nK\\n9\\nI\\nSun\\nm\\nh\\nn\\ni\\no\\nj\\nFig. 2.\\nThe stars are arranged in Space somewhat as in the picture, only not in\\na plane, but throughout a solid.\\nwards through Space. Space contains them all, and there\\nis room for countless millions more. The spaces between\\nthem are empty.\\nLet us try to realize this in another way. Think first\\nof the Sun it is 870,000 miles in diameter. Then think\\nof the nearest star. It is 20,000,000,000,000 miles from\\nthe Sun. Then imagine a whole universe of countless\\nmillions of stars no one nearer to another than twenty\\nbillion miles. All these stars may be thought of as a\\ngreat cluster in the shape of a globe. Imagine this cluster\\nto shrink and shrink, to get smaller and smaller. The\\nstars will come nearer and nearer to each other, and the\\nglobe of the Sun (870,000 miles in diameter, remember)", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0040.jp2"}, "39": {"fulltext": "SPACE-TEE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 17\\nwill also grow smaller at the same time and in the same\\nproportion. Let the shrinking go on till the universe is\\n2,300,000,000 times smaller than at first\u00e2\u0080\u0094 till the Sun s\\nglobe is only two feet in diameter,* and then stop the\\nshrinking.\\nWe shall have a model of the universe with everything\\nin its true proportions, only the San will be two feet\\nin diameter instead of 870,000 miles. Now how far\\noff will the nearest star to the Sun be, in this shrunken\\nmodel of the universe? It will be as far from the Sun as\\nthe city of Peking is from the city of New York The\\nnearest star will be so far off. The other stars will be ar-\\nranged in order out beyond this one, and none of them\\nwill be any nearer, in this model, to its neighbors than the\\ndistance from China to New York. And the model must\\ncontain millions of stars. Even this model will be incon-\\nceivably large. The real universe Space is inconceiva-\\nbly larger than the model. An illustration like this en-\\ntirely fails to give a measure of the size of Space, but it\\ncertainly does give some conception of its immense exten-\\nsion. In thinking of the universe of stars you must try\\nto realize it in this way. The Sun and all the stars lie in\\nspace, none of them near together, with immense empty\\nregions between the different bodies. Each star is incon-\\nceivably far from its nearest neighbors, and there are mil-\\nlions upon millions of stars. It is not at all easy to have\\nclear ideas of an infinite extension but it is absolutely\\nnecessary in beginning the study of Astronomy to have\\nsome idea of the space in which the Sun, all the planets,\\nand all the stars exist.\\nWhy do we call Space empty How far away is the Moon\\nfrom the Earth The diameter of the globe of the Sun is how much\\nlarger than this distance Is the Sun a star Space contains mil-\\nTwo feet is 3000 1 000 th part of 870,000 miles.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0041.jp2"}, "40": {"fulltext": "18 ASTRONOMY.\\nlions upon millions of stars. Each star is at least twenty millions of\\nmillions of miles from its nearest neighbors. Are the spaces be-\\ntween tliem empty of large bodies? Suppose you could make an\\nexact model of Space with each star in its right place, and suppose\\nyou could make this model shrink until the 870,000 miles of the\\nSun s diameter had shrunk to two feet how far off would the star\\nnearest to the Sun be from the Sun itself Would these words do\\nfor a definition of Space Space is indefinite extension If you have\\na dictionary, look up the word and see how it is defined there.\\n8. The Celestial Sphere. In what has just been said\\nabout Space we have spoken of the universe as it really is.\\nThe stars are scattered all about through Space at enormous\\ndistances one from another. That is the way the universe\\nreally is. ISTow we have to ask how does it appear to be to\\nus If you look at the heavens on a clear night what do\\nyou see In the first place you see hundreds of stars, some\\nvery bright, some less bright. They all seem to be at the\\nsame distance from you. They look as if they were bright\\npoints fastened to the inside surface of a great hollow globe\\nthe celestial sphere hung over the Earth. You see the\\nbright points. The surface on which you imagine them\\nto lie is called the celestial sphere. There is, in fact, no\\nsuch surface, but there seems to be one. Let us make a\\nformal definition of it which is to be learned by heart.\\nThe Celestial Sphere is that surface to which the stars seem\\nto be fastened. No one ever thinks of the stars as if they\\nwere outside of the celestial sphere and shining through it.\\nIn Fig. 3 the black square is a part of Space.* There\\nare a few stars in it, namely p, q, r, s, t, t, t, u, v. In\\nrespect to the immense distances of the stars, the Earth, 0,\\nmay be considered as a mere point. The configurations of\\nthe stars are the same whether you are at Lisbon or at\\nThe student must remember here and throughout the book that\\nthe drawings have to be on a small scale. All the Universe has to\\nbe drawn on a few square inches.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0042.jp2"}, "41": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 19\\nNew York. No change of place on the Earth alters the\\ngrouping of the stars. You are on the Earth looking out\\nat the sky at night and you see all these stars. If you look\\nat the star which is really at q you are looking along the\\nline Oq and see it as if it were on the surface of the celes-\\ntial sphere at Q. If you look at r and s, you see them at\\nThe Earth is supposed to be at O, a few of the stars at p, q, r, s, f, t, t, u, v.\\nThese stars are seen by us as if they were all on the surface of the celes-\\ntial sphere at P, Q, R, S, T, U, V.\\nR and S. If you look at u and v you see them at U and\\nV. All of them appear to be at one and the same distance\\nfrom you, though they really are at very different dis-\\ntances. The point Q is in the line Oq prolonged; the\\npoints R, 8, U, V are in the lines Or, Os, Ou, Ov pro-\\nlonged. Now suppose there happened to be three stars, t,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0043.jp2"}, "42": {"fulltext": "20 ASTRONOMY.\\nt, t, in a line. They would all three appear on the celes-\\ntial sphere at T. You would never know there were three\\nseparate stars, because you could only see one bright point\\nat T. You do not see the other stars r, s, etc.,\\nwhere they really are, but at places on the celestial sphere\\nat E, S, V.\\nFig. 4.\u00e2\u0080\u0094 The Earth (n, q, s) in the Centre op the Celestial\\nSphere.\\nOn the surface of the celestial sphere meridians and parallels are sup-\\nposed to he drawn corresponding to meridians and parallels on the Earth.\\nWhat you see in a dark night is stars apparently studded\\nover the inner surface of the celestial sphere. It is only\\nby reasoning about it that you know they are not on this\\nsurface but scattered about inside of the sphere. The an-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0044.jp2"}, "43": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 21\\ncient astronomers thought that the sphere actually existed\\nand that the stars were really fastened to it. Although it\\ndoes not exist, the idea can be made to serve a useful pur-\\npose. For instance, if we want to know the angle be-\\ntween the two lines Or and Os (the angle between the two\\nlines joining the Earth and two distant stars) all we have\\nto do is to measure the arc RS on the celestial sphere.\\nThe arc RS is the measure of the angle rOs in space.\\nThe sphere has other uses, too. Just as there is a ter-\\nrestrial equator on the globe of the Earth (and terrestrial\\nmeridians, etc.), so there is a celestial equator (and celes-\\ntial meridians, etc.) on the celestial sphere. The simplest\\npart of astronomy deals with the apparent places of stars\\nas they seem to be on the celestial sphere it is called\\nSpherical Astronomy for that reason. It is only after we\\nhave learned about the apparent places and motions of\\nstars and planets that we can go on to study their real\\nmotions. So that the idea of a celestial sphere will be use-\\nful. Whenever you go out at night you will see it it is\\nthe dark sphere on which the bright stars seem to rest.\\nImagine that the stars are not there yet the sphere will\\nremain. Every one imagines the blue vault of the sky in\\nthe daytime as if it were a hollow sphere hanging over us.\\nThe Sun seems to be on its inner surface. When you see\\nthe Moon in the daytime it, too, seems to lie on the celes-\\ntial sphere.\\nThe stars really are at very different distances from us all are\\nvery far away, but some are much further away than others do\\nthey seem to be at different distances when you look at them at\\nnight Do they seem to lie on the inner surface of a sphere What\\nis the celestial sphere Is it a sphere that really exists, or only one\\nthat appears to exist Does the celestial sphere seem to exist in the\\ndaytime as well as at night?\\n9. Some Mathematical Terms used in Astronomy. It", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0045.jp2"}, "44": {"fulltext": "22\\nASTRONOMY.\\nis convenient to nse a few mathematical terms in speaking\\nabout the geometrical parts of Astronomy. All of the\\nmathematical ideas here introduced are simple, but it may\\nbe well to set them down in order. If they are under-\\nstood by the student he will have no difficulty in compre-\\nhending the astronomical matters that are to be spoken of.\\nIf they are not thoroughly understood some points will not\\nbe as clear as they should be.\\nAxgles: Their Measurement. An angle is the\\namount of divergence of two lines. For example, the\\nangle between the two lines S 1 E\\nand S 2 E is the amount of diver-\\ngence of these lines. The angle\\nS 6 ES* is the amount of divergence\\nof the two lines S*E and S 4 E.\\nThe eye sees at once that the\\nangle S*ES* in the figure is\\ngreater than the angle S l ES*,\\nand that the angle S*ES 3 is\\ngreater than either of them.\\nFig. 5. \u00e2\u0080\u0094Angles their\\nMeasurement.\\nIn order to compare them and to obtain their numerical ratio, we\\nmust have a unit-angle.\\nThe unit-angle is obtained in this way The circumference of any\\ncircle is divided into 360 equal parts. The points of division are\\njoined, with the centre. The angles between any two adjacent radii\\nare called degrees. In the figure, SES* is about 12\u00c2\u00b0, S 3 ES 4 is about\\n22\u00c2\u00b0, S^ES* is about 30\u00c2\u00b0, and S l ES* is about 64\u00c2\u00b0. The vertex of the\\nangle is at the centre E the measure of the angle is on the circum-\\nference S^S^S^S 4 or on any circumference drawn from \u00c2\u00a3asa centre.\\nIn this way we have come to speak of the length of one three-\\nhundred-and-sixtieth part of any circumference as a degree, because\\nradii drawn from the ends of this part make an angle of 1\u00c2\u00b0.\\nFor convenience in expressing the ratios of different angles the\\ndegree has been subdivided into minutes and seconds.\\nOne circumference 360\u00c2\u00b0 21600 1296000\\n1\u00c2\u00b0 60 360\\n1 60", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0046.jp2"}, "45": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 23\\nSmaller angles than seconds are expressed by decimals of a second.\\nThus one-quarter of a second is 0 .25; one-quarter of a minute\\nis 15\\nThe Radius of the Circle in Angular Measure. If R is\\nthe radius of a circle, we know from geometry that one\\ncircumference 2 nE, where n 3.1416. That is,\\n2 nR 360\u00c2\u00b0 21600 1296000\\nor R 57\u00c2\u00b0.3 3437 7 206264 .8.\\nBy this we mean that if a flexible cord equal in length\\nto the radius of any circle were laid round the circumfer-\\nence of that circle, and if two radii were then drawn to the\\nends of this cord, the angle of these radii would be 57\u00c2\u00b0. 3,\\n3437 .7, or 206264 .8.\\nIt is important that this should be perfectly clear to the\\nstudent.\\nFor instance, how far off must you place a foot-rule in order\\nthat it may subtend an angle of 1\u00c2\u00b0 at your eye? Why, 57.3 feet\\naway. How far must it be in order to subtend an angle of a min-\\nute? 3437.7 feet. How far for a second? 206264.8 feet, or over 39\\nmiles.\\nAgain, if an object subtends an angle of 1\u00c2\u00b0 at the eye,, we know\\nthat its diameter must be a s great as its distance from us. If it\\n5^.o\\nsubtends an angle of 1 its distance from us is over 200,000 times as\\ngreat as its diameter.\\nThe instruments employed in astronomy may be used to\\nmeasure the angles subtended at the eye by the diameters\\nof the heavenly bodies. In other ways we can determine\\ntheir distance from us in miles. A combination of these\\ndata will give us the actual dimensions of these bodies in\\nmiles. For example, the sun is about 93,000,000 miles\\nfrom the Earth. The angle subtended by the sun s diam-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0047.jp2"}, "46": {"fulltext": "u\\nASTRONOMY.\\neter at this distance is 1922 What is the diameter of the\\nsun in miles (1 is about 451 miles.)\\nAn idea of angular dimensions in the sky may be had\\nby remembering that the angular diameters of the Moon\\nand of the Sun are about 30 It is 180\u00c2\u00b0 from the west\\npoint to the east point counting through the point immedi-\\nately overhead. How many moons placed edge to edge\\nwould it take to reach from horizon to horizon? The\\nstudent may guess at the answer first and then com-\\npute it.\\nIt is convenient to remember that the angular distance\\nbetween the two Pointers in the Great Bear (see Fig. 1)\\nis about 5\u00c2\u00b0.\\nPlane Triangles. The angles of which we have spoken are\\nangles in a plane. In any plane triangle there are three angles A, B,\\nand three sides a, b, c six parts. If any three of the parts are given\\n(except the three angles) we can construct the triangle. For in-\\nFig. 6. \u00e2\u0080\u0094A Plane\\nTriangle.\\nFig.\\n7. Two Similar Plane\\nTriangles.\\nstance, if you know the three sides a, b, c, you can make one triangle,\\nand only one, with these sides. If you only know the three angles\\nyou can make any number of triangles with three such angles. All\\nof them will have the same shape, but they will have different sizes.\\n(See Fig. 7.)\\nThe Sphere its Planes and Circles. In Fig.\\n8, is the centre of the sphere. Suppose any plane", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0048.jp2"}, "47": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 25\\nas AB to pass through the centre of the sphere. It will\\ncut the sphere into two hemispheres. It will intersect the\\nsurface of the sphere in a circle AEBF which is called a\\ngreat circle of the sphere. A great circle of the sphere is\\none cut from the surface by a plane passing through the\\ncentre of the sphere. Suppose a right line POP perpen-\\ndicular to this plane. The points P and P in which it\\nintersects the surface of the sphere are every where 90\u00c2\u00b0 from\\nthe circle AEBF. They are the poles of that circle. The\\npoles of the great circle CEDE are Q and Q It is\\nproved in geometry that the following relations exist be-\\ntween the angles made in the figure\\nFig. 8. The Sphere its Great Circles their Poles.\\nI. The angle POQ between the poles is equal to the in-\\nclination of the planes to each other.\\nII. The arc BD which measures the greatest distance\\nbetween the two circles is equal to the arc PQ which\\nmeasures the angle POQ.\\nIII. The points E and F, in which the two great circles\\nintersect each other, are the poles of the great circle", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0049.jp2"}, "48": {"fulltext": "26 ASTRONOMY.\\nPQACP Q BD which passes through. the poles of the first\\ntwo circles.\\nThe Spherical Triangle. In the last figure there are\\nseveral spherical triangles, as EDB, FAC, ECP Q B, etc.\\nIn astronomy we need consider only those whose sides are\\nformed by arcs of great circles. The angles of the trian-\\ngle are angles between two arcs of great circles; or what is\\nthe same thing, they are angles between the two planes\\nwhich cut the two arcs from the surface of the sphere.\\nIn spherical triangles, as in plane, there are six parts,\\nthree angles and three sides. Having any three parts the\\nother three can be constructed.\\nThe sides as well as the angles of spherical triangles are\\nexpressed in degrees, minutes, and seconds.\\nIf the student has a school globe, let him mark on it the triangle\\nwhose sides are\\na 10\u00c2\u00b0, b 7\u00c2\u00b0, c 4\u00c2\u00b0.\\nIts angles will be (A is opposite to a, B to b, to c)\\nA 128\u00c2\u00b0 44 45 1\\nB 83\u00c2\u00b0 11 12\\nC= 18\u00c2\u00b0 15 31 1\\nLatitude and Longitude of a Place on the\\nEarth s Surface. According to geography, the latitude\\nof a place on the Earth s surface is its angular distance\\nnorth or south of the Earth s equator.\\nThe longitude of a place on the Eartli s surface is its an-\\ngular distance east or west of a given first meridian (the\\nmeridian of Greenwich, for example).\\nIf P in Fig. 9 is the north pole of the earth, the lat-\\nitude of the point B is 60\u00c2\u00b0 north;, of Z it is 30\u00c2\u00b0 north; of\\nit is 27^\u00c2\u00b0 south. All places having the same latitude are\\nsituated on the same parallel of latitude. In the figure\\nthe parallels of latitude are represented by straight lines.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0050.jp2"}, "49": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE- DEFINITIONS 27\\nAll places having the same longitude are situated on the\\nsame meridian. We shall give the astronomical definitions\\nof these terms further on.\\nIt is found convenient in astronomy to modify the geo-\\ngraphical definition of longitude. In geography we say\\nthat Washington is 77\u00c2\u00b0 ivest of Greenwich, and that Syd-\\nney (Australia) is 151\u00c2\u00b0 east of Greenwich. For astronom-\\nFig. 9. Latitude and Longitude of Places on the Earth s\\nSurface.\\nical purposes it is found more convenient to count the\\nlongitude of a place from the first meridian always towards\\nthe west. Thus Sydney is 209\u00c2\u00b0 west of Greenwich (360\u00c2\u00b0\\n151\u00c2\u00b0 209\u00c2\u00b0).\\nThe Earth turns on its axis once in 24 hours. In a day\\nof 24 hours every point on the Earth s surface moves\\nonce round a circle (its parallel of latitude). Every point", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0051.jp2"}, "50": {"fulltext": "28 ASTRONOMY.\\nmoves 360\u00c2\u00b0 in 24 hours, or at the rate of 15\u00c2\u00b0 every hour\\n(360\u00c2\u00b0 divided by 24 is 15\u00c2\u00b0).\\nHence we can measure the longitude of a place in de-\\ngrees or in hours, just as we choose. Washington is 5 h 8 m\\nwest of Greenwich (77\u00c2\u00b0) and Sydney is 13 h 56 m west of\\nGreenwich (209\u00c2\u00b0). In the figure suppose F to be west of\\nthe first meridian. All the places on the meridian PQ\\nhave a longitude of 15\u00c2\u00b0 or 1 hour all those on the merid-\\nian Pb h Q have a longitude of 75\u00c2\u00b0 or 5 hours and so on.\\nWhat is an angle? What is a degree? What is a minute of\\narc? a second? The radius of a circle, if wrapped around the cir-\\ncumference of a circle, would cover an arc of how many degrees\\nWhat is the angular diameter of the Moon of the Sun How far\\napart in arc are the two pointers of the Great Bear? What is the\\ndifference between a plane triangle and a spherical triangle? Give\\nan example of a plane triangle of a spherical triangle. Define the\\nlatitude of a place on the Earth s surface. Define the longitude of\\na place on the Earth s surface.\\n10. The Points and Circles of the Celestial Sphere.\\nThe Horizon. We only see\\none half of the celestial sphere;\\nnamely, the half above our\\nheads. If we are at sea, or in\\na large open conntry on land,\\nthe concave vault of the day-\\ntime sky seems to rest on a flat\\nplain, and this plain seems to\\nbe bounded by a circle. The\\nflat plain is called the plane of\\nFig. IO.-Hu.f of the Ce- the horizon (pronounced hor-i\\nlestial Sphere, Studded Z on). Its bounding circle is the\\nwith Stars. .circle of the horizon. A point\\nThe sphere seems to rest on the\\nplane of the horizon. The horizon on the celestial sphere directly\\nseems to he hounded by the circle\\nnhs. n is the north point, s is overhead is called the zenitn-\\nthe south point of the horizon. Z\\nis the zenith-point or the point point, or more briefly tne\\ndirectly overhead.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0052.jp2"}, "51": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 29\\nzenith. A line joining the observer and the zenith-point\\nis perpendicular to the plane of the horizon. If you wish\\nto describe the situation of a star you can say that its\\nzenith-distance is so many degrees 50\u00c2\u00b0 for example. The\\nstar S in the figure is distant from the zenith Z by an arc\\nZS. Its zenith-distance is 50\u00c2\u00b0. The arc from the zenith\\nto the horizon is 90\u00c2\u00b0. That is, the zenith-distance of the\\nhorizon is everywhere 90\u00c2\u00b0. The altitude of a star is its\\nangular distance above the horizon. The altitude of the\\nstar 8 in the figure is HS 40\u00c2\u00b0.\\nThe zenith-distance and the altitude of a star are meas-\\nured on a vertical circle, i.e., on a circle passing through\\nthe star and perpendicular to the horizon.\\nThe zenith-distance of any star -f- the altitude of the star 90\u00c2\u00b0.\\nFig. 11.\u00e2\u0080\u0094 The Earth s Axis and the Plane op its\\nEquator EQ.\\nNP is the earth s north pole SP is the south pole eg is the earth 1\\nequator EQ is the plane of the celestial equator.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0053.jp2"}, "52": {"fulltext": "30 ASTBONOMY.\\nThe Celestial Equator. In the figure there is a pic-\\nture of the Earth. NP is its north pole, SP is its south\\npole, and the line joining them is the Earth s axis, eg is\\nthe Earth s equator. It is a circle round the Earth. If\\nwe imagine the plane of that circle to continue out beyond\\nthe Earth on all sides till it reaches the celestial sphere the\\nshaded surface EQ (a circle) will represent it. This sur-\\nface is the plane of the equator of the celestial sphere or\\nmore briefly, it is the plane of the celestial equator. If we\\nimagine the axis of the earth prolonged both ways till it\\nmeets the celestial sphere the prolonged line is the axis of\\nthe celestial sphere.\\nIf we imagine the planes of the meridians and parallels\\non the Earth to be prolonged outwards to meet the celes-\\ntial sphere, they will meet it in circles that are the merid-\\nians and parallels of that sphere. They are not drawn in\\nthe last figure, so as to avoid confusing it but some of\\nthem are drawn in the next figure. In this n is the north\\npole of the Earth, NP the north pole of the celestial\\nsphere; eq is the equator of the Earth, EQ the equator of\\nthe celestial sphere the celestial equator; the plaues of the\\nmeridians of the Earth are prolonged and make the merid-\\nians of the celestial sphere the plaues of the parallels on\\nthe Earth make the parallels ML, EQ (for the equator is\\na parallel of latitude), and SO.\\nZ is the zenith-point of the observer it is the point of\\nthe celestial sphere directly over his head. N is the nadir-\\npoint of the observer it is the point of the celestial sphere\\ndirectly beneath his feet. HR is a plane through the cen-\\ntre of the Earth and perpendicular to the line ZN. We\\nshall now define the plane of the horizon to be that plane\\npassing through the centre of the Earth which is perpen-\\ndicular to the line joining the observer s zenith- and nadir-\\npoints. On page 28 the horizon was described as the flat\\nplain on which the observer stands and on which the up-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0054.jp2"}, "53": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 31\\nper half of the celestial sphere rests. Such a plane is\\ncalled the plane of the sensible horizon (i.e., of the horizon\\nevident to the senses). HE through the centre of the\\nEarth divides the celestial sphere into two equal parts. It\\nis called the rational horizon. The sensible and the ra-\\ntional horizons are parallel to each other.\\nFig. 12. The Earth (n, q, s, e) surrounded by the Celestial\\nSphere {N, Q, S, E).\\nThe meridians and parallels on the celestial sphere serve\\nthe same purpose as the meridians and parallels on the\\nEarth. The latitude of a place on the Earth is its angular\\ndistance north or south of the terrestrial equator. The\\nlongitude of a place on the Earth is the angular distance\\nof that place tvest of the first meridian. If we know the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0055.jp2"}, "54": {"fulltext": "32 ASTRONOMY.\\nlatitude and longitude of a place on the surface of the\\nEarth we know all that can be known of its situation.\\nJust in the same way we describe the situations of stars\\non the surface of the celestial sphere. The declination (like\\nlatitude) of a star is its angular distance north or south of\\nthe celestial equator. The right-ascension (like longi-\\ntude) of a star is its angular distance east of the first me-\\nridian. Declinations in the sky are like latitudes on the\\nEarth. Eight-ascensions in the sky are like longitudes on\\nthe Earth. The names are different, but the principle of\\nmeasurement is the same.\\nDeclination of a Star. The declination of a star is\\nits angular distance north or south of the celestial equator.\\nFig. 13.\u00e2\u0080\u0094 Declination and Right- ascension of a Star.\\nIn the figure EVQ is the equator of the celestial sphere the celes-\\ntial equator. The Earth is not shown in the picture. If it were\\nshown it would be a dot at the centre of the sphere. PAa is a nierid.\\nian of the celestial sphere passing through the star A. The angular\\ndistance of the star A north of the celestial equator is Aa. Aa is\\nthe north declination of that star. PbB is a meridian of the celestial\\nsphere passing through the star B. This star is south of the celes-\\ntial equator by an angular distance measured by bB. bB is the south\\ndeclination of the star B.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0056.jp2"}, "55": {"fulltext": "SPACE\u00e2\u0080\u0094 THE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 33\\nIf for a moment we should take the sphere PEQ to represent the\\nEarth and EQ the equator of the Earth, then the terrestrial north\\nlatitude of A would be measured by a A and the south latitude of B\\nby bB. The declination of a point on the surface of the celestial\\nsphere corresponds to the latitude of a point on the surface of the\\nEarth. PA is the north polar-distance of A P B is the south polar-\\ndistance of B\\nThe polar-distance of a star -f- the star s declination 90\u00c2\u00b0.\\nRight-ascension of a Star. The right- ascension of a\\nstar is its angular distance east of a first meridian.\\nFig. 13 Ms.\\nIn the figure P V is the first meridian. PAa is the meridian\\nthrough the star A. This meridian is east of the first meridian by\\nthe angle VPa, which is measured by the arc Va. Va is the right-\\nascension of the star A. PbB is the meridian through the star B.\\nThis meridian is east of the first meridian by the angle VPb, which\\nis measured by the arc Vb. Vb is the right-ascension of the star B.\\nIf for a moment we should take the sphere PEQ to represent the\\nEarth, and EQ the equator of the Earth, and PV the meridian of\\nGreenwich, (east) terrestrial longitude of a place A would be Va;\\nthe longitude of a place B would be bB. The right-ascension of a\\npoint on the surface of the celestial sphere corresponds to the longi-\\ntude of a point on the surface of the Earth.\\nIt is very important to understand these matters at the beginning,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0057.jp2"}, "56": {"fulltext": "34 ASTRONOMY.\\nand it is necessary for the student to memorize the following defini-\\ntions:\u00e2\u0080\u0094\\nThe plane of the horizon is a plane through the centre of the Earth\\nperpendicular to the line joining the zenith and the nadir of the ob-\\nserver. The zenith of an observer is the point of the celestial sphere\\ndirectly over his head. Therefore each person has a different zenith-\\npoint. The nadir of an observer is the point of the celestial sphere\\ndirectly beneath his feet. The zenith and nadir are points on the\\nsurface of the celestial sphere not points on the Earth. The zenith-\\ndistance of a star is its angular distance from the zenith. The alti-\\ntude of a star is its angular distance above the horizon. A vertical\\ncircle is a great circle of the sphere whose plane is perpendicular to\\nthe plane of the horizon. The axis of the celestial sphere is the line\\nof the Earth s axis prolonged. The equator of the celestial sphere\\nthe celestial equator\u00e2\u0080\u0094 is that great circle cut from the celestial sphere\\nby the plane or the Earth s equator extended. The declination of a\\nstar is its angular distance north or south of the celestial equator.\\nThe right- ascension of a star is its angular distance east (not west) of\\nthe first meridian of the celestial sphere. (This first meridian has\\nnothing to do with the meridian of Greenwich on the Earth, as we\\nshall soon see.)\\nThe terrestrial meridian of an observer is that great cir-\\ncle of the Earth that passes through the observer and\\nthrough the Earth s axis. All terrestrial meridians pass\\nthrough the north and south poles of the Earth.\\nThe celestial meridian of an observer is that great circle\\nof the celestial sphere that passes through the zenith of\\nthe observer and through the axis of the celestial sphere.\\nAll celestial meridians pass through the north and south\\npoles of the celestial sphere.\\nIn figure 14 n, e, q, s is the earth, and some terrestrial meridians are\\ndrawn upon it. Some celestial meridians are drawn on the celestial\\nsphere NP, E, Q, SP. Z is the zenith of the observer. Where must\\nhe be in the figure He must be on the surface of n, e, q, s, where\\naline ZN (zenith to nadir) intersects it. Make a pinprick at this\\npoint. His terrestrial meridian is the little circle n, e, q, s (because\\nit passes through the observer s place and through n and s). His\\ncelestial meridian is NP, Z, SP (because it contains his zenith and\\nthe two celestial poles).", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0058.jp2"}, "57": {"fulltext": "SPACE\u00e2\u0080\u0094 TEE CELESTIAL SPHERE\u00e2\u0080\u0094 DEFINITIONS. 35\\nFig. 14.\u00e2\u0080\u0094 Correspondence of the Terrestrial and Celestial\\nMeridians of an Observer.\\nFig. 15. The Celestial Sphere.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0059.jp2"}, "58": {"fulltext": "38\\nASTRONOMT.\\nzon. P is the north celestial pole. PZS is the observer s celestial\\nmeridan. XXI, XXII O, I, II is the celestial equator,\\nand is the vernal equinox the origin of right-ascensions. Paral-\\nlels of declination are shown (circles parallel to the celestial equator)\\nevery 10\u00c2\u00b0 both north and south of the equator. Meridians of the\\ncelestial sphere {hour-circles) are drawn every 15\u00c2\u00b0; every hour. They\\npass from pole to pole across the celestial sphere and cross the equa-\\ntor at the points marked XXI, XXII, I, II Every star on\\nthe hour-circle i has a right ascension of 15\u00c2\u00b0, or 1 hour on of\\n30\u00c2\u00b0, or 2 h on XXII of 330\u00c2\u00b0, or 22 hours and so on.\\nAll stars on the parallel of declination marked A have a north\\ndeclination of 40\u00c2\u00b0 (-f- 40\u00c2\u00b0) on the parallel C, of 4- 30\u00c2\u00b0 on the equa-\\ntor, of 0\u00c2\u00b0 on the parallel Bof\u00e2\u0080\u0094 30\u00c2\u00b0. The student should mark the\\nfollowing places on the figure\\nR. A. 22 h and Decl. 80\u00c2\u00b0\\n23 30\u00c2\u00b0\\n24 0\u00c2\u00b0\\no h 40\u00c2\u00b0\\nR. A. h and Decl. 40 c\\n1 +60\u00c2\u00b0\\n2 h =+40\u00c2\u00b0\\n2 30\u00c2\u00b0", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0060.jp2"}, "59": {"fulltext": "CHAPTER III\\nDIURNAL MOTION OF THE SUN, MOON, AND STARS.\\n11. The Diurnal Motion of the Sun, Moon, and Stars.\\nIt is a familiar fact to all of us that the Sun rises and sets\\nevery day. The Moon rises and sets. Stars also rise above\\nthe eastern horizon; they appear to move across the sky\\nand to come to their greatest altitude on the meridian\\nro,/\\nFig. 18.-\\n-The Apparent Motion of the Sun from Rising to\\nSetting.\\nand then they appear to decline to the west and set below\\nthe western horizon. Every one is familiar with the Sun s\\nrising and setting. It is too splendid a spectacle to be\\noverlooked. We are all more or less familiar with the mo-\\ntion of the Moon from rising to setting. We may know\\n39", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0061.jp2"}, "60": {"fulltext": "40\\nASTRONOMY.\\nthe fact that groups of stars also rise and set. But to thor-\\noughly understand their motions we must actually observe\\nsome particular stars carefully. The student should him-\\nself make the observations that are described here so far as\\nhis time and opportunities will allow.\\nDiurnal Motions of Southern Stars. Let the\\nstudent go out into a field or park at night where he can\\nsee the sky from his zenith towards the southern horizon,\\nIII\\nFig. 19.\u00e2\u0080\u0094 Diurnal Motion of a Group of Southern Stars.\\nThe right hand of this picture is west the left hand is east.\\nand where he can command an unobstructed view of the\\neastern and western horizon. Let him select a group of\\nbright stars that are not very far apart, and that are not\\nvery far above the eastern horizon. He must learn the\\ngroup so well that he can always recognize it in the sky no\\nmatter where it may be. Let him stand with his back\\ntoward the north. The group is rising, let us say (the\\nlower left-hand circle in Fig. 19) when he begins to", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0062.jp2"}, "61": {"fulltext": "DIURNAL MOTION: SUN, MOON, AND STARS. 41\\nobserve it. If he watches the group looking at it every\\nhalf hour or so he will see that it is continually rising\\nabove the eastern horizon and getting higher in the heav-\\nens.\\nAbout three hours after the rising of this group it\\nwill be towards the southeast (the second circle counting\\nfrom the left of Fig. 19). About six hours after rising,\\nthe group will be just south of him and at its highest\\nat its greatest altitude. The point in the sky where a\\nstar (or a group of stars) has its greatest altitude is called\\nits point of culmination. It is due south of the observer\\nat culmination (the uppermost circle, S, in the last figure).\\nIt requires about six hours for a group of southern stars to\\nmove from the eastern horizon, where it rises, to the point\\ndue south, where it culminates. Six hours of watching is\\nquite as long as can be given by the student. But if he\\nshould watch longer than this, he would see the group of\\nstars decline to the west and finally set (as in the two right-\\nhand circles of the last figure).\\nHunters, sailors, shepherds, as well as astronomers, have\\nobserved facts like these thousands and thousands of times.\\nAny one who wishes can observe them whenever he likes on\\nany clear night. So that the student can prove them for\\nhimself if he chooses; and we may take them as proved\\nfacts. The picture shows what actually does happen for a\\ngroup of southern stars. When it is due south it looks\\nlike the upper circle, marked S. It is at its culmination.\\nIt is at its greatest altitude. Three hours before the time\\nof culmination the group was as in the circle next S, to\\nthe left. Six hours before this time it was as in the lower\\nleft-hand circle. Three hours after the time of culmina-\\ntion the group has declined towards the west (see the\\nfigure), and six hours after this time it is setting in the\\nwest, as in number V.\\nIt is not to be expected that a schoolboy will have the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0063.jp2"}, "62": {"fulltext": "40\\nASTRONOMY.\\nthe fact that groups of stars also rise and set. But to thor-\\noughly understand their motions we must actually observe\\nsome particular stars carefully. The student should him-\\nself make the observations that are described here so far as\\nhis time and opportunities will allow.\\nDiurnal Motions op Southern Stars. Let the\\nstudent go out into a field or park at night where he can\\nsee the sky from his zenith towards the southern horizon,\\nIII\\nII\\nFig. 19.\u00e2\u0080\u0094 Diurnal Motion of a Group of Southern Stars.\\nThe right hand of this picture is west the left hand is east.\\nand where he can command an unobstructed view of the\\neastern and western horizon. Let him select a group of\\nbright stars that are not very far apart, and that are not\\nvery far above the eastern horizon. He must learn the\\ngroup so well that he can always recognize it in the sky no\\nmatter where it may be. Let him stand with his back\\ntoward the north. The group is rising, let us say (the\\nlower left-hand circle in Eig. 19) when he begins to", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0064.jp2"}, "63": {"fulltext": "DIURNAL MOTION: SUN, MOON, AND STARS. 41\\nobserve it. If he watches the group looking at it every\\nhalf hour or so he will see that it is continually rising\\nabove the eastern horizon and getting higher in the heav-\\nens.\\nAbout three hours after the rising of this group it\\nwill be towards the southeast (the second circle counting\\nfrom the left of Fig. 19). About six hours after rising,\\nthe group will be just south of him and at its highest\\nat its greatest altitude. The point in the sky where a\\nstar (or a group of stars) has its greatest altitude is called\\nits point of culmination. It is due south of the observer\\nat culmination (the uppermost circle, S, in the last figure).\\nIt requires about six hours for a group of southern stars to\\nmove from the eastern horizon, where it rises, to the point\\ndue south, where it culminates. Six hours of watching is\\nquite as long as can be given by the student. But if he\\nshould watch longer than this, he would see the group of\\nstars decline to the west and finally set (as in the two right-\\nhand circles of the last figure).\\nHunters, sailors, shepherds, as well as astronomers, have\\nobserved facts like these thousands and thousands of times.\\nAny one who wishes can observe them whenever he likes on\\nany clear night. So that the student can prove them for\\nhimself if he chooses; and we may take them as proved\\nfacts. The picture shows what actually does happen for a\\ngroup of southern stars. When it is due south it looks\\nlike the upper circle, marked S. It is at its culmination.\\nIt is at its greatest altitude. Three hours before the time\\nof culmination the group was as in the circle next S, to\\nthe left. Six hours before this time it was as in the lower\\nleft-hand circle. Three hours after the time of culmina-\\ntion the group has declined towards the west (see the\\nfigure), and six hours after this time it is setting in the\\nwest, as in number V.\\nIt is not to be expected that a schoolboy will have the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0065.jp2"}, "64": {"fulltext": "42 ASTRONOMY.\\nleisure to watch throughout a whole night. If he were to\\ndo so he would see the group move as in the figure if he\\nused a long winter s night for his observation and began\\nhis watch as soon as the sky grew dark. There is a sim-\\nple experiment that he can try, however, which will make\\nthe diurnal motion of the southern stars quite easy to un-\\nderstand. Let him provide himself with a hammer and\\nwith a bundle of common laths, and let him sharpen one\\nend of each lath so that it can be easily driven into the\\nground. Let him choose a spot of ground to stand on that\\nis soft, so that the laths can be set in place without too\\nmuch trouble. Let him select some one bright star that\\nis near the eastern horizon, and remember it well so as\\nnot to mistake it for any other star.\\nNow he should kneel down, set the sharp end of a lath\\non the ground, and sight along the lath until it points ex-\\nactly to the star. The lath is to be sighted at the star just\\nas a rifle is pointed at a deer. The lath is now to be driv-\\nen into the ground firmly; and after this is done it is well\\nto take another sight along the lath at the star to be sure\\nthat it still points correctly. When all is right the ob-\\nserver should look at his watch and note the time and\\nwrite it down, like this:\\nFirst lath set at 8 h m p.m.\\nThings will look as in Fig. 20. The lath 01 will\\npoint to the star at 8 h m\\nThe observer need pay no more attention to the star for\\na couple of hours. A little before ten o clock he should\\ntake another lath and make the same observation on the\\nsame star. He will find that the star has moved towards\\nthe west and upwards. Leaving the first lath in place, he\\nmust now fix a second one so as to point at the star at 10\\no clock. Its point will have to be set a few inches away", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0066.jp2"}, "65": {"fulltext": "DIURNAL MOTION: SUN, MOON, AND STABS. 43\\nEast\\nWest\\nThe\\nGround\\nFig. 20.\u00e2\u0080\u0094 A Pointer Directed at a Star.\\nB^\\nEast\\nWest\\nThe\\nGround\\nTig. 21. A Pointer Directed at a Star.\\nEast\\nThe\\nIII\\nWest\\nGround\\nFig. 22. A Pointer Directed at a Star.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0067.jp2"}, "66": {"fulltext": "44 ASTRONOMY.\\nfrom the point of the first one, so as not to interfere with\\nit. It will appear as in Fig. 21.\\nHe should make a second record, thus:\\nSecond lath set at 10 h m p.m.\\nNow the observer can go to sleep if he likes, setting\\nhis alarm-clock to wake him about quarter before twelve.\\nAt 12 h he should set a third lath to point at the same star.\\nIt will be like Fig. 22.\\nHis note-book will read\\nThird lath set at 12 h m p.m.\\nHe should do the same thing at 2 o clock in the morning,\\nand the fourth lath will point as in the next figure.\\nFourth lath set at 2 a.m.\\nEast West\\nThe [_ Ground\\nFig. 23.\u00e2\u0080\u0094 A Pointer Directed at a Star.\\nThese four observations will be enough, though the more\\nthat are made the clearer the motion of the star will be.\\nThe chief practical trouble will be that the points of the\\nlaths cannot be set very close together without interfering\\nwith each other. If they could be set just right and if a\\ngreat number of them were so set, things would look like\\nthe group of laths, B, in the next figure, where the flat", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0068.jp2"}, "67": {"fulltext": "DIURNAL MOTION: SUN, 310 ON, AND STABS. 45\\ntable represents the ground, and the lines in the circle B\\nrepresent a number of laths accurately set at the point 0.\\nThis figure makes everything clear. The laths have\\nbeen set at equal intervals of time and they are at equal\\nangles apart. This proves that the apparent motion of the\\nstar B is such that it moves through equal angles in equal\\ntimes. Its motion is uniform.\\nIf the observer had chosen to select a star very far south\\n(A, for example) and had set laths for it, also, the group\\nFig. 24 \u00e2\u0080\u0094A Model to show how Stars seem to move from\\nRising to Setting in their Diurnal Paths.\\nof pointers for this star would look like the cone of rays\\nmarked A in the figure. All the laths would lie in the\\nsurface of a cone, and the vertex of this cone would\\nbe at 0. If he had chosen a star nearer to his zenith\\n((7, for example) and had set the laths for it, just as\\nbefore, they would also lie in the surface of a cone C, as\\nin the figure. Finally, if he had chosen a star much fur-\\nther north (Z), for example) the pointers to that star would\\nall lie in the cone D. The line OP is the axis of all these\\ncones, and it points to the north pole of the heavens.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0069.jp2"}, "68": {"fulltext": "46\\nASTRONOMY.\\nThe north pole of the heavens is that point where the axis\\nof the Earth, prolonged, meets the celestial sphere.\\nDiurnal Motions of Northern Stars. After the\\nmotions of southern stars, from their rising to their setting,\\nhave been caref ally observed and are thoroughly under-\\nFig. 25. The Northern Heavens;\\nas they appear to an observer in the United States in the early evening\\nduring August. The right-hand side of the picture is east.\\nstood, the motions of northern stars must be observed.\\nThey can be studied in the same way as before. The\\ndrawings of the cones C and D in the last figure show ex-\\nactly what would be observed. In every one of these\\ncones, for any and every star in the sky, experiments will", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0070.jp2"}, "69": {"fulltext": "DIURNAL MOTION: SUN, MOON, AND STABS. 47\\nprove that the star moves through equal angles in equal\\ntimes. The diurnal motions of all the stars are uniform.\\nThe time required for the star D to go completely round\\nits cone once and to come back to the starting-point again\\nis 24 hours, one day; and the same is true for any and\\nevery star.\\nIn Fig. 25 the stars of the northern sky are shown\\nas they appear to an observer in the middle regions of the\\nUnited States in the early evening in August. The same\\nstars are visible all the year round, but they will not always\\nbe at the same altitudes above the horizon at the same hour\\nof the night. No matter what hour of the night, or what\\ntime of the year you read this paragraph, you can see the\\nstars of this picture (if the night is clear) by going noio\\nout-of-doors and looking towards the north. In order to\\nmake the picture look right you may have to turn the page\\nof the book round somewhat (in the direction of the arrows)\\nso as to put a different part of the page uppermost. But\\nby taking a little pains you can hold the picture in such a\\nposition that it will agree with the configuration of the\\nstars in the sky.\\nThe first set of stars to find in the sky is the Great Bear\\nUrsa Major the Great Dipper, as it is often called. It\\nis made up of seven stars arranged somewhat as in the\\nnext figure:\\nPolaris.\\nv*\\ni\\n*y\\nFig. 26.-\\n\u00e2\u0080\u0094Ursa Major and Polaris.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0071.jp2"}, "70": {"fulltext": "48\\nASTRONOMY.\\nThey are called by these names a [Alpha) TTrsse ma-\\njoris; (3 {Beta) Ursse majoris; y {Gamma) Ursae majoris;\\nS {Delta) Ursae majoris; e (Epsilon) Ursae majoris rj\\n{Eta) Ilrsae majoris; C (Zeta) TTrsse majoris. The letters\\na, (3, y, d, e, rj, C are the first seven letters of the Greek\\nalphabet. The stars themselves are a part of the constel-\\nlation or group of stars named Ursa Major the Great Bear\\nby the ancients (see Fig. 25). After you have found\\nthem you must notice that two of them a and j3 (they are\\ncalled the pointers point to another star, not so\\nbright, which is itself called Polaris the pole-star the\\nstar near the north pole of the celestial sphere.\\nIt is well to form the habit of glancing up at the north-\\nFig. 27.\u00e2\u0080\u0094 The Stars of the Dipper;\\nas they appear in the early hours of the evening in the month of May.\\nern heavens every time you go out of doors on a clear\\nnight, so as to be able to find Ursa Major, Polaris, and\\nCassiopea quickly and easily.\\nIf you study the motions of the northern stars you will\\nfind that Polaris the polar star seems to be almost sta-\\ntionary. If it were exactly at the north pole of the heav-\\nens (which it is not) it would be absolutely stationary; but\\nit is very nearly so. All the other northern stars seem to", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0072.jp2"}, "71": {"fulltext": "DIURNAL MOTION: SUN, MOON, AND STARS. 49\\nmove round Polaris in circles. They move from the east,\\nthen upwards, then to the west, then downwards, then to\\nthe east again (in the direction of the arrows in Fig. 25),\\nand so on forever. It takes 24 hours for each and every\\nstar to move once completely round the pole. Its motion\\nhas a period of one day hence the name diurnal motion.\\nThe diurnal motions of all the stars can be described in\\nthree theorems (following), and you should learn these the-\\norems by heart, because that is the quickest way to get a\\nperfectly definite and correct statement of the appearances\\nin the sky. Recollect that the north-polar- distance\\n(N.P.D.) of a star is its angular distance from the north\\ncelestial pole.\\nThe following are the laws of the diurnal motion:\\nI. Every star in the heavens appears to describe a circle\\naround the pole as a centre in consequence of the diurnal\\nmotion.\\nII. The greater the star s north-polar -distance the larger\\nis the circle.\\nIII. All the stars describe their diurnal orbits in the\\nsame period of time, which is the time required for the earth\\nto turn once on its axis (twenty-four hours).\\nThese laws are true of the thousands of stars visible to\\nthe naked eye, and of the millions upon millions seen by\\nthe telescope.\\nThe circle which a star appears to describe in the sky in\\nconsequence of the diurnal motion of the earth is called\\nthe diurnal orbit of that star (an orbit is a path in the\\nsky).\\nThese laws are proved by observation. The student can\\nsatisfy himself of their correctness on any clear night.\\nIf the star s north-polar-distance is less than the altitude\\nof the pole, the circle which the star describes will not\\nmeet the horizon at all, and the star will therefore neither\\nrise nor set, but will simply perform an apparent diurnal", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0073.jp2"}, "72": {"fulltext": "50\\nASTRONOMY.\\nrevolution round the pole. Such stars are shown in\\nFig. 25. The apparent diurnal motion of the stars is in\\nthe direction shown by the arrows in the cut. Below the\\nnorth pole the stars appear to move from left to right, west\\nto east above the pole they appear to move from east to\\nwest.\\nThe circle within which the stars neither rise nor set is\\ncalled the circle of perpetual apparition. Within it the\\nFig. 28.\u00e2\u0080\u0094 The Stars of the Dippek;\\nas they appear at different times during their daily revolution round\\nthe pole.\\nstars perpetually appear are visible. The radius of this\\ncircle is equal to the altitude of the pole above the horizon\\nor to the north-polar-distance of the north point of the\\nhorizon.\\nWhen a photographic camera is directed to the north\\npole of the heavens at night and an exposure of about 12\\nhours is given the developed plate will look like Fig. 29.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0074.jp2"}, "73": {"fulltext": "DIURNAL MOTION: SUN, MOON. AND STARS. 51\\nThe plate has remained stationary; the stars have in 12\\nhours moved one-half round their diurnal orbits. In\\nmoving they have left trails on the plate. Each trail\\nis an arc of a circle, and the centre of all these circles is\\nFig. 29.\\nFrom a photograph of the motion of the stars near the north pole of the\\nheavens. The exposure-time was 12 hours. The bright trail nearest the\\npole was made by Polaris.\\nthe same. It is the north celestial pole. If the camera\\nhad been directed to the equator the trails of the stars\\npassing across the plate would have been straight lines.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0075.jp2"}, "74": {"fulltext": "52 ASTRONOMY.\\nIf the student is a photographer, he should try these ex-\\nperiments for himself, using the longest-focus lens that he\\ncan obtain.\\nWe have now to inquire wliy do the stars rise and set ac-\\ncording to these laws. What explanations can be given of\\ntheir motions Of all the possible explanations, which is\\nFig. 30.\\nFrom a photograph of the trails of stars near the celestial equator.\\nthe right one It is possible to explain the rising and set-\\nting of the stars in several ways. Let us give three such\\nways.\\n(A.) The Earth and the observer are at rest and each and\\nevery star has a particular motion of its own, each star", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0076.jp2"}, "75": {"fulltext": "DIURNAL MOTION: SUN, MOON, AND STARS. 53\\nmoving at just such a rate as actually to move completely\\nround the Earth back to its starting-point in 24 hours.\\nThere are at least a hundred million stars, in all possible\\nsituations. It is incredible that each one of them has a\\nspecial rate of motion of its own just as a railway train\\nhas its own rate of motion and that the 100,000,000 mo-\\ntions are so nicely regulated as to obey the laws of the di-\\nurnal motion exactly. This explanation is too complicated.\\nIt must be rejected.\\n(B.) All the stars are set in a huge sphere above us all\\nof them are at the same distance from us; the sphere itself\\nturns round the Earth once in 24 hours, while the Earth\\nand the observer remain at rest. This was the explanation\\ngiven by the ancients and it was a perfectly good explana-\\ntion so long as it was not known that the stars were sit-\\nuated at very different distances from us; so long as it was\\nnot known that some stars were comparatively near and\\nsome much further off. As soon as we know this one fact\\nit is impossible to suppose the stars to be set all in one\\nsphere. There would need to be a sphere for each star\\n(since no two stars are at exactly the same distance from\\nus). Moreover the planets {Venus, Jupiter, etc.) and the\\ncomets, are sometimes at one distance from us and some-\\ntimes at another. So that the explanation adopted by the\\nancients must also be given up, since the planets and comets\\nrise and set like the stars.\\n(C.) The simplest explanation possible is that the stars\\nare fixed and do not move at all that the whole Earth\\nwith the observer on its surface revolves round an axis once\\nevery 24 hours; so that the actual turning of the Earth\\nfrom west to east makes the stars (and the planets and\\ncomets) appear to move from east to west from rising to\\nsetting. This is the true explanation. It is not true be-\\ncause it is the simplest nor is there any one simple and\\nconclusive proof of its truth. It is true because it com-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0077.jp2"}, "76": {"fulltext": "54 ASTRONOMY.\\npletely and thoroughly explains every single one of millions\\nand millions of cases some of them very different from\\nothers. There are some rather complicated proofs of it,\\nbut no simple ones suitable to be given here. We must\\naccept it as true because it explains completely and thor-\\noughly every case that has arisen in the past and because\\nthere are millions and millions of such cases. Or, let us\\nsay that we will accept it as true until we come to some\\ncase which is not explained by it.\\nThe real motion of the horizon of an observer among the stars makes\\nthem aimear to rise and set.\\nthem appear to rise and set.\\nThe observer on the Earth is unconscious of its rotation,\\nand the celestial sphere appears to him to revolve from\\neast to west around the Earth, while the Earth appears to\\nremain at rest. The case is much the same as if he were\\non a steamer which was turning round, and as if he saw the\\nharbor-shores, the ships, and the houses apparently turn-\\ning in an opposite direction.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0078.jp2"}, "77": {"fulltext": "DIURNAL MOTION: SUN, MOON AND STARS. 55\\nFig. 31 is intended to explain the apparent diurnal motion of\\nthe stars which is caused by the real rotation of the Earth on its axis.\\nThe little circle N is the Earth, seen as it would be by a spectator\\nvery far away. The circle WZE is one of the circles of the celestial\\nsphere. W is towards the west and ^towards the east. The Earth\\nrevolves from west to east in the direction of the arrow. Suppose a\\nto be the situation of an observer on the Earth. Z will be his zenith\\nin the heavens. HH will be his horizon (since it is a plane through\\ntha cemtre of the Earth perpendicular to the line joining his zenith\\nand nadir). After a while the observer will have been carried on-\\nwards by the rotation of the Earth and his zenith will be at Z His\\nhorizon will have moved to HH It will have moved below all the\\nstars in the space HEH and these stars will have risen\\nthey will have come above his horizon. His horizon will have\\nmoved above all the stars in the space HWIT and these stars\\nwill have set they will have sunk below his horizon.\\nIt is really the horizon that moves and the stars that are at\\nrest but in common language we say that one group of stars\\nhas risen above his horizon, and that the second group has set. A\\nlittle later the observer on the rotating Earth will be at the point b\\nhis zenith will be at Z and his horizon at H H His horizon will\\nhave sunk below a new group of stars in the east (and these stars will\\nhave risen and his horizon will have moved above a group of\\nstars in the west (and this group will have set\\nThe zenith of an observer moves once round the celestial sphere\\neach day. His horizon (which is perpendicular to the line joining\\nhis zenith and nadir) moves once round the celestial sphere each\\nday, likewise. Therefore, stars in the east rise, culminate (come to\\ntheir greatest altitude), and set daily. This is the apparent diurnal\\nmotion of the stars, and it is explained by the actual motion of the\\nEarth on its axis.\\nBefore leaving this figure one important thing must be noticed.\\nSuppose there are two observers on the Earth, one at a and one at b.\\nTheir zeniths would be at Z and at Z on the celestial sphere at\\nsome instant. Their horizons would be, at this instant, HH and\\nH H The observer to the eastward (b) would see a whole group of\\nstars that are yet invisible to the other observer further west (a).\\nThat is, an observer at Greenwich at ten o clock at night (for ex-\\nample) will see groups of stars then invisible to an observer at\\nWashington. The horizon of the Washington observer has not yet\\nmoved below them they have not yet risen to him. If the Wash-\\nington observer waits for several hours these groups will, by and by,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0079.jp2"}, "78": {"fulltext": "56 ASTRONOMY.\\nrise. But the Greenwich observer always sees stars rise before they\\nhave risen at Washington.\\nWhat is the diurnal motion of the stars Describe the course of\\na southern star from its rising to its setting. At what point does such\\na star attain its greatest altitude above the horizon? What number\\nwill express the altitude (in degrees, for instance) of a star when it\\nis rising? What is the point of culmination of a star? The word\\nculmination is often used to express a lime as well as a definite point\\nin the sky what time How can stakes set in the ground be used\\nto demonstrate the diurnal motion of the stars? Is the motion of the\\nstars from rising to setting uniform? How do you know? The\\nsouthern stars all rise and set. What stars do not rise and set\\nWhat stars, then, are always above the observer s horizon The\\nnorth-polar- distance of every star that never sets must be less than\\nthe altitude of what point Make a sketch of the seven stars of\\nthe Great Bear. Which two are the pointers Where would Polaris\\nbe in this sketch Hold the paper on which the sketch is made be-\\ntween the thumb and finger of your left hand with Polaris covered\\nby your thumb. Now turn the paper round slowly, taking hold of\\nthe outer edges of it. If you face the north while doing this you\\nwill see that you are imitating, by a model, the actual diurnal mo-\\ntions of the northern stars. Define the north pole of the heavens. In\\nwhich direction (west to east, or east to west) do such stars move\\nwhen they are above the pole? When they are below below the pole\\nHow do they move (up or down when they are furthest east Fur-\\nthest west\\nDefine in a brief and accurate phrase the north-polar-distance in\\nstars?\\nGive the three laws of the diurnal motion. I. Every star in the\\nheavens II. The greater the star s N.P.D. III. All\\nthe stars describe their diurnal orbits in the same which is the\\nWhat is the diurnal orbit of a star? How can you know that\\nthese laws are true? What is the circle of perpetual apparition?\\nWhy is it so called?\\nThe foregoing laws, I, II, III, are true, as we know from observa-\\ntion. These are the appearances. What is the real cause of these\\nappearances? How do we know that the stars are not actually set in\\na huge sphere above our heads, and that this sphere does not turn\\naround the fixed Earth once every day (motions of planets, comets,\\netc.) The Earth turns on its axis once in 24 hours do you feel it\\nturning If the Earth turns, and the observer stays at one place (say\\nin New York) on its surface, does he move in space If the observer", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0080.jp2"}, "79": {"fulltext": "DIURNAL MOTION: SUN, MOON, AND STABS. 57\\nmoves round a circle every day, will his zenith, move on the surface\\nof the celestial sphere? his nadir? Will his horizon move among the\\nstars? When his horizon moves below a group of stars in the east,\\nthose stars will When his horizon moves above a group of\\nstars in the west those stars will\\nFig. 32. Part of a Celestial Globe:\\nShowing the principal circles of the celestial sphere.\\nIn this figure Z is the zenith of the observer, and iVT^/Shis horizon.\\nP is the north celestial pole, and XX, XXI 0, I the celes-\\ntial equator. is the vernal equinox. All stars on the hour circle\\nof II hours are on the celestial meridian of the observer (PZS). The\\nstar C (whose R. A.= 22 h is 4 hours west of the meridian the star\\nD (R.A. 20 h is 6 h west nearly to the western horizon.\\nIn Fig. 38 Z, P, NWS, etc., have the same meaning as in Fig.\\n32. In fact, the picture represents the same globe after it has\\nbeen turned one hour towards the west. The stars C and D are\\nin the same places on the celestial sphere as before, but G is now 5 U", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0081.jp2"}, "80": {"fulltext": "58\\nASTRONOMY.\\nwest of the meridian, and D is just setting 7 h west of the meridian.\\nIn Fig. 32 A and B (whose right ascensions are 2 h were on the\\ncelestial meridian of the observer here they are l h west of the\\nmeridian.\\n1\\n\u00e2\u0080\u00a2^fA^X ^/^Ov\\np\\nM$j w\\ntea\\nB@HQfiS\u00c2\u00abKfiS#JB^iQ^E\\nr x ^v v\\nFig. 33.\u00e2\u0080\u0094 Part of a Globe:\\nShowing the principal circles of the celestial sphere.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0082.jp2"}, "81": {"fulltext": "CHAPTER IV.\\nTHE DIURNAL MOTION TO OBSERVERS IN DIFFERENT\\nLATITUDES, ETC.\\n12. The Latitude of an Observer on the Earth. The al-\\ntitude of the celestial pole above the horizon of any place on\\nthe Earth s surface is equal to the latitude of that place.\\nLet L be a place on the Earth PEpQ, Pp being the\\nEarth s axis and EQ its equator. Z is the zenith of the\\nplace, and HR its sensible horizon. Its celestial or rational\\nFig. 34.\\nhorizon would be represented by a line through parallel\\nto HR. LOQ is the latitude of L according to ordi-\\nnary geographical definitions i.e., it is the angular\\ndistance of L from the Earth s equator. Prolong OP in-\\ndefinitely to P and draw LP parallel to it. P and P\\n59", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0083.jp2"}, "82": {"fulltext": "60 ASTRONOMY.\\nare points on the celestial sphere infinitely distant from L.\\nIn fact they appear as one point since the dimensions of\\nthe Earth are vanishingly small compared with the radios\\nof the celestial sphere.* We have then to prove that\\nLOQ P LH.\\nPOQ and ZLH are right angles, and therefore equal.\\nZLP ZOP by construction. Hence ZLH ZLP\\nP LH POQ ZOP LOQ, or the latitude of the\\npoint L is measured by either of the equal angles LOQ or\\nP LH.\\nIn Geography, which deals only with the Earth, it is\\nconvenient to define the latitude of an observer anywhere\\non the surface to be the angular distance of the point\\nwhere he stands from the terrestrial equator. The lati-\\ntude of an observer at L is LOQ\\nIn Astronomy, which deals chiefly with the heavens, it\\nis convenient to define the latitude of an observer anywhere\\non the Earth s surface to be the altitude of his celestial pole\\nabove his horizon. The latitude of an observer at L is\\nP LH the altitude of the pole or we might say, the lat-\\nitude of an observer is the N.P.D. of the north point of\\nhis horizon (if he is in the northern hemisphere). The\\nlatitude of an observer at L is P LHm Fig. 34.\\nIt is often more convenient, in Astronomy, to define the\\nlatitude of an observer by describing the place of his zenith\\non the celestial sphere and to say, the latitude of an ob-\\nserver anywhere on the Earth s surface is the declination\\nof his zenith.\\nFig. 35 represents the celestial sphere HZRN. The\\nEarth is a point at the centre of the circle. Some ob-\\nserver on the Earth has a zenith Z, a nadir N, a horizon\\nHR. P is the pole of the heavens and E a point of the\\ncelestial equator.\\nTwo lines drawn from the star Polaris to the points L and\\nmake an angle with each other of less than gTnnfTni tu \u00c2\u00b0f", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0084.jp2"}, "83": {"fulltext": "LATITUDE.\\n61\\nIn the figure PH measures the latitude of the observer,\\nbecause PH is the north-polar-distance of the north-point\\nof his horizon. Z is his zenith, EZ is the declination of\\nhis zenith (it is the angular distance of Z from the celestial\\nequator)\\nNow the arc PH the arc EZ because the arc ZH is\\n90\u00c2\u00b0, and PH =.90\u00c2\u00b0 PZ\\\\ moreover, the arc PE is 90\u00c2\u00b0,\\nand EZ 90\u00c2\u00b0 PZ. Therefore PH (the observer s lati-\\ntude) is measured by EZ (the declination of his zenith).\\nFig, 35.\\nThe latitude of an observer is measured by the declination of his Zenith.\\nIn Fig. 12 the latitude of the observer is measured either by\\n(NP) H or by QZ.\\nIn Fig. 16 the latitude of the observer is measured either by the\\nangle PON or by the angle COZ (or by the arcs PN and CZ).\\nIn Fig. 36 the latitude of the observer whose zenith is Z is\\nthe elevation of the north pole of the heavens (P) above his\\nhorizon (NWS) 40\u00c2\u00b0 it is measured by the declination of his zenith\\n(Z) 40\u00c2\u00b0.\\nDefine the latitude of an observer on the Earth according to\\nGeography. Define the latitude of an observer on the Earth ac-\\ncording to Astronomy in three ways I. The altitude of the North\\nPole above the observer s horizon is the of the observer, II,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0085.jp2"}, "84": {"fulltext": "62\\nASTRONOMY.\\nThe N.P.D. of the north point of an observer s horizon is the\\nof the observer. III. The declination of an observer s zenith\\nis the of that observer.\\nFig. 36.\\nSo far we have only spoken of observers in the northern\\nhemisphere of the Earth. The northern hemisphere is\\nthe most important to us, because all the more intelligent\\nnations of the globe lived in it for centuries and all astron-\\nomy was perfected there. Later on, our definitions will\\nbe extended to cover all cases.\\n13. The Horizon of an Observer Changes as He Moves\\nfrom Place to place on the Earth. The theorem that has\\njust been written is easily proved. As the observer travels\\nfrom place to place on the Earth his zenith moves on the\\ncelestial sphere. It is the point directly over his head.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0086.jp2"}, "85": {"fulltext": "DIURNAL MOTION IN 34\u00c2\u00b0 NORTH LATITUDE. 63\\nHis horizon is the plane always perpendicular to the line\\njoining his zenith and nadir. As this line moves with the\\nmotion of the observer his horizon must move.\\nIt is so important to understand just how the horizon of\\nan observer moves and just how the appearances of his sky-\\nare changed, that it is well worth while to take space to\\nconsider several cases.\\nFig. 37.\\nThe circles of a celestial sphere for an observer in north latitude PN or CZ.\\nThe student must pay particular attention to this figure.\\nWhen he understands just what it means he has mas-\\ntered all the more important theorems of spherical astron-\\nomy. The large circle stands for the celestial sphere.\\nThe Earth is a point at 0. P is the north pole of the\\nheavens (and p the south pole), and hence D WOE must be\\nthe celestial equator (since its plane is perpendicular to the\\nline joining the poles). The celestial sphere is full of stars.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0087.jp2"}, "86": {"fulltext": "64 ASTRONOMY.\\nNow let us suppose there is an observer on the Earth (0)\\nat some point in the northern hemisphere. If he is in the\\nnorthern hemisphere his zenith must be somewhere be-\\ntween C and P. Let us suppose that the observer is on\\nthe parallel of 34\u00c2\u00b0 north latitude, say on the parallel of Wil-\\nmington, N. C, or of Los Angeles, California. His lati-\\ntude is 34\u00c2\u00b0 then, and his zenith must be at Z, just 34\u00c2\u00b0\\nnorth of C. His nadir must beat n; his horizon must\\nbe NS. Suppose that we are looking at the celestial\\nsphere, as drawn in the figure, from a point outside of it\\nand west of it. W will be his ivest point; Ehis east point;\\nthe line EWis drawn so that it looks (in perspective) per-\\npendicular to NS, the observer s north and south line.\\nThe Earth will turn round once a day on the axis joining\\nthe poles P and p. The stars in the celestial sphere will\\nappear to rise above his eastern horizon NES they will\\nculminate on his meridian NZS they will set below his\\nwestern horizon NWS. A star which rises at E will cul-\\nminate at C and set at W. If he could see below his hori-\\nzon this star would seem to him to move from W to D\\nand then from D to E again. The interval of time be-\\ntween two successive risings would be 24 hours. Some\\nstars in the north would never set. All of them would lie\\nwithin the circle of perpetual apparition EN. Im is the\\ndiurnal orbit of a circumsolar star. Some stars would\\nnever rise to this observer. His horizon would hide them.\\nAll the stars further south than the circle SB, (the circle of\\nperpetual occultation) would never be seen. A star near\\nthe south pole would have a diurnal orbit like or.\\nThe student should notice that a part of this drawing is\\nquite independent of the situation of the observer. We\\ncan draw the celestial sphere, the celestial poles, the equa-\\ntor, the earth, and they will be the same for any and every\\nobserver; they will be the same whether any observer exists\\nor not. But the instant we imagine an observer on the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0088.jp2"}, "87": {"fulltext": "DIURNAL MOTIONS AT THE NORTH POLE. 65\\nearth anywhere on the earth his zenith is fixed. It\\nmust be at a point on the celestial sphere distant from the\\ncelestial equator by an arc equal to the observer s latitude.\\nSo soon as the zenith is fixed a horizon is fixed. As soon\\nas the horizon is fixed we know that some stars will never\\nrise above it, and that some stars will never set below it.\\nIf we draw the celestial sphere as it is for any particular\\nobserver we shall be able to say just how the stars will ap-\\npear to move for him; just what stars he can see, and just\\nwhat others he can never see.\\nThe student should exercise himself in making diagrams of the\\ncelestial sphere for observers in different latitudes. Let him make\\nsuch a diagram, placing the observer s zenith (Z) at K in the last\\nfigure, and another placing the observer s zenith at I.\\nFig. 38.\\nThe circles of the celestial sphere and the diurnal motions of the stars\\nas they appear to an observer at the north pole of the earth.\\nThe Diurnal Motion of Stars as Seen by an Observer at\\nthe North Pole of the Earth. An observer at the north\\npole of the Earth is in terrestrial latitude 90\u00c2\u00b0; the altitude\\nof the north celestial pole above his horizon will be 90\u00c2\u00b0.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0089.jp2"}, "88": {"fulltext": "66 ASTRONOMY.\\nHis zenith and the north celestial pole will coincide. The\\nstar Polaris will be neatly at his zenith.\\nFig. 38 shows the celestial sphere as it would appear\\nto an observer at the north pole of the Earth. The zenith\\nof the observer will be exactly overhead, of course, and\\nthe pole will coincide with his zenith. His horizon and\\nthe celestial equator will coincide, therefore. As all the\\nstars perform their diurnal revolutions in circles parallel\\nto the celestial equator, no matter what the latitude, in this\\nparticular latitude they will revolve parallel to the horizon.\\nNone of the stars of the southern half of the celestial\\nsphere will be visible at all. All the stars of the northern\\nhemisphere will be constantly visible. They will not rise\\nand set, but they will revolve in diurnal orbits parallel to\\nthe horizon.\\nArctic explorers who travel from temperate regions to-\\nwards the north find the north celestial pole constantly\\nhigher and higher above their horizon. When they are in\\nlatitude 50\u00c2\u00b0, the altitude of the pole (of the star Polaris)\\nwill be 50\u00c2\u00b0; when they are in latitude 70\u00c2\u00b0, the altitude of\\nPolaris will be 70\u00c2\u00b0; if they reach the pole of the Earth,\\nthe altitude of Polaris will be 90\u00c2\u00b0.\\nThe student may know that from March to September of every\\nyear the Sun is north of the celestial equator (in north declination)\\nand that from September to March the Sun is south of the celestial\\nequator (in south declination). From March to September, then,\\nthe Sun is a star of the northern hemisphere.; from September to\\nMarch the Sun is a southern star, An observer at the north pole\\nof the Earth sees all the northern stars revolve in diurnal orbits par-\\nallel to his horizon, and he will thus have the Sun above the horizon\\nfor six entire months, and for the next six months he will not see\\nthe Sun at all. An observer at the south pole of the Earth will\\nhave the Sun constantly above his horizon from September to\\nMarch; constantly below it from March to September. The Fig. 39\\nwill illustrate the diurnal orbit of the Sun to an observer at the\\nnorth pole of the Earth. The Sun is at the point (near W) on\\nMarch 22, and from March to June travels every day about 1\u00c2\u00b0 along", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0090.jp2"}, "89": {"fulltext": "DIURNAL MOTIONS AT THE EQUATOR.\\n6\\nthe lowest broken line of the figure. The Sun is on the\\nhour circle 7 on April 6, on II on April 22, on 111 (near E) on May\\n8, on IV on May 23 (and always on the dotted curve). The student\\nshould trace out in the picture the diurnal orbits of the Sun on the\\ndates just given.\\nThe Diurnal Motion of Stars as Seen by an Observer at\\nthe Earth s Equator. If the observer is at any point on\\nFig. 39.\\nA globe so set as to show the circles of the celestial sphere for an observer\\nat the north pole of the earth.\\nthe Earth s equator his terrestrial latitude will be 0\u00c2\u00b0 the\\nelevation of the north celestial pole above his horizon will\\nbe 0\u00c2\u00b0 the star Polaris will be in his horizon.\\nFig. 40 shows the celestial sphere as it appears to an", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0091.jp2"}, "90": {"fulltext": "68 ASTRONOMY.\\nobserver on the Earth s equator. The zenith of the ob-\\nserver is in the celestial equator. The latitude of the ob-\\nserver is 0\u00c2\u00b0 and hence the altitude of the north celestial\\npole (of Polaris) is 0\u00c2\u00b0 that is, the north and south celes-\\ntial poles are in his horizon. All the stars appear to move\\nin their diurnal orbits parallel to the celestial equator, no\\nmatter what may be the observer s latitude. In this case\\nthey will all appear to revolve in circles perpendicular to\\nthe horizon. All the stars of the sky, those in both halves\\nFig. 40.\\nThe circles of the celestial sphere and the diurnal motions of the stars as\\nthey appear to an observer on the earth s equator.\\nof the celestial sphere, will be visible, for all of them will\\nrise, every day, above the eastern horizon and will pass\\nacross the sky and set below the western horizon. Every\\nstar will be above the horizon exactly half a day 12 hours.\\nIn Fig. 41 the diurnal paths of all stars are perpendicular to the\\nhorizon, and every star is 12 h above and 12 h below it. Stars whose\\nright-ascension is 6 h are on the meridian in the picture The star B\\nis 3 b the stars A, B, are 4 h west of the meridian. The vernal equi-\\nnox (0) is 6 h west.\\nThe ecliptic (the path of the Sun) is marked on the northern celes-\\ntial hemisphere by a broken line from towards E,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0092.jp2"}, "91": {"fulltext": "DIURNAL MOTION OF THE SUN. 69\\netc. The Sun is at. on March 22 on the hour-circle I, April 6\\non II, April 22 on III, May 8 on IV, May 23 (and always on the\\ndotted curve). The student should trace out the diurnal orbits of the\\nSun for the dates just given. It is clear that the Sun will cross the\\ncelestial meridian of an observer at the Earth s equator north of his\\nzenith when the Sun is in north declination (March to September),\\nand south of it whenever the Sun is in south declination In our\\nlatitudes the Sun is never seen north of the zenith, as may be seen by\\ninspecting Fig. 33, where the dotted line is the Sun s path.\\nFig. 41.\\nA globe so set as to show the circles of the celestial sphere for an ob-\\nserver at the earth s equator. Z is his zenith P the north celestial pole\\nNWS his horizon.\\nIf now the observer travels southward from the equator,\\nthe south pole will, in its turn, become elevated above his\\nhorizon, and in the southern hemisphere appearances will\\nbe reproduced that have been already described for the\\nnorthern, except that the direction of the motion will, in", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0093.jp2"}, "92": {"fulltext": "10 ASTRONOMY.\\none respect, be different. The heavenly bodies will still\\nrise in the east and set in the west, but those near the\\ncelestial equator will pass north of the zenith of the ob-\\nserver instead of south of it, as in our latitudes. The sun,\\ninstead of moving from left to right, there moves from\\nright to left. In the northern hemisphere of the Earth\\nwe have to face to the south to see the sun while in the\\nsouthern hemisphere we have to face to the north to see it.\\nIf the observer travels west or east on a parallel of lati-\\ntude of the Earth s surface, his zenith will still remain at\\nthe same angular distance from the north pole as before\\n(since his terrestrial latitude remains unchanged), and as\\nthe phenomena caused by the diurnal motion at any place\\ndepend only upon the altitude of the elevated pole at that\\nplace, these will not be changed except as to the times of\\ntheir occurrence.\\nFig. 42.\\nThe risings of the stars to an observer on the earth are earlier the\\nfarther east he is. East is in the direction of the arrow, since the earth\\nrevolves from west to east.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0094.jp2"}, "93": {"fulltext": "DIURNAL MOTIONS IN DIFFERENT LATITUDES. 11\\nA star that appears to pass through the zenith of his\\nfirst station will also appear to pass through the zenith of\\nthe second (since each star remains at a constant angular\\ndistance from the pole), but later in time, since it has to\\npass through the zenith of every place between the two sta-\\ntions. The horizons of the two stations will intercept\\ndifferent portions of the celestial sphere at any one instant,\\nbut the Earth s rotation will present the same portions suc-\\ncessively, and in the same order, at both. An observer at\\nb (east of a) will see the same stars rise earlier than an ob-\\nserver at a. (See Fig. 42.)\\nChange of the Position of the Zenith of an Observer by\\nthe Diurnal Motion. If the student has mastered what\\nhas gone before he can solve any questions relating to the\\ndiurnal motion. The following presentation of these ques-\\ntions will be found useful in relation to problems of longi-\\ntude and time, that are to be considered shortly.\\nIn Figure 43 nesq is the Earth NESQ is the celestial sphere. An\\nobserver at n will have his zenith at NP, and his borizon will coin-\\ncide with the celestial equator. The stars will appear to revolve\\nparallel to his horizon (the celestial equator), as we have seen. If\\nthe observer is at s, his zenith is at SP. If the observer is in 45\u00c2\u00b0\\nnorth latitude (the latitude of Minneapolis), his zenith will be at Z in\\nthe figure. The Earth revolves on its axis once daily, and the ob-\\nserver will be carried round a circle. His zenith (Z) will move round\\na circle of the celestial sphere (ML) corresponding to the parallel of\\n45\u00c2\u00b0 on the Earth. If the observer is on the earth s equator at q, his\\nzenith will be at Q, and it will move round the circle EQ of the celes-\\ntial sphere once daily. If the observer is at 45\u00c2\u00b0 south latitude on\\nthe Earth, his zenith will be at S, and the zenith will move round a\\ncircle of the celestial sphere (SO) once daily, and so on. Thus, for\\neach parallel of latitude on the Earth we have a corresponding circle\\non the celestial sphere (a parallel of declination), and each of these\\nlatter circles lias its poles at the celestial poles.\\nNot only are there circles of the celestial sphere that correspond\\nto parallels of latitude on the Earth, but there are also celestial\\nmeridians which correspond to the various terrestrial meridians. The\\nplane of the meridian of any place contains the zenith of that place", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0095.jp2"}, "94": {"fulltext": "72\\nASTRONOMY.\\nand the two celestial poles. It cuts from the earth s surface the ter-\\nrestrial meridian, and from the celestial sphere that great circle\\nwhich we have denned as the celestial meridian.\\nTo fix the ideas, let us suppose an observer at some one point of the\\nEarth s surface. A north and south line on the Earth at that point\\nis the visible representative of his terrestrial meridian. A plane\\nthrough the centre of the Earth and that line contains his zenith, and\\nFig. 43.\\nThe change of the position of the observer s zenitb on the celestial sphere\\ndue to the diurnal motion.\\ncuts from the celestial sphere the celestial meridian. As the Earth\\nrotates on its axis his zenith moves round the celestial sphere in a par-\\nallel, as ZL in the last figure.\\nSuppose that the east point is in front of the picture, the west\\npoint being behind it. Then as the Earth rotates the zenith Z will\\nmove along the line ZL from Z towards L. The celestial meridian\\nalways contains the celestial poles and the point Z, wherever it may", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0096.jp2"}, "95": {"fulltext": "DIURNAL MOTIONS IN DIFFERENT LATITUDES. 73\\nbe. Hence, the arcs of great circles joining N.P. and S.P. in the fig-\\nure are representatives of the celestial meridian of this observer,\\nat different times during the period of the Earth s rotation. They\\nhave been drawn to represent the places of the meridian at intervals\\nof 1 hour. That is, 12 of them are drawn to represent 12 consecutive\\npositions of the meridian during a semi-revolution of the Earth.\\nIn this time Z moves from Z to L. In the next semi-revolution\\nZ moves from L to Z, along the other half of the parallel ZL. In 24\\nlio irs the zenith Zof the observer has moved from Z to L and from\\nL back to Z again. The celestial meridian lias also swept across the\\nheavens from the position N P., Z, Q, S, S.P., through every inter-\\nmediate position to N.P., L, E, 0, S.P and from this last position\\nback to N.P., Z, Q, S, S.P. The terrestrial meridian of the observer\\nhas been under it all the time.\\nThis real revolution of the celestial meridian is incessantly repeated\\nwith every revolution of the Earth. The sky is studded with stars\\nall over the sphere. The celestial meridian of any place approaches\\nthese various stars from the west, passes them, and leaves them.\\nThis is the real state of things. Apparently the observer is fixed.\\nHis terrestrial and celestial meridians seem to him to be fixed, not\\nonly with reference to himself, as they are, but to be fixed in space.\\nThe stars appear to him to approach his celestial meridian from the\\neast, to pass it, and to move away from it towards the west. When\\na star crosses the celestial meridian it is said to culminate. The pass-\\nage of the star across the meridian is called the transit of that star.\\nThis phenomenon takes place successively for each observer on the\\nEarth.\\nSuppose two observers, A and B, A being one hour (15\u00c2\u00b0) east of B\\nin longitude. This means that the angular distance of their terres-\\ntrial meridians is 15\u00c2\u00b0 (see page 28). From what we have just learned\\nit follows that their celestial meridians are also 15\u00c2\u00b0 apart. When B s\\nmeridian is N.P., Z, Q, R, S.P., A s will be the first one (in the fig-\\nure) beyond it when B s meridian has moved to this first position,\\nA s will be in the second, and so on, always 15\u00c2\u00b0 (one hour) in advance.\\nA group of stars that has just come to A s meridian will not pass B s\\nfor an hour. When they are on B s meridian they will be one hour\\nwest of A s, and so on. A s zenith is always one hour west of B s.\\nThe same stars successively rise, culminate, and set to each observer\\n(A and B), but the phenomena will be presented earlier to the eastern\\nobserver.\\nIf the student has access to a celestial globe all the prob-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0097.jp2"}, "96": {"fulltext": "74 ASTRONOMY.\\nlems that have been considered in this chapter can be\\nquickly solved by its use.\\nIn Figure 44 Z is the zenith, iVthe nadir, and W the west point\\nof the observer. P is the north celestial pole, X, XI, XIV, XV,\\nthe celestial equator. The dotted line from P through XII to\\nthe south celestial pole is the hour-circle of 12 hours. The dotted\\nline inclined to the equator by an angle of 23\u00c2\u00b0 is the sun s path the\\necliptic. Stars whose right-ascension are 17 h are on the observer s\\ncelestial meridian.\\nThe star K (K.A. 13\\\\ Decl. 20\u00c2\u00b0) is 4 h west of the merid-\\nian the star B (R.A. 10 h Decl. 30\u00c2\u00b0) is just setting the\\nstars north of Decl. 50\u00c2\u00b0 are circumpolar they never set.\\nProve that as an observer moves from place to place his hori-\\nzon must change. If an observer is in the northern hemisphere of\\nthe Earth his zenith is in the northern half of the celestial sphere.\\nProve it by a diagram. What is a circumpolar star Draw a dia-\\ngram representing the celestial sphere with its poles, its equator.\\nNow, suppose an observer on the Earth in 30\u00c2\u00b0 north latitude\\nwhere will his zenith be on the diagram Draw a circle to show\\nwhat stars will always be above his horizon. Suppose an observer\\nin 86\u00c2\u00b0 north latitude (the highest latitude reached by Nansen in\\n1895); where will his zenith be? Draw circles to show how the\\nstars appeared to move in their diurnal orbits to Nansen. The hori-\\nzon of an observer in some latitude is the same as the celestial equa-\\ntor in what latitude? An observer at the north pole of the Earth\\nwould have the Sun constantly above his horizon for six months\\nprove it. All the stars are successively visible to an observer on the\\nEarth s equator prove it.\\nThe Celestial Globe. A celestial globe is a globe marked\\nwith the lines and circles of the celestial sphere the celes-\\ntial poles, the celestial equator, the celestial meridians and\\nparallels, etc., and with the principal stars, each one in its\\nproper right-ascension and declination. The Figs. 32, 33,\\n39, 41, and 44 represent such a globe with the stars omit-\\nted. Every school should own a celestial globe, because all\\nthe problems of spherical astronomy can be simply ex-\\nplained or illustrated by its use. In text-books we are\\nobliged to use diagrams. They are necessarily drawn on a", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0098.jp2"}, "97": {"fulltext": "THE CELESTIAL GLOBE.\\n75\\nFig. 44.\\nView of a globe showing the circles of the celestial sphere for an\\nobserver in 40\u00c2\u00b0 north latitude (the latitude of Philadelphia, Columbus, O.,\\nQuincy, 111., Denver, etc.).", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0099.jp2"}, "98": {"fulltext": "76 ASTRONOMY.\\nflat surface, and the student has to imagine the spherical\\nsurface. The school-globe shows the surface as it really is.\\nThe celestial globe must be set so that the elevation of\\nthe north celestial pole (if the observer lives in the north-\\nern hemisphere) above the horizon is the same as the lati-\\ntude of the observer. (His latitude can be taken from any\\ngood map.) Then the celestial globe will represent his ce-\\nlestial sphere just as it really is, when the line NS is placed\\nnorth and sooth, N to the north. Any one of the problems\\nof this chapter can be illustrated by turning the celestial\\nglobe about the axis. For instance, let the student point\\nout the circnmpolar stars, those that never rise and set\\nto him. Let him take a star a little further south and\\nturn the globe till the star is at the eastern horizon just\\nrising. By turning the globe slowly he will see exactly how\\nthis particular star moves in its apparent diurnal orbit from\\nrising to culmination, and from culmination to setting.\\nLet him particularly notice how its altitude increases from\\nzero at rising to a maximum at culmination; and how it\\ndecreases from culmination to zero at setting.\\nAfter he has studied the diurnal motion of one star, let\\nhim choose another one and trace its course from rising to\\nsetting. He should study, in this way, the diurnal mo-\\ntions of stars in all parts of the sky. If he has his globe\\nby him while he is observing the real stars in the sky, the\\nglobe will help him to understand quickly, in a few min-\\nutes, motions that the real stars require 24 hours to make.\\nOther problems can be, and should be, studied in the same\\nway.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0100.jp2"}, "99": {"fulltext": "CHAPTER V.\\nCO-ORDINATES-SIDEREAL AND SOLAR TIME.\\n14. Systems of Co-ordinates to define the Place of a Star\\nin the Celestial Sphere. Let us now briefly consider some\\nof the ways in which the position of a star in the celestial\\nsphere may be described. Many of them are already fa-\\nmiliar.\\nFig. 45.\u00e2\u0080\u0094 Systems of Co-ordinates on the Celestial Sphere.\\nAny great circles of the celestial sphere which pass\\nthrough the two celestial poles are called how-circles.\\nEach hour-circle is the celestial meridian of some place on\\nthe Earth.\\n77", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0101.jp2"}, "100": {"fulltext": "78 ASTRONOMY.\\nThe hoar-circle of any particular star is that one which\\npasses through the star at the time. As the Earth re-\\nvolves, different hour-circles, or celestial meridians, come\\nto the star, pass over it, and move away towards the east.\\nIn Fig. 45 let be the position of the Earth, in the centre of the\\ncelestial sphere NZ8D. Let Z be the zenith of the observer at a\\ngiven instant, and P, p, the celestial poles. By definition PZSpnNP\\nis his celestial meridian. NS is the horizon of the observer at the\\ninstant chosen. PON is his latitude. If P is the north pole, he is\\nin latitude 34\u00c2\u00b0 north, because the angle PON 34\u00c2\u00b0.\\nEG WD is the celestial equator E and W are the east and west\\npoints. The Earth is turning from Wto E. The celestial meridian,\\nwhich at the instant chosen in the picture contains PZp, was in the\\nposition P V about three hours earlier.\\nPC, PB, PV, PD are parts of hour-circles. If J. is a star, PB is\\nthe hour-circle passing through that star. As the Earth turns PB\\nturns with it (towards the east), and directly PB will have moved\\naway from A towards the top of the picture, and soon the hour-circle\\nPV will pass through the star A. When it does so, PF will be the\\nhour-circle of the star A. At the instant chosen for making the\\npicture PB is its hour-circle.\\nWe are now seeking for ways of denning the position of\\na star, of any star, on the celestial sphere. We define the\\nposition of a place on the Earth by giving its latitude and\\nlongitude. These two angles are called the co-ordinates of\\nthis place. Co-ordinates are angles which, taken together,\\ndetermine the position of a point. If we say that the\\nlongitude of a city is 77\u00c2\u00b0 and that its latitude is 38\u00c2\u00b0 53 N\\nwe know that this city is Washington. These two num-\\nbers determine its position. The place of this city is de-\\nscribed by them and no other city can be meant.\\nTo describe and determine the place of a star on the\\ncelestial sphere we may employ several different pairs of\\nco-ordinates. Those spoken of here will all be needed in\\nwhat is to follow.\\nNorth-polar-distance and Hour-angle. The north-\\npolar-distance (N.P.D.) of the star A is PA. Tlie hour-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0102.jp2"}, "101": {"fulltext": "CELESTIAL CO-ORDINATES.\\n79\\nangle of a star is the angular distance between the celes-\\ntial meridian of the observer and the hour-circle passing\\nthrough that star. The honr-angle is counted from the\\nmeridian towards the west from 0\u00c2\u00b0 to 360\u00c2\u00b0 (or from h to\\n24 L The hour-angle of a star at A at the instant chosen\\nfor making the picture is ZPB. The hour-angle of a star\\nat iT is 0\u00c2\u00b0. The hour-angle of a star at Fis ZPV; of a\\nstar at D is ZPB 180\u00c2\u00b0 12 h and so on.\\nThe hour-angle is measured by the arc of the celestial\\nFig 45 bis.\\nequator between the celestial meridian of the observer and\\ni he foot of the hour-circle through the star. The arc CB\\nis the measure of the angle ZPB. Knowing the two co-\\nordinates PA and CB the place of the star A is described\\nand determined.\\nNorth-polar-distance and Right-ascension. The north-\\npolar-distance of the star A is PA, measured along the\\nhour-circle PB. Let us choose some fixed point V on the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0103.jp2"}, "102": {"fulltext": "80 ASTRONOMY.\\nequator to measure our other co-ordinate from, and let us\\nalways measure it on the equator towards the east from 0\u00c2\u00b0\\nto 360\u00c2\u00b0 (from h to 24 h That is, from V through B 9 C,\\nE, D, IF, successively.\\nVB is the right-ascension of A. The right-ascension of\\na star is the angular distance of the foot of the hour-circle\\nthrough the star from the vernal equinox, measured on the\\ncelestial equator, toivards the east.\\nExactly what the vernal equinox is we shall find out\\nlater on; for the present it is sufficient to define it as a\\ncertain fixed point on the celestial equator.*\\nIf we have the right-ascension and north-polar-distance\\n(E.A. and N.P.D.) of a star, we can point it out. Thus\\nVB and PA define the position of A.\\nThe right-ascension of the star K is VC. Of a star at\\nE it is VOB; of a star at D it is VCED of a star at W it\\nis VCEDWfund so on.\\nRight-ascension and Declination. It is sometimes con-\\nvenient to use in place of the north-polar-distance of a star\\nits declination.\\nThe declination of a star is its angular distance north or\\nsouth of the celestial equator.\\nThe declination of A is BA, which is 90\u00c2\u00b0 minus PA.\\nThe relation between N.P.D. and 6 is\\nN.P.D. 90\u00c2\u00b0 S; d 90\u00c2\u00b0 N.P.D.\\nNorth declinations are -f- south declinations are\\njust as geographical latitudes are -f (north) and (south).\\nAltitude and Azimuth.\u00e2\u0080\u0094 A vertical plane with respect to any ob-\\nserver is a plane that contains his vertical line. It must pass through\\nhis zenith and nadir, and must be perpendicular to his horizon. A\\nvertical plane cuts the celestial sphere in a vertical circle.\\nIt is, in fact, that point at which the Sun passes the celestial\\nequator in moving 1 from the southern half of the heavens to the\\nnorthern half. The Sun is south of the celestial equator from Sep-\\ntember 22 to March 21 and north of it from March 21 to September 22.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0104.jp2"}, "103": {"fulltext": "CELESTIAL CO-ORDINATES.\\n81\\nFig. 46.\\nAs soon as we imagine an observer to beat any point on the Earth s\\nsurface his horizon is at once fixed his zenith and nadir are also\\nfixed. From his zenith radiate a\\nnumber of vertical circles that\\ncut the celestial horizon perpen-\\ndicularly, and unite again at his\\nnadir.\\nSome one of these vertical cir-\\ncles will pass through any and\\nevery star visible to this observer.\\nThe altitude of a heavenly body\\nis its angular elevation above the\\npl.t ne of the horizon measured on\\na vertical circle through the star.\\nThe zenith distance of a star is\\nits angular distance from the\\nzenith measured on a vertical\\ncircle.\\nIn the figure, ZS is the zenith distance of S, and HS (a) is its\\naltitude. ZSH is an arc of a vertical circle.\\nZSH a C 90\u00c2\u00b0; C 90\u00c2\u00b0 a a 90\u00c2\u00b0\\nThe azimuth of a star is the angular distance from the point where\\nthe vertical circle through the star meets the horizon from the north (or\\nsouth) point of the horizon. Nil ox SH is the azimuth of S in Fig.\\n46. The prime-vertical of an observer is that one of his verti-\\ncal circles that passes through his east and west points. The azi-\\nmuth of a star on the prime- vertical is 90\u00c2\u00b0.\\nCo-ordinates of a Star. In what has gone before we\\nhave described various ways of expressing the apparent\\npositions of stars on the surface of the celestial sphere.\\nThat one most commonly used in Astronomy is to give\\nthe right-ascension and north-polar-distance (or declina-\\ntion) of the star. The apparent position of the star on the\\ncelestial sphere is fixed by these two co-ordinates jnst as\\nthe position of a place on the Earth is fixed by its two co-\\nordinates, latitude and longitude.\\nIf the student has a celestial globe he can set it so as to make the\\npreceding definitions very clear. The north pole of the globe must\\nbe above the horizon of the globe by an angle equal to the latitude,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0105.jp2"}, "104": {"fulltext": "82 ASTRONOMY.\\nIn the figure Z is tlie observer s zenith, as before. The star A has\\nthe following co-ordinates R.A. 2 h hour-angle l h west, Decl.\\n40\u00c2\u00b0, N.P.B. 50\u00c2\u00b0, zenith distance the arc ZA, altitude 90\u00c2\u00b0\\nZA, azimuth, the arc measured on the horizon S WN f rom\\nthrough W to to the foot of a vertical circle from Z through A\\nthe azimuth of A is something more than 90\u00c2\u00b0. The student should\\npoint out the corresponding co-ordinates for the stars B, G, and B.\\nFig. 47.\\nA globe showing the circles of the celestial sphere as they appear to an\\nobserver in 40\u00c2\u00b0 north latitude.\\nStudents mast try to realize the circles that have been\\ndescribed in the book as they actually exist in the sky.\\nThey are in the sky first; and in the book only to explain\\nthe appearances in the sky. On a starlit night let him\\nfirst find the north celestial pole (near the star Polaris).\\nAll hour-circles pass through this point. Next he must", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0106.jp2"}, "105": {"fulltext": "CELESTIAL CO-OBDINATES. 83\\nfind his zenith. All vertical circles pass through this\\npoint. The great circle in the sky that passes through the\\nnorth pole of the heavens and his own zenith is his own\\ncelestial meridian. Let him trace it out in the sky from\\nthe north point of his horizon to the south point; and\\nimagine it extending completely round the earth as a great\\ncircle. Let him choose a star a little to the west of his\\nmeridian and decide 1st. What is the N.P.D. of this\\nstar? 2d. What is its hour-angle? Next he should select a\\nstar far to the west, and decide what its N.P.D. and hour-\\nangle are. Then he should take a star a little to the east\\nof his meridian and decide the same points for this star.\\nA little practice of this sort will make all the circles of the\\nsky quite familiar.\\nDefine hour-circles of the celestial sphere. What is the hour-\\ncircle of a star? Does a star have different hour-circles at different\\ninstants What are the two co-ordinates that determine the position\\nof a point on the surface of the Earth What pairs of co-ordinates\\nmay be used to determine and describe the position of a star on the\\ncelestial sphere Define the hour-angle of a star. What is the\\nmeasure of the hour-angle on the celestial equator Define the\\nright- ascension of a star. Hour-angles are counted from the celestial\\nmeridian of a place towards the The right- ascension of a star\\nis counted, on the celestial equator, towards the\\n15. Measurement of Time; Sidereal Time; Solar Time;\\nMean Solar Time Sidekeal Time. The Earth rotates\\nuniformly on its axis and it makes one complete revolution\\nin a sidereal day.\\nA sidereal day is the interval of time required for the\\nEarth to make one complete revolution on its axis, or, what\\nis the same thing, it is the interval between two successive\\ntransits of the same star over the celestial meridian of a\\nplace on the Earth. A sidereal day 24 sidereal hours.\\nA sidereal hour 60 sidereal minutes. A sidereal minute", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0107.jp2"}, "106": {"fulltext": "84 ASTRONOMY.\\n60 sidereal seconds. In a sidereal day the earth turns\\nthrough 360\u00c2\u00b0, so that\\n24 hours 360\u00c2\u00b0; also,\\n1 hour 15\u00c2\u00b0; 1\u00c2\u00b0 4 minutes.\\n1 minute 15 1 4 seconds.\\n1 second 15 1 0.066 second.\\nWhen a star is on the celestial meridian of any place its\\nhour-angle is zero, by definition (seepage 79). It is then\\nat its transit or culmination.\\nAs the Earth rotates, the meridian moves away (east-\\nwardly) from this star, whose hour-angle continually in-\\ncreases from 0\u00c2\u00b0 to 360\u00c2\u00b0, or from hours to 24 hours.\\nSidereal time can then be directly measured by the hour-\\nangle of any star in the heavens which is on the meridian\\nat an instant we agree to call sidereal hours. When this\\nstar has an hour-angle of 90\u00c2\u00b0, the sidereal time is 6 hours;\\nwhen the star has an hour-angle of 180\u00c2\u00b0 (and is again on\\nthe meridian, but invisible unless it is a circumpolar star),\\nit is 12 hours when its hour-angle is 270\u00c2\u00b0, the sidereal\\ntime is 18 hours and, finally, when the star reaches the\\nupper meridian again, it is 24 hours or hours. (See Fig.\\n48, where EC WD is the apparent diurnal path of a star in\\nthe equator. It is on the meridian at C.)\\nInstead of choosing a star as the determining point whose\\ntransit marks sidereal hours, it is found more conven-\\nient to select that point in the sky from which the right\\nascensions of stars are counted the vernal equinox the\\npoint V in Fig. 48. The sidereal time at any instant is\\nmeasured by the hour-angle of the vernal equinox. The\\nfundamental theorem of sidereal time is: The hour-angle\\nof the vernal equinox, or the sidereal time, is equal to the\\nright-ascension of the meridian; that is, CV VC.\\nTo avoid continual reference to the stars, we set a clock\\nso that its hands shall mark hours minutes seconds", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0108.jp2"}, "107": {"fulltext": "SIDEREAL TIME.\\n85\\nat the instant the vernal equinox is on the celestial merid-\\nian of the place; and the clock is regulated so that exactly\\n24 hours of its time elapses during one revolution of the\\nEarth on its axis.\\nIn this figure PZCS is the celestial meridian of the observer whose\\nzenith is Z. Vis the vernal equinox. It is that point on the celes-\\ntial sphere from which right-ascensions are counted. We shall soon\\nsee how to determine it.\\nPig. 48.\u00e2\u0080\u0094 Measurement op Sidereal Time.\\nSuppose that there were a very bright star exactly at V. (There is\\nno star exactly at the vernal equinox.) Such a star would rise (at E)\\\\\\nculminate (at 0); and set (at W). When it is on the celestial merid-\\nian of the observer its hour-angle is O h O m s (at C). Two hours\\nlater the star V will have moved 30\u00c2\u00b0 to the westward, towards set-\\nting. Its hour-angle ZPB will then be 2 h The sidereal time of the\\nobserver whose zenith is Z will then be 2 h Six hours after its cul-\\nmination (at C) the star V will have moved to TFand its hour-angle\\nwill be 6 b The sidereal time of the particular observer whose zenith", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0109.jp2"}, "108": {"fulltext": "86\\nASTRONOMY.\\nis Z will then be 6 h When Fhas moved to D, the sidereal time will\\nbe 12 1 When V has moved to E, the sidereal time will be 18 h\\nWhen V has moved to Cthe sidereal time will be 24 h (or 1 again)\\nand a new sidereal day will begin and so on forever.\\nWhen the hour-angle of V is 2 h and the vernal equinox\\nis at B, the right-ascension of the celestial meridian (of the\\n1\\nm m\\n\u00c2\u00abr T_T\\n^^^^\u00e2\u0096\u00a0HSKSSBftWaK^SWHHH\\n^^k ji^y V\\nJ \u00c2\u00a3^J!L~y m 7* s /\\\\J\\\\;\\nt X//^ c /t\\\\/^\\nIIY^Oy^jAX\\nP^l\\n/f y i v/\\nrtky ,/v x\\nV _ ,CJ\\n^P(aa//^ ^/v\\nWX/\\\\/V /Y\\n[P\\ny^ \\\\S OQ jT^Z /C\\n^Sc S y^\\ny y/ X\\nFig. 49.\\nThe hour-angle of the vernal equinox, O, in this figure is 2 hours west.\\nThe sidereal time is therefore 2 hours. The R.A. of the observer s merid-\\nian is 2 hours.\\npoint C) is 2 h The right-ascension of any star on the\\nmeridian at that instant must be 2 hoars. Speaking gen-\\nerally, when the vernal equinox is anywhere (as at V in\\nFig. 48) the right-ascension of the celestial meridian (of the\\npoint C in the figure will be VC. The sidereal time is\\nthe angle ZP V measured by the arc CV. The right-ascen-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0110.jp2"}, "109": {"fulltext": "SIDEREAL TIME.\\n87\\nsion of the meridian is VC. The right-ascension of any\\nstar on the meridian at that instant will be VC.\\nConversely if a star C is on the celestial meridian of a\\nplace at any instant the right-ascension of that star is ex-\\npressed by the same number of degrees (or of hours) as the\\nhour-angle of the vernal equinox or as the sidereal time.\\nFig. 50.\\nThe hour-angle of the vernal equinox, 0, in this figure is 3 hours west.\\nThe sidereal time is therefore 3 hours. The R. A. of the observer s merid-\\nian is 3 hours.\\nSuppose then that we had a catalogue of the right-ascensions of\\nstars like this\u00e2\u0080\u0094 and we have such catalogues. See Table V for a\\nspecimen of the sort\\nThe R. A. of the star Aldebaran is 4 30 m\\nSirius is\\n6 h 41 m\\nRegulus is\\n10 h 3 m\\nSpica is\\n13 20 m\\nArcturus is\\n14 ll ra\\nVega is\\n18 34 m\\nFomalhaut is 22 h 52 m", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0111.jp2"}, "110": {"fulltext": "88 ASTRONOMY.\\nSuppose further that we had a way of knowing when a star was\\non our celestial meridian, that is, exactly south of us (and we have\\nsuch a way, as will soon be seen), then if an observer noticed that\\nSirius was on his celestial meridian at a certain instant he would know\\nthat the sidereal time at that instant must be 6 h 41 m (For the R.A.\\nof Sirius is 6 h 41\u00e2\u0084\u00a2 and this is the R.A. of the meridian, and this is\\nequal to the hour-angle of the vernal equinox; and, finally, this is\\nFig. 51.\\nThe hour-angle of the vernal equinox, 0, in this figure is 6 hours west.\\nThe sidereal time is therefore 6 hours. The R.A of the observer s merid-\\nian is 6 hours.\\nthe sidereal time at that instant). If the star Fomalhaut is on the\\ncelestial meridian of an observer at another instant, the sidereal time\\nat that instant must be 22 h 52 ra and so on. The sidereal clock must\\nshow on its dial 6 1 41 m when Sirius is on the meridian and it must\\nshow 22 h 52 m when Fomalhaut is on the meridian, and so on. As\\nsoon as we know the right-ascension of one star we can set the hands\\nof the sidereal clock correctly. When Sirius is on the meridian on", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0112.jp2"}, "111": {"fulltext": "SIDERIAL TIME.\\n89\\nFig. 52.\\nThe hour-angle of the vernal equinox in this figure is 17 hours. The\\nsidereal time is therefore 17 hours. The R.A. of the observer s meridian\\nis 17 hours.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0113.jp2"}, "112": {"fulltext": "90 ASTRONOMY.\\nMonday they must point to 6 h 41 m When Sirius comes to the merid-\\nian on Tuesday they must again mark 6 1 41 m And it is just the\\nsame for other stars. Any star whose right-ascension is known will\\nenable us to set the hands of the sidereal clock correctly as soon as\\nwe know the direction of our meridian in space. The hour-hand of\\nthe clock must move over 24 h every day, from one transit of the star\\ntill the next succeeding transit.\\nSolar Time. Time measured by the hour-angle of the\\nsun is called true (or apparent) solar time. An apparent\\nsolar day is the interval of time betiveen two consecutive\\ntransits of the Sun over the celestial meridian. The instant\\nof the transit of the Sun over the meridian of any place is\\nthe apparent noon of that place, or local apparent noon.\\nWhen the Sun s hour-angle is 12 hours or 180\u00c2\u00b0, it is lo-\\ncal apparent midnight.\\nThe ordinary sun-dial marks apparent solar time. As a\\nmatter of fact, apparent solar days are not equal. In in-\\ntervals of time that are really equal the hour-angle of the\\ntrue Sun changes by quantities that are not quite equal.\\nThe reason for this will be fully explained later. Hence\\nour clocks are not made to keep this kind of time.\\nMean Solar Time. A modified kind of solar time is\\ntherefore used, called mean solar time. This is the time\\nkept by ordinary watches and clocks. It is sometimes\\ncalled civil time, because it regulates our civil affairs.\\nMean solar time is measured by the hour-angle of the mean\\nSun, a fictitious body which is imagined to move uniformly\\nin the equator. We have tables that give us the position\\nof this imaginary body at any and every instant, just as cat-\\nalogues of stars give us the right-ascensions of stars. We\\nmay therefore speak of the transit of the mean Sun as if\\nit were a bright shining point in the sky. A mean solar\\nday is the intei val of time between two consecutive transits\\nof the mean Sun over the celestial meridian. Mean noon at\\nany place is the instant when the mean Sun is on the ce-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0114.jp2"}, "113": {"fulltext": "MEAN SOLAR TIME. 91\\nlestial meridian of that place (at (7 in Fig. 48). Twelve\\nhours after local mean noon is local mean midnight. The\\nmean sun is then at D in Fig. 48. The mean solar day is\\ndivided into 24 hours of 60 minutes each.\\nAstronomers begin the mean solar day at noon and count\\nround to 24 hours. It happens to be convenient for them\\nto do so. In ordinary life the civil day is supposed to be-\\ngin at midnight, and is divided into two periods of 12\\nhours each. When the mean Sun is at j9, in Fig.\\n48, it is midnight (12 h of Sunday Monday begins.\\nWhen the mean Sun is at (7, it is mean noon (12 h of Mon-\\nday. When the mean Sun has again reached D it is mid-\\nnight (12 h Tuesday begins, and so on. It is more con-\\nvenient, in ordinary life, to change the date the day at\\nmidnight, when most persons are asleep.\\nEverything that is here said about the measurement of time can be\\nclearly illustrated by the use of a celestial globe. Set the globe to\\ncorrespond to the observer s latitude. The vernal equinox is marked\\non every globe. Place the vernal equinox on the meridian of the ob-\\nserver. It is now sidereal h Rotate the globe slowly to the west.\\nThe hour angle of the vernal equinox measures the sidereal time.\\nTrace the course of the equinox throughout a whole revolution that\\nis, throughout a sidereal day.\\nAgain, suppose the sun to be in north declination 15\u00c2\u00b0, and in R. A. 2 h\\n31 m (its approximate position on May 1 of each year). Find this point\\non the globe (see Fig. 50), and trace the sun s course from rising to\\nsetting, and to rising again that is, throughout 24 h You will see\\nthat the sun rises north of the east point on May 1 and reaches a high\\naltitude at noon for observers in the northern hemisphere of the\\nEarth.\\nAgain, suppose the Sun to be in south declination 15\u00c2\u00b0, and in R.A.\\n14 h 34\u00e2\u0084\u00a2, its approximate position on November 3 of each year (see\\nFig. 52). Find this point on the globe, and trace the Sun s\\ncourse from rising to setting, and to rising again. You will see that\\nthe Sun rises south of the east point on November 3, and that its alti-\\ntude at noon is considerably less in November than in May.\\nThe student should also try to realize all these explana-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0115.jp2"}, "114": {"fulltext": "92 ASTRONOMY.\\ntions regarding time by conceiving the appearances in the\\nsky. On a starlit night he should face southwards and he\\nwill see some star on hi.s celestial meridian. If the right\\nascension of that star is 3 h 24 m 16. 93 9 then, at that instant,\\nthe sidereal time is 3 h 24 m 16.93 s a second later it is 3 h\\n24 m 17.93 s an hoar later still it is 4 h 24 m 17.93% and so\\non. Let him trace out in the sky the position of the ce-\\nlestial equator. The vernal equinox must be west of his\\nmeridian by an arc of 3 h 24 m etc., or of 51\u00c2\u00b0. Let him\\nfix in his mind a point of the equator 51\u00c2\u00b0 west of the me-\\nridian. The vernal equinox is there. In an hour it will\\nbe 15\u00c2\u00b0 farther to the west; in two hours it will be 30\u00c2\u00b0 fur-\\nther, and so on. In 24 hours it will have made the circuit\\nof the sky and have returned to its former place once more.\\nThe same kind of exercises should be gone through with\\nin the daytime, so as to realize the motions of the mean\\nSun. The mean Sun is never very far away from the true\\nSun. At noon the Sun is due south, on the celestial me-\\nridian. At 2 p.m. the hour-angle of the mean Sun is 2 1\\nat 3 p.m. it is 3 h at midnight it is 12 h\\nDefine a sidereal dav. Wbat is the measure of the sidereal time\\nat any instant? When the vernal equinox is on the celestial merid-\\nian of a place, what is the sidereal time at that instant What is the\\nrelation between the sidereal time at any instant and the right ascen-\\nsion of the meridian at that instant Draw a diagram that will show\\nthat relation. If a star whose R.A. is 6 h 41 m is on the celestial merid-\\nian of a place at a certain instant, what is the siderealtime of that\\nplace at that instant If you knew that the R A. of Sirius was 6 h\\n-I l m how could you set the hands of a clock so as to mark the correct\\nsidereal time? What is true solar time? What kind of time is marked\\nby a sun-dial? How is mean solar time measured? Is the mean\\nSun a body that really exists Is there any objection to imagining\\nsuch a body to exist in the sky, and to supposing that it has motions\\nfrom rising and setting like the stars? What is a mean solar day?\\nDefine the instant of mean noon. How many hours in a mean solar\\nday In civil life we divide a mean solar day into groups of\\nhours each. If you have a celestial globe use it so as to illus-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0116.jp2"}, "115": {"fulltext": "TIME. 93\\ntrate what you have learned about different kinds of time. Stand up\\nand imagine yourself out of doors on a starlit night. Point at your\\nzenith (Z). Point out your horizon. Point out the north celestial\\npole (P) (it is at an altitude equal to your latitude). Point out\\nthe celestial equator. Choose some point of the equator to be the\\nvernal equinox V. What is the hour- angle of 7? (Answer It is\\nZP V\u00e2\u0080\u0094 poiut out this angle.) In an hour from now where will Fbe?\\nin two hours in 24 hours Why does V have different positions\\nin the sky at different instants In speaking of sidereal time we\\nrefer everything to F= the vernal equinox. JSow, suppose that in-\\nstead of considering the motions of V }o\\\\x think of the motions of the\\ntrue Sun. Describe those motions as well as you know them, and say\\nwhat the apparent solar time is. Do the same things for the mean\\nSun. Do you now thoroughly understand that the hour-angle of the\\nmean Sun is measured by the motion of the hour-hand of your watch\\nThe hands of your watch point to 4 p.m. What event took place 4\\nhours ago (supposing your watca to be keeping local mean solar\\ntime)", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0117.jp2"}, "116": {"fulltext": "CHAPTER VI.\\nTIME\u00e2\u0080\u0094 L0NG1TITUDE.\\n16. Time Terrestrial Longitudes. We have seen that\\ntime may be reckoned in at least three ways. The natural\\nunit of time is the day.\\nA sidereal day is the time required for the Earth to turn\\nonce on its axis; it is measured by the interval between\\ntwo successive transits of the same star (sidereus is the\\nLatin for a star or a group of stars) over the same celestial\\nmeridian.\\nA solar day is the interval of time between two succes-\\nsive transits of the true Sun over the same celestial merid-\\nian. It is longer than a sidereal day, because the Sun ap-\\npears to be constantly moving eastwards among the stars\\n(as we shall soon see), so that if the Sun has the same\\nright-ascension as the star Sirius on Monday noon, by\\n^o\\nEast West\\nMonday Tuesday\\nTuesday noon it will have moved about a degree to the\\neast of Sirius. Therefore Sirius will come to the celes-\\ntial meridian on Tuesday a little earlier than the Sun, and\\nhence the solar day will be a little longer than the sidereal\\nday. The eastward motion of the true Snn in right-\\nascension is not uniform, so that intervals of time that are\\nreally equal are not measured by equal angnlar motions of\\nthe true Sun. The true Sun moves in the ecliptic not\\nin the celestial equator. Hence a mean Sun has been", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0118.jp2"}, "117": {"fulltext": "TIME. 95\\ninvented, as it were. The mean Sun is an imaginary\\npoint like a star moving uniformly along the celestial\\nequator so as to make one complete circuit of the heavens in\\na year.\\nA mean solar day is the interval of time between two\\nsuccessive transits of the mean Sun over the same celestial\\nmeridian. As the mean Sun moves eastwards among the\\nstars, a mean solar day is longer than a sidereal day. The\\nexact relation is:\\n1 sidereal day 0.997 mean solar day,\\n24 sidereal hours 23 h 56 4 s 091 mean solar time,\\n1 mean solar day 1.003 sidereal days,\\n24 mean solar hours 24 h 3 m 56 s 555 sidereal time,\\nand\\n366.24222 sidereal days 365.24222 mean solar days.\\nLocal Time. When the mean Sun is on the celestial\\nmeridian of any place, as Boston, it is mean noon at Bos-\\nton. When the mean Sun is on the celestial meridian of\\nSt. Louis, it is mean noon at St. Louis. St. Louis being\\nwest of Boston, and the Earth rotating from west to east,\\nthe local noon of Boston occurs earlier than the local noon\\nat St. Louis. The local sidereal time at Boston at any\\ngiven instant is expressed by a larger number than the local\\nsidereal time of St. Louis at that instant.\\nThe sidereal time of mean noon can be calculated before-\\nhand (as we shall see) and is given in the astronomical\\nephemeris (the Nautical Almanac, so called) for every day\\nof the year. We can thus determine the local mean solar\\ntime when we know the sidereal time. In what precedes\\nwe have shown (page 84) how to set and regulate a sidereal\\nclock. A mean-solar clock can be regulated by comparing\\nit with a sidereal time-piece as well as by direct observa-\\ntion of the Sun. After the student understands the con-\\nstruction and use of astronomical instruments we shall re-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0119.jp2"}, "118": {"fulltext": "96 ASTRONOMY.\\nturn to this matter of time and show exactly how the mean\\nsolar time of oar clocks and watches is determined.\\nTerrestrial Longitudes. Owing to the rotation of the\\nEarth, there is no such fixed correspondence between merid-\\nians on the Earth and meridians on the celestial sphere as\\nthere is between latitude on the Earth and declination in\\nthe heavens. The observer can always determine his lati-\\ntude by finding the declination of his zenith, but he can-\\nnot find his longitude from the right-ascension of his\\nzenith with the same facility, because that right-ascension\\nis constantly changing.\\nConsider the plane of the meridian of a place extended\\nout to the celestial sphere so as to mark out on the latter\\nthe celestial meridian of the place. Take two such places,\\nWashington and San Francisco, for example; then there\\nwill be two such celestial meridians cutting the celestial\\nsphere so as to make an angle of about forty-five degrees\\nwith each other in this case.\\nLet the observer imagine himself at San Francisco. His\\ncelestial meridian is over his head, at rest with reference to\\nhim, though it is moving among the stars. Let him con-\\nceive the meridian of Washington to be visible on the\\ncelestial sphere, and to extend from the pole over toward\\nhis southeast horizon so as to pass about forty-five degrees\\neast of his own meridian. It would appear to him to be at\\nrest, although really both his own meridian and that of Wash-\\nington are moving in consequence of the Earth s rotation.\\nThe stars in their courses will first pass the meridian of\\nWashington, and about three hours later they will pass his\\nown meridian. Now it is evident that if he can determine\\nthe interval which a star requires to pass from the merid-\\nian of Washington to that of his own place, he will at\\nonce have the difference of longitude of the two places by\\nturning the interval of time into degrees, at the rate of 15\u00c2\u00b0\\nto each hour.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0120.jp2"}, "119": {"fulltext": "LONGITUDE.\\n97\\nThe difference of longitude between any two places depends upon\\nthe angular distance of the terrestrial (or celestial) meridians of\\nthese two places, and not upon the motion of the star or sun which\\nis used to determine this angular difference, and hence the longitude\\nof a place is the same whether expressed as the difference of two\\nsidereal or of two solar times. The longitude of Washington west\\nfrom Greenwich is 5 h 8 ra or 77\u00c2\u00b0, and this is in fact the ratio of the\\nangular distance of the meridian of Washington from that of Green-\\nwich, to 24 hours or 360\u00c2\u00b0. The angle between the two meridians is\\n7 g 7 o of 24 hours, or of a whole circumference.\\nFm. 53.\\n-Relation between Terrestrial Meridians and\\nCelestial Meridians.\\nEvery observer on the earth has a terrestrial meridian on which he\\nstands and a celestial meridian over his head. The latter passes through\\nthe celestial poles and the observer s zenith.\\nThe difference of longitude of any two places on the Earth\\nis measured ly the difference of their simultaneous local\\ntimes", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0121.jp2"}, "120": {"fulltext": "98 ASTRONOMY.\\nIf two stations on the Earth (say Greenwich and Wash-\\nington) have sidereal time-pieces set and regulated properly\\nto the two local times, we shall know the difference of\\nlongitude of the two places as soon as we can compare the\\ntwo time-pieces. The dials will differ by the difference of\\nlongitude.\\nOne way to determine the longitude is actually to carry\\nthe Washington time-piece over to Greenwich and to com-\\npare its dial with that of the Greenwich time-piece. When\\nthe Greenwich time-piece marks 5 h 8 m p.m. the Washing-\\nton time-piece will mark h (noon). We cannot transport\\npendulum clocks by sea and keep them running, so that\\nthe Washington time-piece referred to must be a chro-\\nnometer, which is nothing but a large and perfect watch\\nkept going by the motive power of a coiled spring.\\nA much better way of comparing the two time-pieces is\\nto send the beats of a clock by telegraph from one station\\nto the other. It is possible to arrange things so that an\\nobserver at Greenwich can make a signal that can be ob-\\nserved at Washington. If Greenwich sends a signal at\\n5 h 8 m p.m., Washington will note the face of the standard\\nclock when it is received, and the Washington local time\\nwill be h (noon). A Greenwich signal sent at 6 h 8 m local\\nGreenwich time, will be received at Washington at l h and\\nso on. This is the theory of the method now universally\\nemployed for exact determinations of longitude. It was\\nfirst employed by our Coast and Geodetic Survey between\\nBaltimore and Washington in 1844, and it was called the\\nAmerican method.\\nIt is of vital importance to seamen to be able to deter-\\nmine the longitude of their vessels. The voyage between\\nLiverpool and New York is made weekly by scores of\\nsteamers, and the safety of the voyage depends upon the\\ncertainty with which the captain can mark the longitude\\nand latitude of his vessel upon the chart.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0122.jp2"}, "121": {"fulltext": "LONGITUDES AT SEA. 99\\nThe method used by a sailor is this: with a sextant (see\\nChapter VII) the local time of the ship s position is de-\\ntermined by an observation of the Sun. That is, on a\\ngiven day he can set his watch so that its hands point to\\ntwelve at local mean noon. He carries on his ship a\\nchronometer which is regulated to Greenwich mean time.\\nIts hands always point to the Greenwich hour, minute, and\\nsecond. Suppose that when the ship s time is h (noon)\\nthe Greenwich time is 3 h 20 m The ship is west of Green-\\nwich 3 h 20 m 50\u00c2\u00b0. The difference of simultaneous local\\ntimes measures the difference of longitude. Hence the\\nship is somewhere on the terrestrial meridian of 50\u00c2\u00b0 west of\\nGreenwich. If the altitude of the pole-star is measured,\\nthe latitude of the ship is also known. Suppose the alti-\\ntude of the pole-star above the horizon to be 45\u00c2\u00b0. The\\nship is then in the regular track of vessels bound for Liver-\\npool. Observations like this are made every day.\\nWhen the steamer Faraday was laying the direct cable from\\nEurope to America she obtained her longitude every day by compar-\\ning her ship s time (found by observation on board) with the Green-\\nwich time telegraphed along the cable and received at the end of it\\nwhich she had on her deck.\\nFrom the National Observatory at Washington the beats of a clock\\nare sent out by telegraph along the lines of railway every day at\\nWashington noon at every railway station and telegraph office the\\ntelegraph sounder beats the seconds of the Washington clock. Any\\none who can set his watch to the local time of his station (by making\\nobservations of the sun at his own station), and who can compare it\\nwith the signals of the Washington clock, can determine for himself\\nthe difference of the simultaneous local times of Washington and of\\nhis station, and thus his own longitude east or west from Wash-\\nington.\\nStandard Time in the United States. In a country of\\nsmall area, it is practicable to use the local time of its cap-\\nital city all over the country. Greenwich time (nearly the\\nsame as London time) is the standard time of the whole of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0123.jp2"}, "122": {"fulltext": "100 ASTRONOMY.\\nEngland. The case is not quite the same in a country of\\nwide extent in longitude. San Francisco is about 3 h west\\nof Washington, and it would be inconvenient to use Wash-\\nington local time in San Francisco.\\nThe matter was regulated in 1883 by the railways of the\\nUnited States and Canada, which adopted the system now\\nin use. By this system the continent was divided into\\nfour sections, each 15\u00c2\u00b0 (one hour) of longitude in width\\n(from east to west). Each section extended south from\\nthe Arctic Ocean to Central America and the Gulf. In\\neach section a central meridian was chosen, and the local\\ntime of that meridian was taken for the standard time of\\nall the cities and towns of that section. The meridians\\nchosen as central were:\\nI. The meridian of 75\u00c2\u00b0 W. from Greenwich (it passes\\nwest of Albany and east of Philadelphia).\\nII. The meridian of 90\u00c2\u00b0 W, from Greenwich (it passes\\neast of St. Louis and nearly through New Orleans).\\nIII. The meridian of 105\u00c2\u00b0 W. from Greenwich (it passes\\na little to the west of Denver).\\nIV. The meridian of 120\u00c2\u00b0 W. from Greenwich (it passes\\na little west of Virginia City and of Santa Barbara).\\nThe local time of the 75th meridian was called Eastern Time\\n90th Central Time;\\n105th Mountain Time\\n120th Pacific Time.\\nGreenwich time is 5 hours later than Eastern time\\n6 Central time\\n7 Mountain time\\n8 Pacific time.\\nEastern time is used throughout the New England States, Pennsyl-\\nvania, New Jersey, Delaware, the Virginias, and in the greater por-\\ntion of the Carolinas east of the Blue Ridge.\\nCentral time is used in Florida and Georgia and in the Central\\nStates, including Texas, most of Kansas and Nebraska, and in the\\neastern half of the two Dakotas,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0124.jp2"}, "123": {"fulltext": "STANDARD TIME. 101\\nMountain time is used in the group of States about the Rocky\\nMountains, including most of Arizona, Utah, Idaho, and Montana.\\nPacific time is used in the Pacific States.\\nThroughout the United States and Canada every watch\\nand clock running on standard time should show the same\\nminute and second. The hour hands alone should differ.\\nStandard time is Greenwich time, so far as the minutes\\nand seconds are concerned, with an arbitrary change of\\nwhole hoars in the different sections. All time-pieces in\\nEngland show Greenwich time. The chronometers of most\\nships on the Atlantic run on Greenwich time. All time-\\npieces in the United States run on Greenwich time so far\\nas the minutes and seconds are concerned the only differ-\\nence is a difference in the whole hour. The chronometers\\nof most ships in the Pacific Ocean run on Greenwich time,\\nwith no change in the hour.\\nThe standard time of the Hawaiian Islands will probably be that\\nof the 150th meridian west of Greenwich (10 hours slower than\\nGreenwich time); that of the Philippine Islands will probably be the\\nlocal time of the 120th meridian east of Greenwich (8 hours faster\\nthan Greenwich time). Cape Colony (Cape of Good Hope) time is\\nl h 30 m fast of Greenwich time, and Natal time is 2 h fast The time\\nof West Australia is 8 h of Japan and South Australia 9\\\\ of Victoria\\nand Queensland 10 h and of New Zealand ll h 30 m fast of Greenwich\\ntime. On the Continent of Europe, Belgium and Holland use Green-\\nwich time unchanged, while Norway, Sweden, Denmark, Austria,\\nand Italy employ a standard time l h fast of Greenwich time. France\\nstill holds to the meridian of Paris as standard, and French time is\\n9 m 21 s faster than Greenwich time. The system of standard time is\\nso convenient that it will eventually be extended to all civilized\\ncountries, in all likelihood.\\nChange of the Day to an Observer travelling round\\nthe Earth. Suppose an observer to be at Greenwich.\\nWhen the mean Sun crosses his celestial meridian it is\\nnoon. Let us say it is Monday noon. When the mean\\nSun next crosses his celestial meridian it is Tuesday noon,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0125.jp2"}, "124": {"fulltext": "102 ASTRONOMY.\\nand so on. Whenever the mean Sun crosses the meridian\\nof any observer anywhere on the Earth it is noon for him.\\nIf he is east of Greenwich the San crosses his celestial\\nmeridian before it reaches the Greenwich meridian, and his\\ntime is later than the Greenwich time. If he is west of\\nGreenwich the Sun does not cross his celestial meridian\\nuntil after it has crossed that of Greenwich, and the Green-\\nwich time is later.\\nSuppose a traveller to set out from Greenwich carrying a watcli\\nwith him that shows not only the Greenwich hour and minute, but\\nalso the day. It would be easy to have a watch made with a day-\\nhand that went forward one number (of days) every time the hour-\\nhand marked another 24 hours elapsed. Suppose this observer to\\ncarry a card also, on which he makes a mark, thus every time the\\nSun crosses his celestial meridian. He makes a mark for every one\\nof his noons. Suppose him to travel eastwards round the globe.\\nWhen he comes to Sicily (15\u00c2\u00b0 1 hour of longitude east of Green-\\nwich) the local time will be 1 P.M. of Monday, when his watch shows\\nnoon of Monday. At Alexandria in Egypt (30\u00c2\u00b0 2 hours of longi-\\ntude east of Greenwich) the local time will be 2 p.m. when his watch\\nshows noon, and the day will be the same as the Greenwich day.\\nIf he goes to the Fiji Islands (180\u00c2\u00b0 12 hours of longitude east of\\nGreenwich) he will find the date later there than the date he carries\\nwith him in his watch. The local time at Fiji will be 12 hours later\\nthan his. It will be Monday midnight (and thus the beginning of\\nTuesday) when his watch marks Monday noon. This is natural\\nenough. He is travelling eastwards and the Sun crosses these east-\\nern meridians before it crosses that of Greenwich. When he reaches\\nSt. Louis (270\u00c2\u00b0 18 hours of longitude east of Greenwich) the date\\nthere would be, on the same principle, 18 hours later than the Green-\\nwich date. When his watch marks Monday noon the people there\\nmight call the time 18 hours later that is, Tuesday 6 a.m. (12 h (noon)\\n18 h 30 h and 30 h 24 h 6 h But in fact they call the day Mon-\\nday instead of Tuesday, though they call the hour corresponding to\\nGreenwich noon 6 a.m. Instead of reckoning their time to be 18\\nhours more (later) than Greenwich time, they reckon it to be 6 hours\\nless (earlier). The 18 hours more that they fail to count at all and\\nthe 6 hours less make up 24 hours 1 day. The traveller has thus\\ngained a day on his journey.\\nWhen he finally arrives at Greenwich again his watch agrees", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0126.jp2"}, "125": {"fulltext": "CHANGE OF THE DAY. 103\\nwith the Greenwich reckoning as to hours and minutes. The day-\\nhand of the watch shows that he has been away for 100 days (let us\\nsay), but his card shows 101 marks on it. The Sun has somehow\\npassed his celestial meridian once more than the number of days\\nelapsed. To make the name of his day agree with the name of the\\nday used in Hawaii, the United States, and England he has to drop\\none day. How is it that he has gained a whole day in travelling\\neastwards round the Earth?\\nWhen the Sun crosses the celestial meridian of an observer it is\\nnoon for him. If the observer stays at one spot on the Earth the\\nEarth itself, in turning on its axis eastwardly, brings his celestial\\nmeridian to and past the Sun daily. If the observer travels round\\nthe Earth towards the east to meet the Sun his own travels will move\\nhis celestial meridian eastward a little every day. The Sun will pass\\nhis meridian 101 times if he has himself gone round the Earth in 100\\ndays. One hundred of the transits of the Sun will be due to the\\nrotation of the Earth on its axis. One of them will be due to his own\\ncircumnavigation of the globe.\\nIf instead of going eastwards the observer (with his watch and his\\ncard) should travel westwards round the globe he would find the\\nlocal time at Washington five hours less (earlier) than the Greenwich\\ntime. At St. Louis the local time would be six hours less (earlier).\\nAt San Francisco it would be eight hours less (earlier). When his\\nwatch marks Greenwich noon of Monday the people of San Francisco\\nwill call the date 4 a.m. of Monday eight hours less (earlier) than\\nGreenwich.\\nWhen he reaches India or Germany he will find his Monday is not\\ncalled Monday but Tuesday. When he returns to Greenwich he will\\nfind that his reckoning agrees with the Greenwich reckoning in every\\nrespect but one. His watch will show the Greenwich hour and minute\\nexactly. His watch shows that he has been absent for 100 days, let\\nus say. But his card shows that he has had only 99 noons. In going\\nround the world to the westward, away from the Sun, he has lost one\\nwhole day. If he had remained in Greenwich the Earth s rotation\\nwould have brought his celestial meridian to the Sun and past it 100\\ntimes. But in his journey westward he has carried his celestial\\nmeridian with him and moved it away from the Sun. The Earth has\\nturned round 100 times during his absence, but the Sun has only\\ncrossed his (travelling) meridian 99 times. Thus he has lost a day\\nby travelling completely round the Earth westwards away from\\nsunrise. If he had travelled towards sunrise eastwards he would\\nhave gained a day, as we have just seen.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0127.jp2"}, "126": {"fulltext": "104 ASTRONOMT.\\nThe Earth turns round just 100 times in a certain inter-\\nval of time, and there is never any trouble in keeping the\\naccount. Those persons who stay in one place (as at\\nGreenwich) have simply to count the number of transits of\\nthe Sun over their celestial meridian. Those persons who\\ntravel westwards must add a day when they cross the\\nmeridian of Fiji (180\u00c2\u00b0 from Greenwich). Those persons\\nwho travel eashvards must subtract a day at this meridian,\\nwhich is called the international date-line (meaning change-\\nof-date line).\\nWhen Alaska was transferred from Russia to the United\\nStates it was found that one day had to be dropped. The\\nRussian settlers had brought their Asiatic date with them,\\nwhile we were using a reckoning less by one day because\\nour count was brought from Europe.\\nShips in the Pacific Ocean passing the meridian of 180\u00c2\u00b0\\nadd a day going westivards and subtract a day going east-\\nwards.\\nIt is to be noted that the place where the change of date is made\\ndepends upon civil convenience and not upon astronomical necessity.\\nThe traveller must necessarily change his date somewhere on his\\njourney round the world. It is convenient for trade that two adja-\\ncent countries should have the same day-names; so that the date-line\\nin actual use deflects slightly from the 180th meridian. All Asia is\\nto the west of this line all America, including the Aleutian Islands,\\nis east of it. Samoa is east of it, but the Tonga group and Chatham\\nIsland are west of it.\\nDefine a sidereal day, a so^ar day, a mean-solar day. Which\\nof the three is the shorter Why is a sidereal day shorter than a\\nmean solar day What is local time What measures the difference\\nof longitude between two places on the Earth Describe how to de-\\ntermine the difference of longitude between Boston and San Fran-\\ncisco* by the transportation of chronometers by the comparison of\\nclocks by telegraph. How does a sailor determine his longitude from\\nGreenwich at sea Give an account of standard time as employed in\\nthe United States. Into how many sections is the country divided\\nName the four kinds of time employed. Four watches keeping the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0128.jp2"}, "127": {"fulltext": "LATITUDE.\\n105\\nstandard time of San Francisco, Denver, St. Louis, and Philadel-\\nphia are laid side by side How will their standard times differ?\\nHow will their minutes and seconds compare with Greenwich time\\nWhat time is used by most ships Change of the Day. When is it\\nnoon to any observer? If the observer is E. of Greenwich does his\\nnoon occur earlier or later than the noon of Greenwich? Explain\\nwhy it is that an observer travelling completely round the Earth to\\nthe eastwards towards sunrise gains a day and why an observer\\ntravelling completely round the Earth westwards away from sunrise\\nloses a day.\\n17. Methods of Determining the Latitude of a\\nPlace ok the Earth. Latitude from Circumpolar\\nStars. In the figure suppose Z to be the zenith of the\\nobserver, HZRN his meridian, P the north pole, HR his\\nhorizon. Suppose S and S to be the two points where\\na circumpolar star crosses the meridian, as it moves around\\nFig. 54.\\nThe latitude of a place on the earth can be determined by measuring\\nthe zenith distances of a circumpolar star at its two culminations.\\nthe pole in its apparent diurnal orbit. PS PS in the\\nstar s north-polar-distance, and PH p the latitude\\nof the observer.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0129.jp2"}, "128": {"fulltext": "106\\nASTRONOMY.\\nTherefore\\nZS+ZS\\n2\\n90\u00c2\u00b0\\nZP 90\u00c2\u00b0 0.\\nZS+ ZS\\ni 2\\nZS and ZS can be measured by the sextant or by the\\nmeridian-circle, as will be explained in the next chapter.\\nGranted that these arcs can be measured, it is plain that\\nthe latitude of a place is known as soon as they are known.\\nLatitude by the Meridian Altitude of the Sun or of a\\nStar. In the figure Z is the observer s zenith, P the pole,\\nHH the horizon, PZH the\\nobserver s meridian, Q a\\npoint of the celestial equa-\\ntor. The star S is on the\\nmeridian (and just at its\\ngreatest altitude at that in-\\nstant). Its altitude HS can\\nbe measured by one of the\\ninstruments described in the\\nZS is there-\\n1 fore known, for ZS 90\u00c2\u00b0\\nFig. 55.\\nThe latitude of a place on the earth Hex t chapter.\\nv of a ship at sea) can be determined\\nby measuring the meridian altitude\\nof the sun (or of a star). HS.\\ntion of the Sun (or of a star), and\\nNautical Almanac.\\nZS QS QZ the declination of the observer s zenith,\\nor\\nQS is the declina-\\nQS is given in the\\nQ d the latitude of the observer.\\nIf the star (or Sun) S culminates north of the zenith\\nQS ZS QZ,\\nat/S\\nor\\n6 C 0.\\nThis is the method uniformly used at sea, where the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0130.jp2"}, "129": {"fulltext": "PARALLAX.\\n107\\nmeridian altitude of the Son is measured every day with\\nthe sextant. The meridian altitudes of stars are often\\nmeasured at sea, by night, to determine the latitude.\\nExplain how to determine the latitude of a place on the Earth\\nby measuring the zenith distances of a circumpolar star at its upper\\nand at its lower culmination. Draw a diagram to illustrate the\\nmethod. Explain how to determine the latitude of a place on the\\nEarth by measuring the meridian altitude of the Sun.\\n18. Parallaxes of the Heavenly Bodies. The apparent\\nposition of a body (a planet, for instance) on the celestial\\nsphere remains the same as long as the observer is fixed. If\\nthe observer changes his place and the planet remains in\\nthe same position, the apparent position of the planet will\\nchange. The change in the apparent position of a planet\\ndue to a change in the position of the observer is called the\\nHT\\nc\\nk\\nI\\nFig. 56.\u00e2\u0080\u0094 Parallax.\\nChange in the apparent position of a star due to a change in the place of\\nthe observer.\\nparallax of the planet. To show how this is let CH be\\nthe Earth, C being its centre. S and S are the places\\nof two observers on the surface. Z and Z are their\\nzeniths in the celestial sphere H x P P is a planet. (P is\\ndrawn near to the Earth to save space in the figure. If\\nit were drawn at its proper proportional distance for the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0131.jp2"}, "130": {"fulltext": "108 ASTRONOMY.\\nMoon, which is the nearest celestial body to the Earth\\n(240,000 miles distant), the drawing would show P more\\nthan two feet distant from C.)\\nS will see P in the apparent position P 8 will see\\nP in the apparent position P That is, two different\\nobservers will see the same object in two different appar-\\nent positions. If the observer S moves along the surface\\ndirectly to the apparent position of P on. the celestial\\nsphere will appear to move from P to P This change\\nis due to the parallax of P.\\nIf the observers S and S could go to the centre of the\\nEarth (C) they would both see the planet P in the posi-\\ntion P x\\nAstronomical observations made by observers at points\\non the Earth s surface (as at Greenwich and Washington)\\nare corrected, therefore, by calculation, so as to reduce\\nthem to what they would have been had the observers been\\nsituated at the centre of the Earth, from w r hich point the\\nplanet would be seen always in one position on the celestial\\nsphere.\\nThe student can try an experiment in the classroom that will illus-\\nstrate what parallax is (See Fig. 57). Let him set up a pointer some-\\nwhere in the middle of the room and look at it from a point near the\\nsouth-west corner of the room I. The line joining his eye and the\\npointer will meet the opposite wall in a point 1. One of his class-\\nmates under his direction should mark the point 1. Now let the\\nobserver go to another station, II. He will see .the pointer projected\\nagainst the opposite wall at 2, and this point should be marked also.\\nIf he goes to III the pointer will be seen projected at 3, and so on.\\nThe change in the apparent position of the pointer on the opposite\\nwall due to the change in the observer s place is the parallax of the\\npointer. The real position of the pointer has not changed at all.\\nWhile the observer has moved from I to III the apparent posi-\\ntion of the pointer has moved from 1 to 3. Any one who is making\\na railway journey can find many examples of parallactic changes of\\napparent position by fixing his eye on points in the landscape. They\\nwill appear to move relatively to each other as the observer moves.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0132.jp2"}, "131": {"fulltext": "PARALLAX.\\n109\\nIn Fig. 58 suppose that G represents the Sun, around which the\\nEarth S moves in the nearly circular orbit 8 S H S C is no\\nlonger 4000 miles as in the last example, but it is 93,000,000 miles.\\nSuppose P to be a star. When the Earth is in the position S the\\niN .E ,S.E.\\nN.W,\\n1\\n1\\nIII\\nf\\n2\\nPointer\\nII\\n1\\nI\\n3\\nI\\ns.w.\\nFig 57.\\nTo illustrate the parallax of a body.\\nstar will be projected on the celestial sphere at P when the Earth\\nhas moved to S the star will be projected on the celestial sphere at\\nP While the Earth is moving from S to S the star P will appear\\nto move from P to P It will not really move in space at all, but\\nFig\\nThe Annual Parallax of a Star.\\nits apparent position on the celestial sphere will appear to move be-\\ncause the observer moves. If the observer were at the Sun (C) in-\\nstead of on the Earth (at S he would see the star at Pi if the ob-\\nserver S were at the Sun (C) he, also, would see the star at P x", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0133.jp2"}, "132": {"fulltext": "110 ASTRONOMY.\\nObservations made at different points of the Earth s orbit (at dif-\\nferent times of the year, that is) are reduced, by calculation, to what\\nthey would have been if the observer had made them from the Sun\\ninstead of from the Earth.\\nOne important point should be especially noted here. If the dis-\\ntance of Pfrom C, in the last figure, increases the changes in its posi-\\ntions P P due to changes in the position of the observer (S S etc.)\\nwill be less and less. The student can prove this by drawing the\\nfigure three times, making the small circle and the points S S the\\nsame in each figure. In the first drawing let him make CP 1 inch,\\nin the second make CP 2 inches, in the third make CP 3 inches.\\nThe greater the distance of a body from the observer, the less the\\nchange in the body s apparent position due to a given change in the\\nobserver s place.\\nThe Moon is 240,000 miles away from the Earth. An\\nobserver at Greenwich will see the Moon projected on the\\ncelestial sphere in a place quite different from the Moon s\\nplace as seen from the Cape of Good Hope. Jupiter is\\nover 400,000,000 miles away from the Earth. Observers\\nat Greenwich and at the Cape of Good Hope will see it at\\ndifferent apparent positions on the celestial sphere, but\\nthese positions will not be very far apart. Sirius is over\\n200,000,000,000,000 miles away from the Earth. Observers\\nat Greenwich and at the Cape of Good Hope will see it in\\nthe same position. That is, we have no telescopes that\\nwill measure its exceedingly small change of place. An\\nobserver at Greenwich looking at Sirius in January will\\nsee it in a position on the celestial sphere only a very\\nlittle different from the place in which the same observer\\nwill see it in July. Yet the observer has travelled half\\nround the Earth s orbit meanwhile, and his place in July\\nis about 186,000,000 miles distant from his place in\\nJanuary. (The distance from the Earth to the Sun is\\nabout 93,000,000, and twice that is 186,000,000.) It is\\nclear that if we can measure the amount of displacement\\nof the Moon, of Jupiter, of Sirius, due to a known change\\nin the observer s place, there must be a way to calculate", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0134.jp2"}, "133": {"fulltext": "PARALLAX. Ill\\nhow far off these bodies are to suffer the observed changes\\nin their apparent positions.\\nWhat is the parallax of a star (or of the Sun, or of a planet)?\\nTo what point of the Earth are observations made on its surface re-\\nduced? Why are they so reduced? Describe a simple experiment\\nto illustrate parallactic changes. Is there a change in the apparent\\nposition of stars due to the revolution of the Earth round its orbit\\nDraw a figure to illustrate this. To what point within the Earth s\\norbit are observations reduced to avoid such parallactic changes?\\nProve by three drawings that the further a star is from the observer\\nthe less are its parallactic changes due to a given change in the\\nobserver s place.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0135.jp2"}, "134": {"fulltext": "CHAPTER VII.\\nASTRONOMICAL INSTRUMENTS.\\n19. Astronomical Instruments Telescopes, Celestial\\nPhotography The Nautical Almanac. The instruments\\nof astronomy are telescopes that enable us to see faint\\nstars which otherwise we should not see at all or telescopes\\nand circles combined, that enable us to measure angles;\\nor timepieces (chronometers and clocks) that enable us to\\nmeasure intervals of time with exactness; spectroscopes,\\nthat enable us to analyze the light from a heavenly body\\nand to say what chemical substances it is made of, etc.\\nAll these instruments have been gradually perfected until\\nmost of them are now extremely accurate, but many of\\nthem had very humble beginnings.\\nClocks. The first timepieces were sun-dials,* water-\\nclocks, etc. The ancients noticed that the shadow of an\\nobelisk moved during the day. When the Sun was rising\\nin the east the shadow of an obelisk lay opposite to the\\nSun towards the west. As the Sun rose higher in the sky\\nand moved towards the meridian the shadow moved towards\\nthe north and grew shorter. When the San was exactly\\nsouth of the obelisk (on the meridian due south of the ob-\\nserver and at its greatest altitude) the shadow lay exactly\\nto the north and it was the shortest. As the Sun drew\\ntowards the west the shadow moved towards the east and\\nWe know that a Sun-dial was set up in Rome B.C. 263. Plau-\\ntus speaks of a slave who complained of Sun-dials and the new-\\nfangled hours. In old time, he says, he used to eat when he was\\nhungry now the time when he gets his meals depends on the Sun\\n\\\\n", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0136.jp2"}, "135": {"fulltext": "Fig. 59. Galileo.\\nBorn 1564; died 1642.\\n113", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0137.jp2"}, "136": {"fulltext": "114\\nASTRONOMY.\\ngrew longer; and as the Sun was setting in the west the\\nshadow pointed towards the east. A circle was traced on\\nthe ground round the obelisk\\nand the north point of the circle\\nwas marked. When the shadow\\nfell at this point the Sun was\\ndue south at noon and the day\\nwas half over. This was the\\nfirst timepiece. By dividing\\nthe circle into smaller parts the\\nday was likewise divided into\\nparts. Some of the churches\\nin Italy have sun-dials laid out\\non their floors so that a spot of\\nsunlight admitted through the\\nsouth wall traverses an arc\\ndivided into hours and minutes.\\nThe student should set up a verti-\\ncal pole and trace a circle around it\\nand divide the circle into parts, using\\nhis watch to get the hour marks. The\\ncircular dial of Fig. 60 is horizontal\\nFig. 60.\u00e2\u0080\u0094 A Sun dial. and X II is towards the north.\\nIt was not easy, in ancient times, to mark the places on\\nthe dial that corresponded to the hours and to the smaller\\ndivisions of time. These were often counted by water-\\nclocks or sand-clocks, in which water or sand poured from\\na box through a hole in the bottom. The lowering of the\\nupper surface of the water or sand marked the passage of\\ntime. The common hour-glass is a sand-clock. Candles\\nwere marked by lines at equal intervals and equal intervals\\nof time were counted by the burning of equal lengths of\\nwax. The student can construct timepieces in this way\\nand he can test their accuracy by a watch or clock.\\nGalileo noticed about the year 1600 that a given\\npendulum always made its swings in equal times no matter", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0138.jp2"}, "137": {"fulltext": "ASTRONOMICAL INSTRUMENTS.\\n115\\nwhether it swung through large arcs or small ones. A\\nlong pendulum swung slowly; a short pendulum swung\\nfaster; but each pendulum had its own time of swinging\\nand it always swung in that time. A pendulum about 39^\\ninches long made a swing in one\\nsecond (from its lowest point to its\\nlowest point again in one second).\\nIt made 86,400 vibrations in a\\nmeau solar day.* Intervals of time\\ncould now be accurately divided.\\nThe student should make a pendulum\\nfor himself. A very good method is\\ndescribed in Allen s Laboratory Physics\\nas follows\\nNear 8, which may be the edge of a\\ntable or shelf, is screwed a spool S\\nThe screw is set up until the spool\\nturns with considerable friction. A\\nstring is wound around the spool and is\\nheld in place by passing through the slot\\nof another screw, R, inserted horizontally\\nin the edge of the support. The lower\\nend of the string passes through a hole\\nin a ball B, which forms the pendulum-\\nbob. The length of the pendulum may\\nbe varied by turning the spool so as to Fig^61.\u00e2\u0080\u0094 A Home-made\\nwind or unwind the string. Small\\nadjustments are best made by gently\\nturning the spool.\\nMany improvements have been made in pendulum-clocks\\nsince they were first invented by Huyghens (pronounced\\nhi genz) in 1657, and they are now extraordinarily accu-\\nrate. Chronometers are merely very perfect watches.\\nTheir motive force is a coiled spring, and they can be\\ntransported by sea or land while they are running, which\\nis not true of clocks, of course.\\nPendulum whose\\nLength can be\\nreadily Varied.\\n60 X 60 x 24 86,400,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0139.jp2"}, "138": {"fulltext": "116 ASTRONOMY.\\nCircles. Angles can be measured by circles divided to\\ndegrees, etc. If the arc S S* is so divided and if it has a\\nradial bar ES that can be moved around a pin at the\\ncentre of the circle at E, the angle between any two stars\\ncan be measured in the following way\\n1st. Place the circle so that its\\nplane passes through the two stars\\nS and S* when the eye is at E.\\n2d. Point the bar at S and\\nread the divisions on the circle\\nas 10\u00c2\u00b0 5 for example. The eye\\nwill still be at E, of course.\\n3d. Point the bar at S* and read\\nFig. 62. -Measurement the circle- as 22\u00c2\u00b0 11\\nof Angles by a Circle The angle between the two stars\\nCircle). A PART F A S 2 is 12 6 the difference of\\nthe two readings. In the figure\\nthe angle S ES 2 is about 12\u00c2\u00b0 S *ES 4 is about 22\u00c2\u00b0 S 2 ES 3\\nis about 30\u00c2\u00b0; S ES* is about 64\u00c2\u00b0.\\nBefore the telescope was invented the bar ES was pro-\\nvided with sights like the sights on a rifle. One sight was\\nat E (the place of the eye), the other at the further end of\\nthe bar. The unavoidable error of directing such a bar to\\na star is about 1 of arc, so that the positions of stars before\\nthe telescope was invented were liable to errors of V or so.\\nThe eye cannot detect a change of direction less than about\\none minute of arc. The bar and its sights are nowadays\\nreplaced by a telescope, and the positions of stars deter-\\nmined by such a combination of a circle and a telescope are\\naffected by errors of less than 1 The precision is more\\nthan 60 times greater.\\nThe student will do well to make a half-circle in the following\\nway: Cut a half-circle 8\u00c2\u00a3 inches in diameter out of a piece of thick\\nhard pasteboard, leaving a knob or projection about 1 inch square at\\nC. Through this knob bore a hole with an awl at the exact centre of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0140.jp2"}, "139": {"fulltext": "ASTRONOMICAL INSTRUMENTS. Ill\\nthe circle. Order from Keuffel Esser, opticians, No. 127 Fulton\\nstreet, New York city, a paper circle, 8 inches in diameter, divided\\nto 30 It is No. 1296 of their cata-\\nlogue. It can be sent by mail and\\nwill cost 20 cents. Cut the paper-\\ncircle in two along a diameter and\\nfasten it to the pasteboard, making\\nthe centre of the paper-circle coin-\\ncide with the centre of the paste-\\nboard circle. Make a narrow fiat\\nlight wooden arm for the index-arm, Fig. 63. A Half Circle.\\nlike Fig. 64 A is the centre of the\\ncircle. The arm must revolve about a pin (or a rivet) at A. B and\\nC are the sights. Two common pins will do. D is an index mark, or\\npointer, drawn on the arm. All angles are read from this mark, a,\\nb, c, d, are four divisions of the paper circle. If a 17\u00c2\u00b0, b 18\u00c2\u00b0,\\nc 19\u00c2\u00b0, d 20\u00c2\u00b0, then the reading of the pointer is 18\u00c2\u00a3 degrees. In\\nusing the circle the eye must be at A the observer looks along the\\nA B\\nFig. 64.\u00e2\u0080\u0094 Index Arm for a Divided Circle.\\nsights BC and moves the arm till the sights and the star are in the\\nsame line. To measure the angle between two stars the plane of the\\ncircle must be put in the plane of the eye and of the two stars and\\nkept there. To measure the altitude of the celestial pole (the latitude\\nof the observer) the plane of the circle must be vertical. Two read-\\nings must be made: 1st, when the index arm is horizontal (a level\\nwill show this) and 2d, when the arm points to Polaris. A light\\nplumb line suspended from the centre of the circle will mark the\\nvertical direction, so that zenith distances can be measured.\\nInvention of the Telescope.- The first telescope used in\\nastronomy was invented by Galileo in 1609.* It was like\\na long single-barreled opera-glass. The best of Galileo s\\ntelescopes magnified only about 30 times; but this was\\nEleven years before the Pilgrims landed at Plymouth. Prob-\\nably no one of them had even heard of this invention.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0141.jp2"}, "140": {"fulltext": "118 ASTRONOMY.\\nenough to explain many things that had been mysteries\\nfor two thousand years. The Moon s face was very well\\nshown in Galileo s instruments and the mountains of the\\nMoon were then discovered. The Milky Way was shown\\nto consist of closely crowded stars. If the student will\\nlook at the Moon s face and at the Milky Way with a com-\\nmon opera-glass (which magnifies about 3 times) he will\\nsee far more than with the eye. The true shapes of the\\nplanets Venus and Mercury were made out for the first\\ntime. It was seen that they had phases like the Moon\\n(they were sometimes crescent, sometimes full, etc.), and\\nthis discovery, more than any other, helped to overthrow\\nthe theory of Ptolemy that the Earth was the centre of\\nthe universe, and to establish the theory of Copernicus,\\nthat the centre of our system was the Sum, not the Earth.\\nGalileo discovered four satellites of Jupiter also and\\nshowed, in this way, that the seven planets (Sun,\\nMoon, Mercury, Venus, Mars, Jupiter, Saturn) were\\nseven in number, not because of some mystic law, but\\nsimply because the other bodies of the system happened\\nto be too faint to be seen with the unassisted eye.\\nSeven had been a mystical number since\\nthe times of Pythagoras. There were\\nseven planets, seven days of the week,\\nseven wise men of Greece, seven cardinal\\nvirtues, seven deadly sins, seven notes of\\nmusic in the octave, etc. Men regarded\\nthis number as if it were sacred in itself;\\nand they were not willing to believe their\\nown eyes when more than seven heavenly\\nble- convex bodies were shown to them. The greatest\\nLens of Glass, value of Galileo s discovery was precisely\\nits demonstration that men must accept a scientific fact\\nwhen it is proved; and that Nature was governed by\\nlaws of a different kind from the fanciful analogies of the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0142.jp2"}, "141": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 119\\nimagination. From the time of Galileo men began to\\nthink about Nature in a new way and the discoveries of his\\ntelescope are, for that reason, the most important scientific\\ndiscoveries ever made.\\nConstruction of the Telescope. Long before the time of\\nGalileo glass lenses had been used for spectacles. The\\nEmperor Nero (died a.d. 68) is said to have employed\\nsuch a lens. It was found that a double-convex lens made\\nout of glass not only collected light, but that if it was held\\nin a proper position it magnified the object looked at.\\nThe ordinary hand reading-glass is a familiar example of\\nthis fact.\\nFigure 66 sbows the way in which the reading-glass\\nFig. 66.\\nThe reading-glass C magnifies an object AB to the size ab.\\nmagnifies. AMB is an object viewed by a reading-glass\\nC. From every point of the object AB rays of light issue,\\nand they go in every direction. (The proof of this fact is\\nthat no matter where you stand you can still see AB; and\\nif you see it there must be rays that come from AB and\\nreach your eye.) The bundle of rays that comes from the\\npoint A and falls on the reading-glass C is cAd. No other\\nrays from A fall on the glass. These pass through the\\nglass and come to a focus at a\\\\ a is the image of the point\\nA of the object. The point B of the object is sending out\\nrays in every direction. Some of them fall on the glass", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0143.jp2"}, "142": {"fulltext": "120 ASTRONOMY.\\nnamely the bundle cBd. This bundle of rays passes\\nthrough the glass and comes to a focus at b; b is the\\nimage of the point B of the object. The point M of the\\nobject is giving out rays in every direction. Only those\\nthat fall on the glass can pass through it namely the\\nbundle of rays cMd. This bundle of rays passes through\\nthe glass and comes to a focus at N. N is the image of\\nthe point M of the object. Every point of the object sends\\nout rays, and bundles of rays from every such point pass\\nthrough the glass and each such bundle comes to a focus\\nsomewhere on the line ab and forms an image of the cor-\\nresponding point of the object. All these separate images,\\ntaken together, make one image, a picture, of the object.\\nab is the image, the picture, of AB.\\nNow suppose that with a second hand-glass you should\\nlook at the image ab just as you looked at the object AB\\nwith the first hand-glass. If the second glass is held in a\\nproper position you can magnify the image ab just as the\\nobject AB was originally magnified. A combination of\\nthe two or more lenses to make a magnified image is a tele-\\nscope. Galileo s invention was the use of two lenses in\\ncombination.\\nAll refracting telescopes (telescopes in which rays of light\\nfrom the object are bent refracted by the telescope so as\\nto form an image) consist essentially of two lenses. The\\nfirst lens (that one nearest the star) is made as large as pos-\\nsible so as to collect as much light as possible. All the\\nbundles of rays that fall upon it are bent refracted by\\nthis lens and brought to a focus; and together they make\\nan image a picture of the object. This first lens is\\ncalled the object-lens (or the object-glass). Its sole use is\\nto collect as many rays from the object as possible and to\\nform them into an image a picture at the focus. If\\nyou should hold a piece of ground-glass at the focus of a\\ntelescope you would see a small picture on the glass a pic-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0144.jp2"}, "143": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 121\\nture of the Sun, of the Moon, of a star, according as the\\ntelescope was pointed to the Sun, the Moon, or a star. If\\nyou should put a photographic plate at the focus you could\\nmake a photographic negative of the Sun, the Moon, a\\nstar.\\nThe second lens (it is called the eyepiece) is used to\\nmagnify the image formed by the object-lens. Every tele-\\nscope is provided with several eyepieces. Some of these\\nmagnify more than others. If a powerful eyepiece is used\\nthe telescope may magnify 1000 times. If one of the less\\npowerful is employed it may magnify 100 times. You can\\nchange the magnifying power of a telescope by changing\\nthe eyepiece, therefore; and there is not much point to the\\ncommon qnestion: How much does this telescope mag-\\nnify? The answer is it depends upon what eyepiece\\nyou are using. The tube of a telescope is chiefly for the\\npurpose of keeping the object-glass and the eyepiece at the\\nright distance apart.\\nIt is found that single lenses of glass give imperfect im-\\nages of objects. The images from single lenses are some-\\nwhat distorted and they are bordered with fringes of color.\\nA few experiments witli a common reading-glass will prove\\nthis. Much of the imperfection\\ncan be avoided by making the\\nobject-glasses of telescopes out\\nof two lenses of different kinds\\nof glass close together, as in\\nFig. 67. The light from the\\nstar first falls on a lens of crown- Fig. 67.\u00e2\u0080\u0094 The Achromatic\\nglass and after passing through Object-glass.\\nit falls on a lens of flint-glass. The two lenses act like a\\ncommon convex lens in bringing the rays to a focus to form\\nan achromatic or colorless image. The image from such\\nan object-glass is much more perfect than that formed by\\na single lens. Eyepieces, also, are made of two or more", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0145.jp2"}, "144": {"fulltext": "122 ASTRONOMY.\\nlenses. The telescopes now in use are practically as per-\\nfect as they can be made from the glass we now have.\\nLight-gathering Power of a Telescope. It is not merely\\nby magnifying that the telescope assists vision, bat also by\\nincreasing the quantity of light received from any object\\nfrom a star, for example. When the unaided eye looks at\\nany object, the retina can only receive so many rays as\\nfall upon the pupil of the eye. The eye is itself a little\\ntelescopic lens whose image is received on the sensitive ret-\\nina. By the use of the telescope it is evident that as many\\nrays can be brought to the retina as fall on the entire ob-\\nject-glass. The pupil of the human eye has a diameter of\\nabout one fifth of an inch, and by the use of the telescope\\nit is virtually increased in surface in the ratio of the square\\nof the diameter of the objective to the square of one fifth\\nof an inch that is, in the ratio of the surface of the ob-\\njective to the surface of the pupil of the eye. Thus, with\\na two-inch aperture to our telescope, the number of rays\\ncollected is one hundred times as great as the number col-\\nlected with the naked eye, because\\n(.2) 2 (2) 2 .04 4.0\\n1 100.\\nWith a 5-inch object-glass the ratio is 625 to 1\\n10\\n2,500 to 1\\n15\\n20\\n5,625 to 1\\n10,000 to 1\\n26\\n(i\\n16,900 to 1\\n36\\n32,400 to 1\\nWhen a minute object, like a small star, is viewed, it is\\nnecessary that a certain number of rays should fall on the\\nretina in order that the star may be visible at all. It is\\ntherefore plain that the use of the telescope enables an ob-\\nserver to see much fainter stars than he could detect with\\nthe naked eye, and also to see faint objects much better", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0146.jp2"}, "145": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 123\\nthan by unaided vision alone. Thus, with a 36-inch tele-\\nscope we may see stars so minute that it would require the\\ncollective light of many thousands to be visible to the un-\\naided eye.\\nEeflecting Telescopes.\u00e2\u0080\u0094 One of the essential parts of a refracting\\ntelescope is the object-glass, which brings all the incident rays from\\nan object to one focus, forming there an image of that object. In\\nreflecting telescopes (reflectors) the objective is a mirror of speculum\\nmetal or silvered glass ground to the shape of a paraboloid. Fig.\\n63 shows the action of such a mirror on a bundle of parallel rays,\\nFig. 68. Theory of the Reflecting Telescope.\\nwhich, after impinging on it, are brought by reflection to one focus\\nF. The image formed at this focus can be viewed with an eyepiece,\\nas in the case of the refracting telescope.\\nThe eyepieces used with such a mirror are of the kind already de-\\nscribed. In the figure the eyepiece would have to be placed to the\\nright of the point F, and the observer s head would thus interfere\\nwith the incident light. Various devices have been proposed to rem-\\nedy this inconvenience, of which the most simple is to interpose a\\nsmall plane mirror, which is inclined 45\u00c2\u00b0 to the line AC, just to the\\nleft of F. This mirror will reflect the rays which are moving towards\\nthe focus F (downwards on the page) to another focus outside of the\\nmain beam of rays. At this second focus the eyepiece is placed and\\nthe observer looks into it in a direction perpendicular to A C (up-\\nwards on the page). See Fig. 69.\\nName some of the instruments used in astronomy. Sun-dial.\\nDescribe the motion of the shadow of an obelisk from sunrise to noon,\\nfrom noon to sunset. At what time in the day is the shadow of the\\nobelisk the shortest Prove it by a drawing. At what instant of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0147.jp2"}, "146": {"fulltext": "124 ASTRONOMY.\\nthe day does its shadow point due north? Say how you could make\\na sun-dial with a pole and a common watch. Water-clocks. Tell\\nwhat they were. Pendulums. How can you make a pendulum that\\nswings in a second of time? Divided circles. Say how you could make\\none. Describe how to use it in measuring the angle between two stars\\n(the vertex of the angle is at the eye). Telescopes. When did Galileo\\nconstruct his first telescope? Draw a diagram to show how a com-\\nmon reading-glass forms an image of an object at a focus. Define a\\nFig. 69.\\nThis figure shows the way in which the rays of light move in a reflecting\\ntelescope. They come from a star as a beam of light and cover the whole\\nof the curved mirror at the bottom of the tube (A). This mirror reflects\\nthem towards a focus (like F in the preceding figure). Before the rays\\nreach the focus, they fall on a small flat mirror which turns them at right\\nangles to their former direction and they come to a new focus (G) outside\\nof the telescope-tube. Here the eyepiece is placed.\\ntelescope. Exactly what was Galileo s invention? What is a re-\\nfracting telescope f What is an object-glass an eye-piece What is\\nthe sole purpose of the object-glass Why then is it an advantage\\nto make it as large as may be What is the sole purpose of the eye-\\npiece? What is the answer to the question How much does this\\ntelescope magnify Draw a diagram of a reflecting -telescope.\\n20. The Transit Instrument. The Transit Instrument\\nis used to observe the transits of stars over the celestial\\nmeridian. The times of these transits are noted by the\\nsidereal clock, which is an indispensable adjunct of the\\ntransit instrument. It stands near it so that the dial of\\nthe clock can be seen and so that the beats of the pendu-\\nlum can be heard every second. A skilled observer can\\nestimate the time to the nearest tenth of a second. The\\nfirst transit-instrument was invented in the XVII century.\\nThe transit instrument consists essentially of a telescope TT fast-\\nened to an axis W t right angles to it. The ends of this axis ter-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0148.jp2"}, "147": {"fulltext": "ASTRONOMICAL INSTRUMENTS.\\n125\\nFig. 70. A Transit-instrument.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0149.jp2"}, "148": {"fulltext": "126 ASTRONOMY.\\nminate in accurately cylindrical pivots which rest in metallic bearings\\nVV which are shaped like the letter Y, and hence called the Ys.\\nThe object-glass of the telescope is at the upper end of the tube in\\nthe drawing. The eyepiece is at E. The telescope can be moved\\nso as to point to any point in the celestial meridian to the zenith,\\nthe south point of the horizon, the nadir, the north point, the celes-\\ntial pole.\\nThe Ys are fastened to two pillars of stone, brick, or iron. Two\\ncounterpoises TFTTare connected with the axis as in the plate, so as\\nto take a large portion of the weight of the axis and telescope from\\nthe Ys, and thus to diminish the friction upon them and to render\\nthe rotation about VV more easy and regular. The line VV is\\nplaced accurately level and also perpendicular to the meridian, or in\\nthe east and west line. The plate gives the form of transit used in\\npermanent observatories, and shows the observing chair 0, the re-\\nversing carriage E, and the level L. The arms of the latter have\\nYs, which can be placed over the pivots VV.\\nThe reticle is a network of fine spider-lines placed in the focus of\\nthe objective.\\nIn Fig. 71 the circle represents the field of view of a transit as seen\\nthrough the eyepiece. The seven vertical lines, I, II, III, IV, V,\\nVI, VII, are seven fine spider-lines tightly\\nstretched across a hole in a metal plate,\\nand so adjusted as to be perpendicular to\\nthe direction of a star s apparent diurnal\\nmotion. The horizontal wires, guide-wires,\\na and 6, mark the centre of the field. A\\nstar will move across the field of view\\nparallel to the lines db and will cross the\\nlines I to VII in succession. The field of\\nview is illuminated at night by a lamp\\nFig. 71.\u00e2\u0080\u0094 Spider-lines which causes the field to appear bright.\\nin the Focus of A The wires are dark aga-inst a bright ground.\\nTelescope. Tbe lin sight g ft Une j om j n g the centre\\nof the object-glass and the central one, IV, of the seven vertical\\nwires.\\nThe axis VV is horizontal; it lies east and west. When\\nTT is rotated about FFthe line of sight marks out the\\ncelestial meridian of the place on the sphere.\\nHow the Transit-instrument is Used in Observation.\u00e2\u0080\u0094 It is pointed\\nat the place where a star is about to cross the meridian in its course", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0150.jp2"}, "149": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 127\\nfrom rising to setting. As soon as the star enters the field the\\ntelescope is slightly moved so that the star will cross between the\\nlines a and b. As the star crosses each spider-line, I to VII, the\\nexact time of its transit over each line is noted. The average of\\nthese seven times gives the time the star crossed the middle line IV.\\n(Seven observations are better than one, and this is why seven lines\\nare used.) Let us call this time T. It will be a number giving\\nhours, minutes, seconds and fractions of seconds, as 10 h 25 m\\n37 s 22 for example. T is then the time by the sidereal clock when the\\nstar was on the meridian. When a star is on the celestial meridian\\nof a place the sidereal time is equal to the right-ascension of the star.\\n(See page 88.) Suppose the right-ascension of the star that we\\nhave observed to be known and to be R. A. 10 h 25 m 36 s 18.\\nThis number is the sidereal time at the instant of the transit of the\\nstar. But the clock time was 10 h 25 m 37 s 22. Hence the clock is\\ntoo fast by l 8 04.\\nBy observing the time (T) when a star of Jcnow?i right-\\nascension (E.A.) crosses the meridian we can determine\\nthe correction of the clock. The clock should mark a si-\\ndereal time equal to R.A. It does mark a time T. Hence\\nits correction is R.A. T, because,\\nT (R.A. T) R.A. the sidereal time.\\nIn this way we can set and regulate the sidereal clock, so\\nthat its dial marks the exact sidereal time at any and every\\ninstant. (In practice we do not move the hands but allow\\nfor its errors.) Table V, at the end of the book, gives a\\nlist of the R.A. of a number of stars.\\nNow suppose the sidereal clock to be correct and the\\ntimes of transit T\\\\ T 2 T*, etc., of stars of unknown right-\\nascension to be recorded.\\nThen T x the R.A. of the first star,\\nT second star,\\nT 3 third star, and so on.\\nThe right-ascension of any and every unknown star can\\nbe determined as soon as we have the clock correction. It", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0151.jp2"}, "150": {"fulltext": "128\\nASTRONOMY.\\nFig. 72. A Small Transit-instrument.\\nThe length of the telescope of this instrument is about two feet.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0152.jp2"}, "151": {"fulltext": "ASTRONOMICAL INSTRUMENTS.\\n129\\nis in this way that the transit instrument is employed to\\ndetermine the right ascensions of unknown stars.\\nFig. 73. A Meridian-circle.\\nThe Meridian-circle. The meridian-circle (or transit-\\ncircle) is a combination of the transit-instrument with a", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0153.jp2"}, "152": {"fulltext": "130 ASTRONOMY.\\ncircle (or two circles) fastened to its axis. With the\\ntransit-instrument we can determine the right-ascensioas\\nof stars; with the circle we can measure their declinations.\\nThe picture shows a meridian-circle. Its telescope is\\npointed downwards and the eyepiece is at its upper end.\\nThe instrument differs from the transit in having two\\nfinely divided circles. Each of these circles is read by four\\nlong horizontal microscopes. The axis of the instrument\\nis made level by a hanging-level which is shown in the cut.\\nThe level is, of course, removed when observations of stars\\nare made. Meridian-circles were first made in the XIX\\ncentury.\\nSuch an instrnment can be used as a transit-instrnment\\nprecisely as has been described. Its circle can be used to\\ndetermine the declinations of stars.\\nThe telescope is moved (so as to trace out the meridian)\\nby turning the horizontal axis VV, JVJV, in Fig. 70). As\\nthe axis turns the circles turn with it. The angle through\\nwhich they turn can be determined by noticing how many\\ndegrees, minutes, and seconds, have turned past\\nthe microscopes. In the same room with the meridian-\\ncircle and a few feet south of it there is a small horizontal\\ntelescope. It has a level which rests on top of it, and it\\ncan be made exactly horizontal. If we point the telescope\\nof the meridian-circle at the small horizontal telescope (see\\nthe diagram) the meridian-circle telescope will be horizon-\\nObserver s Telescope of the meridian- Horizontal telescope\\neye. circle pointing south. pointing north.\\nFig. 74. To Determine the Reading of a Meridian-circle\\nWHEN IT IS POINTED HORIZONTALLY.\\ntal when it sees directly down the tube of the horizontal\\ntelescope. The circle must now be read. Suppose its\\nreading in to be H. This reading H is called the\\nhorizontal point. In practice it is more usual to deter-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0154.jp2"}, "153": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 131\\nmine the nadir point instead of the horizontal point H, but\\nit is a little simpler for the student to consider the hori-\\nzontal point as the starting-point.\\nFig. 75.\u00e2\u0080\u0094 Theory of the Meridian- circle.\\nIn the figure HR is the observer s horizon, Z his zenith, PZR his meri-\\ndian, P the pole, E a point of the equator, S and S the two points where a\\ncircumpolar star crosses his meridian.\\nWhen the telescope is pointed south, at R, and is horizontal, the\\ncircle -reading is H. Let us suppose H is equal to 180\u00c2\u00b0 0 0 If the\\ntelescope is pointed to Z the reading will be 90\u00c2\u00b0 0 0 because the\\nzenith is 90\u00c2\u00b0 from the horizon. If the telescope is pointed to the\\npoint H (the north point of the horizon) the reading will be 0\u00c2\u00b0 0 0\\nIf it is pointed to iVthe reading will be 270\u00c2\u00b0 0 0 We need to know\\nthe reading for the polar point P, and for the equator point E.\\nThe star Polaris is not exactly at the North Pole, though it is near\\nit, and so we have no direct way of pointing at the pole. If we\\nknow the latitude of the observer measured by the arc HP, and it is\\n(f then the polar reading P will be p; and the equator reading\\nE will be 90\u00c2\u00b0 p (because the arc PE is 90\u00c2\u00b0).\\nIf we do not know the latitude p we must point the telescope at\\na star S when it is crossing the meridian and determine its zenith dis-\\ntance ZS; and twelve hours later we must again point the telescope\\nat the same star, when it is crossing the meridian again (at S and\\ndetermine the zenith distance ZS Then (as has already been proved\\non page 106),\\nThe latitude of the observer p 90\u00c2\u00b0 gJH~ ZST", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0155.jp2"}, "154": {"fulltext": "132\\nASTRONOMY.\\n0\u00c2\u00b0\\n0\\n37\u00c2\u00b0\\n20\\n24\\n90\u00c2\u00b0\\n0\\n0\\n127\u00c2\u00b0\\n20\\n24\\n180\u00c2\u00b0\\n0\\n0\\n270\u00c2\u00b0\\n0\\nThus, whether the latitude of the observer is known or unknown,\\nwe can determine the reading of the circle when the telescope is\\npointed to any one of the points R, E, Z, P, H.\\nThe latitude of the Lick Observatory is 37\u00c2\u00b0 20 24 p. Its\\nmeridian-circle would then have the following readings:\\nFor the north-point (II m the figure)\\npolar-point (P\\nzenith-point (Z\\nequator-point (E\\nsouth-point (R\\nnadir-point (IV il\\nIf the telescope was pointed to a star 8 as it crossed the meridian,\\nand if the circle reading for 8 was 57\u00c2\u00b0 40 36 the north-polar distance\\nof S would be 20\u00c2\u00b0 20 12 and its declination would be 69\u00c2\u00b0 39 48\\nIts zenith distance north would be 32\u00c2\u00b0 19 24\\nModel of a Meridian circle. The student will do well to make a\\nsimple model of a meridian-circle out of wood. Let him take a piece\\nof wood (planed on all its sides) about a foot long and exactly square,\\nand whittle the ends of it till they are nearly cylindrical. This will\\nserve as the axis. Perpendicular to the axis at its middle point he\\nshould nail on a flat piece of wood, about two feet long, to stand for\\nthe telescope. One end of this last piece should be marked object-\\nglass and the other end eyepiece. One pasteboard circle 8 inches\\nin diameter should be prepared and a paper circle divided to 30\\n(see page 117) should be neatly fastened to this. A square hole\\nshould be cut in the circle, exactly at its centre, and the circle fitted\\nto the axis and fastened securely to it. Two wooden boxes at the\\nright distance apart will serve for piers. On the top of the piers Ys,\\nsawed out of wood, must be fastened to receive the pivots of the\\naxis.\\nFig. 76. Ys of a Meridian circle.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0156.jp2"}, "155": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 133\\nThe line joining the Ys should be east and west. A pointer\\nmust be fastened to the pier, so that it will just touch the divisions of\\nthe circle as they are moved past it. It will be convenient to make\\nthis pointer of rather stiff copper wire bent to the proper shape and\\nfiled to a point at the index end. With a model of this sort the\\nwhole process of observing with the meridian-circle will be very\\nclear.\\nThe telescope of a transit-instrument or of a meridian-\\ncircle can only move in one plane, namely in the plane of\\nthe celestial meridian. As the axis is turned the telescope\\ntraces out the celestial meridian in the sky. Stars can\\nonly be seen with these instruments at the moments when\\nthey are crossing the meridian of the observer. For a\\ncouple of minutes at that time a star is seen moving across\\nthe field of view of the telescope. For the rest of the 24\\nhours (until the next transit) the star cannot be seen.\\nThis arrangement is convenient if the object is to deter-\\nmine the star s position its right-ascension and its decli-\\nnation. It is very inconvenient if we desire to examine the\\nstar (or planet) carefully to determine whether it is a\\ndouble star, whether it is surrounded by a nebula, whether\\nits brightness is changing, and so on. Comets, for ex-\\nample, are very seldom seen far away from the Sun and\\ntherefore are seldom on the meridian during the dark\\nhours. Hence they are not often observable by transit-in-\\nstruments.\\nEquatorial Mountings for Telescopes. For such careful\\nexaminations of the physical appearances of stars and\\ncomets we need to have the telescope mounted on a stand\\nso contrived that we can keep the star in the field of view\\nof the telescope for hours at a time. We wish to be able\\nto point at a star when it is rising in the east and to follow\\nit as long as it is above the horizon, if desirable. A mount-\\ning for a telescope that will permit it to be pointed to any\\nstar above the horizon is called an equatorial mounting.\\nBefore we describe the forms of such mountings that are", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0157.jp2"}, "156": {"fulltext": "134 ASTRONOMY.\\nactually in use let us see if we can make the principles on\\nwhich they must be devised clear.\\nSuppose we had a very large globe like the one shown in\\nFig. 44 Ms. Suppose the observer and the eye piece of the\\ntelescope were in the centre of such a globe and that the\\nobject-glass was set in a hole cut through the surface of the\\nglobe at some point (any point) of the equator. It is clear\\nthat the observer could see any star in the equator so long\\nas it was above the horizon, because he would simply have\\nto turn the globe (and the telescope with it) until it\\npointed to the star and then to move the globe slowly to\\nthe west so as to follow the star as it moved from rising\\ntowards setting. Such a mounting as this would do for a\\nstar in the equator and for no other star; but it would do\\nfor all stars in the equator.\\nIf the object-glass were placed at some point (any\\npoint) in the parallel of 15\u00c2\u00b0 north declination, then all stars\\nin that parallel could be viewed so long as they were above\\nthe horizon by rotating the globe, as before, about its axis\\nthat points to the north pole. The same thing would be\\ntrue for stars on the other parallels of 30\u00c2\u00b0, 45\u00c2\u00b0, 60\u00c2\u00b0. It is\\nplain that the mounting Ave want must have a polar axis\\nlike that of the globe, so that when the telescope is once\\npointed at a star that star can be kept in view from its\\nrising to its setting by simply rotating the polar axis. It is\\nalso plain that the desired mounting must be so contrived\\nthat the telescope can be set to any and every declination.\\nSuch a mounting would be used:\\n1st. By setting the telescope to the declination of the\\nstar we wished to examine:\\n2d. By following that star as long as we pleased by ro-\\ntating the mounting about its polar axis.\\nIf OP in Fig. 77 were the polar axis of the telescope\\nand if the telescope were set on the stars A, B, C, D, in suc-\\ncession, these stars could be followed from rising to setting.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0158.jp2"}, "157": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0159.jp2"}, "158": {"fulltext": "Fig. 78a. The\\n36-inch Refractor of the Lick Observatory of the\\nUniversity of California.", "height": "3582", "width": "2361", "jp2-path": "elementaryastron00hold_0160.jp2"}, "159": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 137\\nThe lines drawn in the different cones A, B, C, D, represent\\ndifferent positions of the telescope. The circles A, B, 0, D,\\nare different parallels of declination. Suppose then that (in\\nthe diagram Figure 78) TTis a telescope mounted on an\\naxis DL so that TT can be revolved about the axis DL so\\nas to point to any declination; and further suppose that\\nDL and TT together can be rotated about the axis SN\\nwhich is pointed to the north pole of the heavens.\\nThe large pictures (Figs. 78a, 80, 81) show a telescope\\nmounted as in the diagram (Fig. 78). The telescope is\\nparallel to the polar axis.\\nIf we moved the upper end of the telescope TT towards\\nthe east to point at another star in another declination\\nsuch a telescope would look as in Fig. 81. If we moved the\\nupper end of the telescope TT towards the south to point\\nat another star such a telescope would look as in Figure\\n78a, where the tube is pointing towards a star south of the\\nzenith, but north of the equator and not very far from the\\nmeridian. In the figure (78a) the polar axis (on top of\\nthe pier) is pointing to the north pole of the heavens. The\\nnorth end of the axis is the highest. The declination axis\\nis fastened to the end of the polar axis, and the telescope\\nis fastened to one end of the declination axis. By taking\\nhold of the eye-end of the telescope it can be pointed to\\nany desired declination whatever it can be made to point\\nsooth (horizontally), to the zenith (vertically), or to the\\npole (as in Fig. 80). After it is pointed to the desired\\ndeclination the polar axis can be rotated in its bearings\\n(about the line NS in figure 78) so that the telescope\\nsees the desired star. The star can be followed from ris-\\ning to setting by slowly rotating the telescope and declina-\\ntion axis (together) towards the west.\\nIf we point such a telescope to a star when it is rising (doing this\\nby rotating the telescope first about its declination axis and then\\nabout the polar axis), we can, by simply rotating the whole apparatus", "height": "3582", "width": "2361", "jp2-path": "elementaryastron00hold_0161.jp2"}, "160": {"fulltext": "138\\nASTRONOMY.\\non the polar axis, cause the telescope to trace out on the celestial\\nsphere the apparent diurnal path which this star will follow from\\nrising to setting. In most telescopes of the sort a driving-clock is\\narranged to turn the telescope round the polar axis at the same rate\\nat which the earth itself turns about its own axis of rotation at the\\nrate at which all stars move from rising to setting. Hence such a\\ntelescope once pointed at a star will continue to point at it so long as\\nthe driving-clock is in operation, thus enabling the astronomer to\\nSOUTH\\nNORTH\\nFig. 79.\\n-A Small Equatorial Telescope Mounted on a\\nPortable Stand.\\nmake an examination or observation of it for as long a time as is re-\\nquired. If we place a photographic plate in the focus of a suitable\\nobjective mounted equatorially we can obtain a long-exposure picture\\nof the star-groups to which it is directed, and so on.\\nThe student should make a model of the essential parts of an\\nequatorial mounting out of wood. The model should have a polar\\naxis NS capable of being turned round the line NS; a declination", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0162.jp2"}, "161": {"fulltext": "ASTRONOMICAL INSTMUMENT8. 139\\nFig. 80. -An Equatorial Telescope Pointed towards the\\nPole.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0163.jp2"}, "162": {"fulltext": "140\\nASTRONOMY.\\nx m:\\nFig. 81.\u00e2\u0080\u0094 An Equatorial Telescope Pointed at a Star in\\nthe North-eastern Region of the Sky.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0164.jp2"}, "163": {"fulltext": "ASTRONOMICAL INSTRUMENTS.\\n141\\naxis DL capable of being turned round the head of the polar axis N;\\na long stick TT to stand for a telescope (mark the object-glass end of\\nit). The whole should be mounted on a box so that NS lies in a\\nnorth and south line, and so that the line NS makes an angle with\\nthe horizon equal to the latitude of the observer. A surveyor s\\ntheodolite becomes an equatorial wlien its horizontal circle is tilted\\nup into the plane of the celestial equator.\\na z\\nI n I\\nFig. 82. The Micrometer.\\nAn apparatus used in connection with a telescope for measuring small\\nangular distances.\\nThe Micrometer. A telescope on an equatorial mount-\\ning is very suitable for long-continued observations, such\\nas the examination of the surface of a planet during the\\ngreater part of a night, but in order to fully utilize it,\\nsome means of measuring must be provided. The equa-\\ntorial cannot be used to measure large arcs with exactness\\nsuch an arc as the difference of declination of two stars\\nseveral degrees apart. When it is provided with a\\nmicrometer it is exactly fitted to measure small distances\\nwith great precision such a distance as that between two\\nstars separated by a few minutes of arc, for example.\\nThe principle of the micrometer is illustrated in figure 82. A\\nmetal box is fitted with two slides b and c and with two accurate\\nscrews A and B. The screw A has a head divided into 100 parts.\\nA hole is cut in each of the slides. A spider-line, n, is stretched\\nacross the hole in the slide moved by the screw A, and a spider-line\\nm is stretched across the hole in the slide moved by the screw B.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0165.jp2"}, "164": {"fulltext": "142\\nASTRONOMY.\\nThe micrometer is fastened to the end of the telescope, at right\\nangles to its axis, so that the lines m and n are in the focus of the\\ntelescope, thus\\nB\\nFig. 83.\\nOP is the object-glass of a telescope whose focus is F; AB is the microm-\\neter.\\nWhen the screw A is moved the spider-line n moves, and the line\\nm moves with the motion of the screw B. The oval hole in Fig. 82\\nrepresents the field of view of the telescope. The observer sees the\\ntwo spider-lines m and n, a fixed spider-line at right angles to them,\\na comb-scale at the bottom of the field and whatever stars the tele-\\nscope is viewing. One complete revolution of the screw A moves the\\nline n from one tooth of the comb-scale to the next tooth and whole\\nrevolutions of A are counted in this way. Fractions of a revolution\\nare counted on the divided head of screw A as its divisions move past\\na fixed index or pointer.\\nSuppose that it is desired to measure the distance between two\\nstars S and Tthat are visible in the field of view. During these\\nmeasures the telescope is driven by the clock so as to follow the stars\\nas they move from rising towards setting.\\nFig. 84.\\nThe micrometer is moved so that its long fixed spider-line passes\\nthrough S and Tthus\\nB-\\nFig. 85.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0166.jp2"}, "165": {"fulltext": "ASTRONOMICAL INSTRUMENTS.\\n143\\nThe lines m and n will appear as in the figure 85. The screw B is\\nthen moved until the line m passes through S and the screw A is\\nmoved till the line n passes through T, thus\\nB-\\nFig.\\nThe reading of the screw A is then taken. Suppose it to be 21\\nwhole revolutions (read on the comb-scale) and f^ of a revolution\\n(read on the divided head the mark 57 being opposite to the index).\\nThe screw A is then moved (B remaining as before) until the line n\\nexactly coincides with the line m, and a second reading of A is\\nmade. Suppose it to be 9 whole revolutions (from the scale) and\\nT (from the index). The distance between the two stars S and\\nT is evidently ST 2K57 9 r .33 12 r .24. If one whole revolu-\\ntion of the screw is known and equal to 11 07 then the distance ST\\n12.24 X 11.07 135 50.\\nWhen the value of one revolution of the screw is known in sec-\\nonds of arc all distances measured in revolutions and parts can be\\nreduced to arc. The value of one revolution in arc is determined\\nonce for all by placing the lines m and n perpendicular to the direc-\\ntion of the diurnal motion of a star and at a known distance say 50\\nrevolutions apart, thus:\\nm\\nS\\nFig. 87.\\nIf the telescope is kept in a fixed position the star, by its diurnal\\nmotion, will move across the field of view in the direction of the\\narrow. The exact times of its transits over n and m are observed.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0167.jp2"}, "166": {"fulltext": "144\\nASTRONOMY.\\nSuppose that it requires 6 m 9. s of sidereal time to pass from the line\\nn to the line m.\\n6 m 9 3 369 553 5 because l 8 15 (see page 84).\\nFifty revolutions of the screw 533 5, therefore, and 1 revolu-\\ntion 11 .07.\\nThe relative position of two stars A and B is not completely de-\\nfined when we know their distance apart and nothing more. We\\nneed to know the angle that the line joining them makes with the\\ncelestial meridian (or with the parallel). To determine this the microm-\\neter is attached to a position-circle, so that the micrometer-box can\\nbe rotated in a plane perpendicular to the axis of the telescope. To\\nmeasure the position-angle of two stars the telescope is kept in a fixed\\nposition and the micrometer-box\\nis turned until one of the stars\\nmoves by its diurnal motion\\nalong the spider-line m. The\\ncircle is then read. Suppose\\nits reading to be 90\u00c2\u00b0. The\\ndirection of the parallel (E~W) is\\nthen 90\u00c2\u00b0 to 270\u00c2\u00b0 of the celestial\\nmeridian (NS) 0\u00c2\u00b0 to 180\u00c2\u00b0. The\\ntelescope is then pointed at A\\nand moved by the driving clock\\nso that A remains at the middle\\nof the field. The micrometer-box\\nis turned until the spider-line m\\npasses through the two stars A\\nand B (see the figure) and the\\ncircle is again read. Suppose\\nFig. 88. Measurement of the the reading to be 46\u00c2\u00b0. This is\\nPosition Angle of Two the measure of the angle NAB\\nStars A and B. _ of tlie ang e t h at t^ i ine j j n _\\ning the two stars makes with the celestial meridian passing through\\nA. When we know the position- angle and the distance of two stars\\nwe know all that can be known about their relative situation.\\nHie diameters of planets can be measured with the micrometer.\\nPhotography. If we put a photographic plate in the focus of the\\ntelescope instead of a micrometer, and if we give the proper ex-\\nposure (the telescope being moved by the driving-clock) we shall\\nhave on the plate a photograph of all the stars in the field.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0168.jp2"}, "167": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 145\\nIf we stop the clock and allow a star to move by its diurnal motion\\nacross part of the field of view it will leave a trail from the\\nFig. 89.\\neast to the west side of the plate. After the plate is developed,\\nwe shall have a map of all the stars and can measure their\\nW E\\nposition-angles one from another, at leisure, and in the daytime.\\nTheir distance apart can be measured in inches and fractions of an\\ninch. The value of one inch on the plate expressed in seconds\\nof arc can be determined once for all by observing transits of a star\\nover two pencil lines ruled on a ground-glass plate in the focus one\\ninch apart. It is clear that a photographic plate will give us first,\\na map of all the stars in the field second, the means of measuring\\ntheir precise relative positions just as measures with the micrometer\\nwill do. One great advantage of the photographic method over\\nvisual measures with the micrometer is that the plate gives a per-\\nmanent record, so that the actual micrometric measurements can be\\nmade at leisure and repeated as often as necessary. Another marked\\nadvantage is that many pairs of stars are photographed at one ex-\\nposure, whereas only one pair can be observed at one time by the\\neye.\\nCelestial Photography. Photographs of the Sun, Moon, Planets,\\nStars, Comets, and Nebulae can be made with telescopes specially con-\\nstructed for photography, and these photographs can be subsequently\\nstudied under microscopes, just as if the object itself were visible.\\nThe intervals of clear sky can be utilized to obtain the photographs,\\nand they can be measured when the sky is cloudy. A great saving\\nof time is thus practicable. A second great advantage of the photo-\\ngraphic plate in Astronomy is that the exposures can be made as\\nlong as desired. Objects can be registered in this way that are too\\nfaint to be seen with the eye using the same telescope. The eye soon", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0169.jp2"}, "168": {"fulltext": "146\\nASTRONOMY.\\nbecomes fatigued with the extreme attention required for astronomi-\\ncal observing. The photographic plate is not subject to fatigue. It\\nhas certain disadvantages that need not be discussed here, and the\\nplate will never supersede the eye. On the other hand, it has\\nalready been of immense importance in Practical Astronomy and is\\ndestined to be employed in many new ways. Some of its applications\\nare mentioned in Part II. of this book.\\nThe Sextant. The sextant is a portable instrument universally\\nFig. 90.\u00e2\u0080\u0094 The Sextant.\\nThe radius of its divided circle is usually from 6 to 10 inches.\\nused by navigators at sea. It was invented by Sm Isaac Newton,\\nand quite independently by Thomas Godfrey, a sea-captain of\\nPhiladelphia. The figure shows its general appearance. Its pur-\\npose is to measure the altitude of a star (or of the Sun), It consists", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0170.jp2"}, "169": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 147\\nessentially of a divided circle of a movable index arm SM which\\ncarries a mirror M (called the index-glass) firmly fastened to it of\\nanother mirror m (called the horizon-glass) fastened to the frame of\\nthe instrument and of a small telescope E. It is held by a handle\\nH. When altitudes are measured, the plane of the instrument is\\nvertical.\\nThe instrument is used daily at sea to measure the altitude of the\\nSun. The chronometer-time at which the altitude is measured is\\nnoted. The method of making the observation is to point the\\ntelescope E at the sea-horizon, which will appear like a horizontal\\nline across its field of view, thus\\nThe rays from the Sun strike the index-glass A (or Mm Fig. 90),\\nand are reflected from it. By moving the index-arm (the glass\\nmoves with it) the reflected rays from the Sun (AB) may be made to\\nFig 91.\u00e2\u0080\u0094 Theory of the Sextant.\\nfail on the horizon-glass B (or m in Fig. 90). When the index-arm\\nhas been moved so that the image of the Sun appears to touch\\nthe horizon the index-arm reads the altitude of the\\nSun on the divided circle.\\nThe altitude of the Sun is also measured daily at apparent noon, that\\nis when the Sun is highest, by every navigator to obtain his latitude.\\nIn the figure Z is an observer on the Earth CP Z Q Z is his\\nzenith, HH his horizon, P is the celestial pole, PZH his celestial\\nmeridian, Q a point of the celestial equator. 5 is the Sun on hi\u00c2\u00a7", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0171.jp2"}, "170": {"fulltext": "148\\nASTRONOMY.\\nFig. 92. The Latitude of an\\nObserver Determined by\\nMeasuring the Meridian\\nAltitude of the Sun.\\nmeridian. The altitude of the Sun HS is measured by the sextant.\\n90\u00c2\u00b0 HS ZS, and ZS is thus a known arc. The declination of the\\nSun at that instant is QS. It is a known quantity because it can be\\ntaken from tables in the Nautical Almanac that have been calculated\\nbeforehand. QZ the declina-\\ntion of the observer s zenith his\\nlatitude ZS QS the sum of\\ntwo known arcs. If the Sun is on\\nthe meridian north of the observ-\\ner s zenith, as it may be in certain\\nlatitudes, QZ the observer s lati-\\ntude QS ZS the Sun s decli-\\nnation (a known quantity) minus\\nthe Sun s zenith distance (which\\nis known as soon as US the alti-\\ntude, has been measured with the\\nsextant). Thus the ship s latitude\\nis determined.\\nThe altitude of the Sun is meas-\\nured daily in the morning (or afternoon, or both) to determine the\\nship s longitude, or rather to determine the local mean time of the\\nship s position. If we know that the local mean time of the ship is\\nTat the instant that the Greenwich time is O the west longitude of\\nthe ship will be O 7 7 (=the difference of the simultaneous local\\ntimes). The Greenwich time is always known, on the ship, from the\\ndial of the Greenwich chronometer that she carries. The local time\\nof the ship is calculated from the triangle ZPA as soon as the Sun s\\naltitude has been measured.\\nIn figure 93 PZS is the celestial sphere, the place of the Earth,\\nZthe zenith of an observer, NS his horizon, E his east point, A the\\nplace of the Sun in the afternoon, BA the Sun s declination, PA the\\nSun s north polar distance, OA the Sun s altitude, ZA the Sun s\\nzenith distance. The angle ZPA is the local apparent solar time\\nbecause it is the hour-angle of the Sun (see page 90). We wish to\\ndetermine ZPA. In the triangle ZPA, PA is known (it is 90\u00c2\u00b0 minus\\nthe Sun s declination which is given in the Nautical Almanac) ZP\\nis known (it is 90\u00c2\u00b0 minus the latitude PN) ZA is known (it is 90\u00c2\u00b0\\nminus the Sun s altitude which has been measured by the sextant).\\nHence every part of the triangle is known and the angle ZPA (ex-\\npressed in hours, minutes and seconds, not in degrees, etc.) corrected\\nfor the difference between mean and apparent solar time gives the\\nlocal time of the ship T. If G is the Greenwich time (from the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0172.jp2"}, "171": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 149\\nGreenwich chronometer) at that instant, the ship s west longitude is\\nO T, a known quantity.\\nThus a little instrument that can be held in the hand enables the\\nnavigator to determine his position on the Earth s surface with con-\\nsiderable accuracy and in a very few minutes. Sextant observations\\nat sea will give the position of a ship to within a mile or so.\\nMake a sketch of the transit instrument and name the impor-\\ntant parts\u00e2\u0080\u0094 the telescope, the axis, the Ys, the piers. When the in-\\nFiu. 93 The Local Time of an Observer Determined by\\nMeasuring the Altitude of the Sun.\\nstrument is revolved, what circle of the celestial sphere does it trace\\nout? If a star of known R.A. A crosses the meridian at a clock\\ntime T, what is the correction of the clock If the sidereal clock is\\ncorrect, and a star of unknown R.A. crosses the meridian at a time\\nB, what is the R.A. of this star?\\nMake a sketch of the important parts of the meridian-circle, and\\nname the parts. How may the horizontal-point H of the circle be\\ndetermined Suppose Hto be known, what is the reading for the\\nzenith-point Z? lor the nadir? Suppose the latitude of the observer\\n0) to be known also, what is the reading for the polar point Pt", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0173.jp2"}, "172": {"fulltext": "150 ASTRONOMY.\\nfor the equator-point E? Describe how a model of a meridian-circle\\ncan be made. For what purpose are transit instruments and merid-\\nian-circles used? Describe the equatorial mounting for telescopes,\\nand say what its advantages are. Draw a diagram of such a mount-\\ning. Explain the construction of a micrometer. How is it used to\\ndetermine the aDgular distance of two stars their position-angle?\\nHow is the value of one revolution of the micrometer determined in\\narc? Explain how a photograph of a group of stars is made. What\\nare some of the advantages of photographic methods of observation\\nWith the sextant the altitude of the Sun (or of a star) can be meas-\\nured. How is the latitude of a ship at sea determined the longitude\\nof the ship\\nThe Nautical Almanac\u00e2\u0080\u0094 The governments of the United States,\\nGreat Britain, France, Germany, and other countries issue annually a\\nNautical Almanac for the use of navigators and others. Copies of\\nthe Nautical Almanac can be purchased through book-dealers. The\\nAlmanac contains\\nTables of the E.A. and Decl. of the Sun, Moon, and Planets for\\nevery day in the year.\\nTables of the K. A. and Decl. of all the brighter stars.\\nTables of all eclipses of the Sun, Moon, and of the satellites of\\nJupiter, as well as many other data of importance to the astronomer\\nand the navigator.\\nTo give the student a better idea of the Nautical Almanac a snail\\nportion of one its pages for the year 1882 is here printed. (See page\\n151.)\\nThe third column shows the R. A. of the Sun s centre at Green-\\nwich mean noon of each day. The fourth column shows the hourly\\nchange of this quantity (9.815 on Feb. 12). At Greenwich hours, on\\nFeb. 12, the sun s R. A. was 21 h 44 m 10 8 .80. Washington is 5 S\\n(5 .13) west of Greenwich. At Washington mean noon, on the 12th,\\nthe Greenwich mean time was 5 h .l3. 9.815 X 5.13 is 50 s 35. This\\nis to be added since the R. A. is increasing. The sun s R. A. at\\nWashington mean noon, on Feb. 12, is therefore 21 h 45 m I s 15. A\\nsimilar process will give the sun s declination for Washington mean\\nnoon. In the same manner, the R. A. and Dec. of the sun for any\\nplace whose longitude is known can be found.\\nThe column Equation of Time gives the quantity to be sub-\\ntracted from the Greenwich mean solar time to obtain the Green-\\nwich apparent solar time (see page 90). Thus, for Feb. 1, the\\nGreenwich mean time of Greenwich mean noon is 0 m 0\\\\ The", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0174.jp2"}, "173": {"fulltext": "AST1WN0M1CAL INSTRUMENTS.\\n151\\ntrue sun crossed the Greenwich meridian (apparent noon) 13 m 51 s 34\\nearlier than this, that is at 23 h 46 m 08 s 66 on the preceding day\\ni.e. Jan. 31. Having the apparent solar-time by observation (see\\npage 148) the mean solar time can be found from this table.\\nAgain, when it was 0 1 m s of Greenwich mean time on Feb. 10,\\nit was 21 h 21 m 50 s 70 of Greenwich local sidereal time (see the last\\nFebruary, 1882\u00e2\u0080\u0094 at Greenwich Mean Noon,\\nDay\\nof\\n5\\no\\na\\ni\\n2\\n3\\nThe Sun\\nS\\nEquation\\nof time\\nto be\\nsubstracted\\nfrom\\nmean\\ntime.\\nU\\nO\\n3\\nSidereal\\ntime\\nor right-\\nthe\\nweek.\\nApparent\\nright-\\nascension.\\nDiff.\\nfori\\nhour.\\nApparent\\ndeclination.\\nDiff.\\nfori\\nhour.\\nascension\\nof\\nmean sun.\\nWed.\\nThur.\\nFri.\\nH. M.\\n21\\n31 4\\n21 8\\ns.\\n13.04\\n16.84\\n19.82\\ns.\\n101.75\\n10.141\\n10.107\\nS 17\\n16\\n16\\n2\\n45\\n27\\n22.4\\n5.4\\n30.9\\n+42.82\\n43.57\\n44.30\\nM. s.\\n13 51 34\\n13 58.58\\n14 5.01\\ns.\\n0.318\\n0.284\\n0.250\\nH. M. s.\\n20 46 21.70\\n20 50 18.26\\n20 54 14.81\\nSat.\\nSun.\\nMou.\\n4\\n5\\n6\\n21 12\\n21 16\\n21 20\\n21.98\\n23.33\\n23.88\\n10.073\\n10.040\\n10.007\\n16\\n15\\n15\\n9\\n51\\n33\\n39.2\\n30.8\\n6.1\\n+44.99\\n45.69\\n46.36\\n14 10.61\\n14 15.41\\n14 19.40\\n0.216\\n0.183\\n0.150\\n20 58 11.37\\n21 2 7.92\\n21 6 4.48\\nTues.\\nWed.\\nThur.\\n8\\n9\\n21 24\\n21 28\\n21 32\\n23.63\\n22.60\\n20.79\\n9.974\\n9.941\\n9.909\\n15\\n14\\n14\\n14\\n55\\n36\\n25.4\\n29.1\\n17.7\\n+47.03\\n47.66\\n48.28\\n14 22.60\\n14 25.01\\n14 26.65\\n0.117\\n0.084\\n0.052\\n21 10 1.03\\n21 13 57.59\\n21 17 54.14\\nFn.\\nSat.\\nSun.\\n10\\n11\\n12\\n21 36\\n21 40\\n21 44\\n18.21\\n14.88\\n10.80\\n9.877\\n9.846\\n9.815\\n14\\n13\\n13\\n16\\n57\\n37\\n51.6\\n11.2\\n16.9\\n48.88\\n49.47\\n50.03\\n14 27.51\\n14 27.63\\n14 26.99\\n0.020\\n0.011\\n0.042\\n21 21 50.70\\n21 25 47.25\\n21 29 43.81\\ncolumn of the table). Having the sidereal time by observation (see\\npage 127), the corresponding mean solar time can be found from this\\ntable.\\nHow to Establish a True North, and South. Line.\u00e2\u0080\u0094 In order to set the\\nhands of a sidereal timepiece correctly we must make them indicate\\nthe hours, minutes, and seconds of any star s right-ascension at the\\ninstant that star is crossing the observer s meridian. In order to\\nmake the timepiece keep sidereal time correctly we must regulate\\nit so that the hands go through 24\u00c2\u00b0 m s in the interval between two\\nsuccessive transits of the same star across the meridian. To make\\nthese observations, we need to know the direction of the meridian,\\nand to mark it permanently.\\nFor students who cannot own a transit instrument it is convenient\\nto mark the meridian by two plumb-lines, A and B, one due north of\\nthe other, thus:", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0175.jp2"}, "174": {"fulltext": "152\\nASTRONOMY.\\n4\\nSouth\\nNorth\\nB\\nFig. 94.\\nThe plumb-lines can be made out of good fishing-line the plumb-\\nbobs out of bits of lead. To prevent them from swinging in the wind\\nit is a good plan to keep the bobs immersed in pails of water. The\\nlines can be suspended from nails driven into walls, trees, etc. The\\nmeridian-line should be marked in a place where a good view of\\nthe whole meridian from north to south can be commanded.\\nThe problem is to place the plumb-lines in a true north and south\\nline. There are several ways of doing this. The following process\\nIMraK\\n3BH\\nBBs\\nM\\ni\\nH\\nMR 1\\nI\\n1\\n\u00c2\u00aba\\ni\u00c2\u00a7\\n1\\n1\\nFig. 95.\u00e2\u0080\u0094 Ursa Majoe.\\nZeta Ursae majoris is the middle star of the handle of the Dipper.\\nis as simple as any. Mark on the ground a line in the direction of the\\nneedle of a common compass. This will be approximately north and\\nsouth. At the north end of this line choose a place for the northern\\nplumb-line A and hang it there. Ten or fifteen feet south of A sus-\\npend the second plumb-bob B from a framework of wood that can\\nbe moved east or west, if necessary. A is always to hang in the\\nplace first chosen. B is to be moved east or west until the right\\nplace is found and then it is to remain there always. The line join-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0176.jp2"}, "175": {"fulltext": "ASTRONOMICAL INSTRUMENTS. 153\\ning A and B (after B is placed correctly) is the meridian line of the\\nobserver.\\nThe plumb-line B is placed correctly when both plumb-lines seem\\nto pass through the two stars Polaris and Zeta Ursce majoris at the\\nsame time.\\nThe right-ascensions of these two stars differ by 12 hours. When\\nPolaris is crossing the meridian from east to west (upper culmina-\\ntion) Ursm majoris is crossing the meridian from west to east (lower\\nculmination). A line joining them at this instant is a\\ncelestial meridian. If we move the plumb-line B until\\nboth plumb-lines A and B pass through both stars then the\\nline joining A and B must be in the plane of the celestial\\nmeridian.\\nThe stars will be approaching their culminations\\nabout 11 P.M. Oct. 20, about 8 P.M. Dec. 5,\\n10 Nov. 5, 7 Dec. 20,\\n9 Nov. 20, 6 Jan. 5,\\nabout 5 P.M. Jan. 20,\\nand these are the hours to prepare to observe them.\\nThe observation consists in moving the support of the\\nplumb-line B (the southern plumb-line) slowly and gently\\neast or west until both stars seem to be on the two plumb-\\nlines at the same time, as in Fig. 96. When they are so\\nlet both plumb-lines rest, and see if the stars stay on the ofi\\ntwo lines for a few minutes. If they do, both lines are\\nin the right position. If they do not, move the southern plumb-\\nline B slightly. After the plumb-line B has been put in the right\\nposition its place must be marked; and the next morning its nail can\\nbe permanently fixed. It will be well to test the meridian-line, so\\ndetermined, by another night s observations. Finally, a meridian-\\nline can be established by this process; and whenever the observer\\nwishes he can observe the transit of any celestial body over the two\\nplumb-lines and note the hour, minute, and second by his sidereal\\ntime piece.* In order to see the plumb-lines in a dark night he\\nshould chalk them well, or paint them white. If this is not enough\\nthey can be illuminated by the light of a lantern placed behind his\\nback (so as not to interfere with his seeing the stars).\\nA cheap watch, regulated to run on sidereal time, is a great con-\\nvenience in making astronomical observations.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0177.jp2"}, "176": {"fulltext": "CHAPTER VIII.\\nAPPARENT MOTION OF THE SUN TO AN OBSERVER ON\\nTHE EARTH\u00e2\u0080\u0094 THE SEASONS.\\n21. Apparent Motion of the Sun to an Observer on\\nthe Earth. Long before the Christian era the ancients\\nknew that there were two classes of bodies to be seen\\nin the sky. The stars the first class rose and set, to\\nbe sure; bat they were always in the same relative posi-\\ntion. They kept their configurations. They were fixed.\\nOne star did not move away from others. The stars of\\nUrsa Major shown in Fig. 1 kept their relative positions\\ntheir grouping century after century. There was another\\nclass of celestial bodies which the ancients called planets or\\nwandering stars. Some of them (Mercury, Venus, Mars,\\nJupiter, Saturn) looked exactly like stars to the naked\\neye, but they mo^ed among the fixed stars, sometimes\\nbeing near to one fixed star, then leaving it and moving\\nnear another star. You can easily observe such motions\\nas these for yourself. Mars or Jupiter moves among the\\nfixed stars with a motion that is quite obvious if you regu-\\nlarly observe its place (and make a sketch of the stars near\\nby). The Moon moves quite rapidly among the stars.\\nThe Sun also moves among the stars, but as the stars\\nare not visible in the daytime, it is necessary to observe\\nthe Sun at sunrise and at sunset in order to prove to\\nyourself that it is moving. The ancients understood this\\nfact very well and they had mapped the path of the Sun\\namong the stars quite accurately. You can do the same\\nthing by observing the Sun at sunrise and sunset each\\n154", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0178.jp2"}, "177": {"fulltext": "APPARENT MOTION OF TEE SUN. 155\\nday and by marking down on a celestial globe, every day,\\nthe position of the Sun. If you continued this process for\\na year you would find that the Sun had apparently made a\\ncomplete circuit of the heavens.\\nIf the Sun were near to a bright star on Jan. 1 (so that\\nthe Sun and the star rose and set at the same time) you\\nwould see that the San moved eastwards so as to set later\\nthan the star on Jan. 2. It would set still later than the\\nstar on Jan. 3, and so on. In July it would set about\\n12 hours later than the star. In half a year the Sun has\\nmoved away from the star by half the circuit of the\\nheavens. In the next January the Sun would be near the\\nsame star again so as to set at the same time with it. The\\nSun then has, in the year, made a complete circuit of the\\nheavens. The ancients proved this and you can prove it\\nfor yourself if you will give a year to the demonstration.\\nThe year is measured by the time required for the Sun to\\nmake this circuit.\\nThe explanation of the apparent motion of the Sun is to\\nbe found in the real motion of the Earth. The Earth\\nmoves round the Sun in a nearly circular orbit (path) and\\ncompletes one revolution in about 365^ days, one year.\\nIn Fig. 97 let represent the Sun, ABGD the orbit of\\nthe Earth around it, and EFGH the sphere of the fixed\\nstars. This sphere is infinitely larger than the circle\\nABCD. Suppose now that 1, 2, 3, 4, 5, 6 are a number\\nof consecutive positions of the Earth in its orbit. A line\\nIS drawn from the Sun to the Earth in any given position\\nis called the radius -vector of the Earth. Suppose this line\\nextended so as to meet the celestial sphere in the point 1\\nIt is evident that to an observer on the Earth at 1 the Sun\\nwill appear projected on the celestial sphere at 1 when\\nthe earth reaches 2 the Sun will appear at 2 and so on.\\nIn other words, as the Earth revolves around the Sun, the\\nlatter will seem to perform a revolution among the fixed", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0179.jp2"}, "178": {"fulltext": "156 ASTRONOMY.\\nstars. The stars do not seem to move because they are at\\nsuch enormous distances that a change of the Earth s place\\nfrom 1 to 6, or from A to C, makes almost no change in\\nthe direction of lines joining the Earth and any star. In\\nspace the lines HA, HC, HD, HB are almost (though not\\nquite) parallel.\\nFig. 97. The Annual Revolution of the Earth about the\\nSun, in the Orbit A BCD.\\nThe diameter of this orhit is about 186,000,000 miles.\\nThe apparent places of the Sun (1 2 3 4 5 6 etc.)\\ncan be denned in the sky by their right-ascensions and\\ndeclinations, or by their distances from the stars there\\nsituated. The right-ascensions and declinations of these\\nstars are known (or if they are not known they can be\\ndetermined by observation).", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0180.jp2"}, "179": {"fulltext": "APPARENT MOTION OF THE SUN. 157\\nIt is plain that an observer on the San would see the\\nEarth projected at points on the celestial sphere exactly\\nopposite to the corresponding points of the Sun s apparent\\npath viewed from the Earth. Moreover, if the Earth\\nmoves more rapidly in some portions of its orbit than iu\\nothers (as it does) the Sun will appear to move more rapidly\\nFig. 98. The Revolution of the Earth in its Orbit about\\nthe Sun.\\namong the stars in consequence. The two motions must\\naccurately correspond one with the other. The apparent\\nmotion of the Sun in the heavens is a precise measure of\\nthe real motion of the Earth in its orbit.\\nThe radius-vector of the Earth (the line joining Earth\\nand Sun) describes a plane surface as the Earth moves.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0181.jp2"}, "180": {"fulltext": "158 ASTRONOMY.\\nIn the figure this is the plane of the paper. In space this\\nplane is called the plane of the ecliptic. This plane will\\ncut the celestial sphere in a great circle; and the Sun will\\nappear to move in this circle. The circle is called the\\necliptic. The plane of the ecliptic divides the celestial\\nsphere into two equal parts. A sidereal year is the interval\\nof time required for the Sun to make the circuit of the sky\\nfrom one star hack to the same star again; or, it is the\\ninterval of time required for the Earth to go once around\\nits orbit.\\nWhen the earth is at 1 in the figure the Sun will appear\\nto be at 1 near some star, as drawn. Now by the diurnal\\nmotion of the Earth the Sun is made to rise, to culminate,\\nand to set successively to every observer on the Earth.\\nThis star being near the Sun rises, culminates, and sets with\\nhim; it is on the meridian of any place at the local noon\\nof that place (and is therefore not visible except in a tele-\\nscope since we cannot see stars in the daytime with the\\nnaked eye). The star on the right-hand side of the figure,\\nnear the line CS1 prolonged, is nearly opposite to the Sun.\\nWhen the Snn is rising at any place, that star will be\\nsetting; when the Snn is on the meridian of the place, that\\nstar is on the lower meridian; when the sun is setting, that\\nstar is rising. It is about 180\u00c2\u00b0 from the Sun.\\nNow suppose the Earth to move to 2. The Sun will be\\nseen at 2 near the star there marked. 2 is east of 1 the\\nSun appears to move among the stars (in consequence of\\nthe earth s annual motion) from west to east. The star\\nnear 2 will rise, culminate, and set with the Sun to every\\nobserver on the Earth. Like things are true of the Sun\\nin each of its successive apparent positions 3 4 5 6 etc.\\nThe student should here notice how our notions of the\\ndirection East and West have arisen. In the first place\\nmen noticed that the Sun rose in one part of the sky\\n(which they named East) and set in another (West).", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0182.jp2"}, "181": {"fulltext": "APPARENT MOTION OF THE SUN. 159\\nSecondly, it was found that these risings and settings were\\ncaused by the daily rotation of the Earth on its axis and\\nthat if the stars appeared to move from east to west the\\nEarth must really turn from west to east. The Sun\\nappears to move, in consequence of the Earth s annual\\nmotion, from west to east among the stars (from V\\ntowards 6 in the figure).\\nThe Earth moves around its circle ABCD in the same\\ndirection that the Sun appears to move around its circle\\nFGHE. Draw an arrow outside of FGHE parallel to\\n1 2 3 4 5 6 with the point near 6 and the feather\\nnear V. Draw another arrow outside of ABCD with the\\npoint near D and the feather near C. These arrows are\\nparallel. Hence the Earth moves in its orbit from west to\\neast. Or, suppose ABCD and FGHE to be two watch-\\ndials and SA and SE to be the hands. When SA points to\\nthe top of its dial {ABCD) its next movement is towards the\\nleft (in the figure). When SE points to the top of its dial\\n(FGHE) its next movement is towards the left, likewise.\\nAs the Sun is observed to move from west to east among\\nthe stars, the Earth must also move from west to east in\\nits orbit.\\nThe apparent position of a body as seen from the Earth\\nis called its geocentric place. The apparent position of a\\nbody as seen from the sun is called its heliocentric place.\\nIn the last figure, suppose the Sun to be at S, and the\\nEarth at 4. 4 is the geocentric place of the Sun, and G\\nis the heliocentric place of the Earth.\\nThe Sun s Apparent Path.\\nIt is evident that if the apparent path of the Sun lay in\\nthe equator, it would, during the entire year, rise exactly\\nin the east and set in the west, and would always cross the\\nmeridian at the same altitude (see page 68). The days\\nwould always be twelve hours long, for the same reason", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0183.jp2"}, "182": {"fulltext": "160 ASTRONOMY.\\nthat a star in the equator is always twelve hours above the\\nhorizon and twelve hours below it. But we know that\\nthis is not the case. The Sun is sometimes north of the\\nequator and sometimes south of it, and therefore it has a\\nmotion in declination.\\nThe Sun was observed with a meridian-circle and a\\nsidereal clock at the moment of transit over the meridian\\nof Washington on March 19, 1879. Its position was found\\nto be\\nEight-ascension, 23 h 55 m 23 s Declination, 0\u00c2\u00b0 30 south.\\nThe observation was repeated on the 20th and following\\ndays, and the results were\\nMarch 20, E.A. 23 h 59 m 2 s Dec. 0\u00c2\u00b0 6 South.\\n21, h 2 m 40 s 0\u00c2\u00b0 17 North.\\n22, h 6 m 19 s 0\u00c2\u00b0 41\\nIf we lay these positions down on a chart, we shall find\\nthem to be as in Fig. 99, the centre of the Sun being south\\nof the equator in the first two positions, and north of it in\\nthe last two. Joining the successive positions by a line,\\nwe shall have a representation of a small portion of the\\napparent path of the Sun on the celestial sphere, or of the\\necliptic.\\nIt is clear that the Sun crossed the equator on the after-\\nnoon of March 20, 1879, and therefore that the equator\\nand ecliptic intersect at the point where the Sun was at\\nthat time. This point is called the vernal equinox, the\\nfirst word indicating the season, while the second expresses\\nthe equality of the nights and days which occurs when the\\nSun is on the equator.\\nIf similar observations are made at any place on the\\nEarth in any year it will be found that the Sun moves\\nalong the ecliptic from the southern hemisphere into the\\nnorthern hemisphere about March 20 of each and every\\nyear*, and the point where the ecliptic crosses the equator", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0184.jp2"}, "183": {"fulltext": "APPARENT MOTION OF THE SUN.\\n161\\nthe vernal equinox can be determined by observation.\\nThe declination of this point is zero (because it is on the\\nequator) and its right-ascension is also zero (because right-\\nascensions are counted from the vernal equinox). From\\nFig. 99 \u00e2\u0080\u0094The Sun Crossing the Equator.\\nMarch to September the Sun is in the northern hemi-\\nsphere. Figs. 49, 50, 51, 52 have the ecliptic marked\\nupon them, and the student should point out the places of\\nthe Sun for the beginning of each month of the year (so\\nfar as is possible) on each figure. (See the next paragraph.)\\nHere for example are the positions of the Sun for the first day of\\nevery month of the year 1898 at Greenwich mean noon:\\n1898 (Jan. 1\\nR. A\\n18 h 49 m\\nBed\\nSouth 23\u00c2\u00b0\\nSouth Feb. 1\\n21 h l m\\n17\u00c2\u00b0\\n(Mar. 1\\n22 b 50 m\\n7\u00c2\u00b0\\nfApr. 1\\nMay 1\\nb 43 m\\nNorth 5\u00c2\u00b0\\n2 h 35 m\\n15\u00c2\u00b0\\nNorth H u ne\\n1 July 1\\n4 h 38 m\\n6 h 42 m\\n22\u00c2\u00b0\\n23*\\n1 Aug. 1\\n8 b 47m\\n18\u00c2\u00b0\\nI Sept. 1\\n10 h 43 m\\ngo\\nOct. 1\\n12 b 31 m\\nSouth 3\u00c2\u00b0\\nSouth Nov. 1\\n14 h 27m\\n15\u00c2\u00b0\\n(Dec. 1\\n16 h 31 m\\n22\u00c2\u00b0\\nOn June 21, 1898,\\nthe S\\n5un\\nhad its greatest\\nnorthern declination\\n-f 23\u00c2\u00b0 27; on December 22,\\n1898, the Sun had its greatest southern\\ndeclination =s 23\u00c2\u00b0\\n37", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0185.jp2"}, "184": {"fulltext": "162\\nASTRONOMY.\\nIf the right-ascensions and\\ndeclinations of the Sun dur-\\ning the months from March\\nto September are laid down\\non a map we shall have a\\ndiagram like Fig. 100. The\\nstraight line represents the\\ncelestial equator. The vernal\\nequinox is at the right-hand\\nside of the picture. The\\nright-ascension of the vernal\\nequinox is zero, and the hours\\nof right-ascension are marked\\nI, II, X, XI. These\\nnumbers increase as you go\\neastwards; hence the point\\nXI is east of the point II.\\nThe Sun crosses the equator\\n(going northwards) at the\\nvernal equinox in the month\\nof March. It continues to\\nmove north until June 21,\\nwhen it reaches its greatest\\nnorthern declination (23\u00c2\u00b0 27\\nFor several days at this time\\nthe Sun moves very little in\\ndeclination and seems (so far\\nas its motion in declination is\\nconcerned) to stand still. For\\nthis reason the ancients called\\nthe Sun s place about June\\n21 the summer solstice (Latin\\nsol the Sun, sistere to\\ncause to stand still). Its right-\\nascension is VI hours.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0186.jp2"}, "185": {"fulltext": "APPARENT MOTION OF TEE SUN. 163\\nFrom June 21 to September 22 the Sun remains north\\nof the equator, but its declination grows less and less\\nduring these months. Finally on September 23 the Sun\\ncrosses the equator once more going southwards at a point\\ncalled the autumnal equinox. Its declination is then zero\\n(because it is on the equator) and its right-ascension is XII\\nhours (because it is 180\u00c2\u00b0 distant from the vernal equinox,\\nFig. 101.\u00e2\u0080\u0094 The Celestial Sphere with .the Equator (AB)\\nand the Ecliptic (CD).\\nP is the north pole of the celestial equator Q is the north pole of the\\nSun s apparent path, the ecliptic.\\nthe zero of right-ascensions). After September 22 and\\nuntil the succeeding March the Sun is in the southern half\\nof the celestial sphere. Its sonth declination continually\\nincreases until December 22, when it is 23\u00c2\u00b0 South, in right-\\nascension XVIII hours. This point is the winter solstice.\\nFrom the winter solstice to the vernal equinox the Sun is\\nmoving northwards (in declination) and always eastwards\\n(in right-ascension) along the ecliptic. Finally in the\\nsucceeding March the Sun again crosses the equator at the\\nvernal equinox (R.A, h Decl. 0\u00c2\u00b0). The point D of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0187.jp2"}, "186": {"fulltext": "164: ASTRONOMY.\\nthe last figure is the summer solstice; the point C is the\\nwinter solstice.\\nThe ecliptic, as well as the equator, is marked on all\\nglobes; and the annual motion of the Sun can be illus-\\ntrated by tracing out the Sun s path day by day. It\\nrequires about 365 days for the Sun to move around the\\n360\u00c2\u00b0 of the ecliptic. Hence the Sun moves eastward\\nFig. 102.\u00e2\u0080\u0094 The Celestial Sphere.\\nEF is the celestial equator, IJ the ecliptic.\\namong the stars about 1\u00c2\u00b0 per day. The Sun s angular\\ndiameter is about half a degree. Therefore the Sun moves\\neach day about two of its own diameters.\\nThe celestial latitude of a star is its angular distance north or south\\nof the ecliptic. The celestial longitude of a star is its angular dis-\\ntance from the vernal equinox, measured on the ecliptic eastwards\\nfrom the equinox. The degrees of celestial longitude for half the.\\nyear are marked on Fig. 100,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0188.jp2"}, "187": {"fulltext": "LENGTH OF THE DAY AT DIFFERENT SEASONS. 165\\nThe sidereal year was defined (page 158) as the interval\\nof time between two successive returns of the Sun to the\\nsame star. Its length is 365 days, 6 hours, 9 minutes,\\n9.3 seconds.\\nThe astronomical year (the year as commonly used) is\\nthe interval between two successive returns of the Sun to\\nthe same equinox. Its length is 365 days, 5 hours, 48\\nminutes, 46 seconds. It is shorter than the sidereal year\\nFig. 103. The Celestial Sphere as it Appears to an\\nObserver in 34\u00c2\u00b0 North Latitude {PON 34\u00c2\u00b0).\\nThe ecliptic is not drawn on this figure.\\nbecause the equinoctial points are not fixed (as the stars\\nare) but move slowly. This will be explained more fully\\nlater on.\\nLength of the Day at Different Seasons of the Year.\\nThe length of time that any star is above the horizon of\\nan observer depends first on the observer s latitude, and", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0189.jp2"}, "188": {"fulltext": "166 ASTRONOMY.\\nsecond on the star s declination. We have jast seen that\\nthe Sun s declination is about 23\u00c2\u00b0 south on January 1, 5\u00c2\u00b0\\nnorth on April 1, 23\u00c2\u00b0 north on July 1, 3\u00c2\u00b0 south on\\nOctober 1.\\nTo every observer the Sun will be above the horizon for\\ndifferent periods at different times of the year. The\\nsummer days will be the longest and the winter days the\\nshortest.\\nFigure 103 represents the celestial sphere to an observer\\nin 34\u00c2\u00b0 north latitude. On January 1 the Sun (Decl.\\nsouth 23\u00c2\u00b0) will cross his meridian 23\u00c2\u00b0 south of the point C\\n(nearly half way from C to S), and will describe a diurnal\\norbit parallel to CWD (the equator). It will remain above\\nthe horizon a short time. The night will be longer than\\nthe daylight hours. On March 20 the Sun will be at V\\n(the vernal equinox). It will cross the meridian at C and\\nwill remain above the horizon (NS twelve hours. The\\ndays and nights will be equal. On July 1 the Sun is in\\ndeclination 23\u00c2\u00b0 north and will cross the meridian 11\u00c2\u00b0 south\\nof Z (GZ= 34\u00c2\u00b0; 34\u00c2\u00b0 23\u00c2\u00b0 11\u00c2\u00b0). The daylight hours\\nwill be long.\\nBy constructing such a diagram for his own latitude and by mark-\\ning the place of the sun for different days of the year the student\\ncan say, beforehand, just what the apparent diurnal path of the sun\\nwill be for any day in any year. A celestial globe set for his latitude\\nwill show the same things. He should notice that the sun rises north\\nof his east point in the summer in the east point at the equinoxes\\nsouth of the east point in the winter. The sun s diurnal path at the\\nequinoxes of Marchand September isthe celestial equator, at the winter\\nsolstice it is the tropic of Capricorn at the summer solstice it is the\\ntropic of Cancer. These tropics are circles of the celestial sphere\\ndrawn parallel to the equator, one (Cancer) 23-\u00c2\u00a3\u00c2\u00b0 north of it, the other\\nCapricorn) 23^\u00c2\u00b0 south of it. They are called tropics because the Sun\\nthere turns from going north (or south) in declination and begins to\\ngo south (or north). They are marked on all globes. The regions\\nof the earth between the latitudes 23$\u00c2\u00b0. north, and south are called the\\ntropics.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0190.jp2"}, "189": {"fulltext": "LENGTH OF THE DAY AT DIFFERENT SEASONS. 167\\nIf the observer is on the equator of the Earth, all the\\naays and nights of the whole year will be equal, no matter\\nwhat the Sun s declination may be. (See Fig. 105.)\\nsouth pole\\nFig. 104.\u00e2\u0080\u0094 The Circles of the Earth.\\nFig. 105.\u00e2\u0080\u0094 The Celestial Sphere as it Appears to an\\nObserver on the Earth s Equator.\\nAll the stars (and the Sun) are always above the horizon 12 hours and\\nbelow it 12 hours. The days and nights are all equal.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0191.jp2"}, "190": {"fulltext": "168\\nASTRONOMY.\\nThe following little table will be found useful and interesting.\\nThe Approximate Time of Sunrise for Observers between\\n30\u00c2\u00b0 and 48\u00c2\u00b0 of North Latitude.\\nN. B. The column of the table headed with the observer s latitude is\\nthe one to be consulted.\\nN. B \u00e2\u0080\u0094The approximate time of sunset is as many hours after noon as\\nthe time of sunrise is before it. For instance on May 1 in latitude 44\u00c2\u00b0 the\\nsun rises at 4 h 51 m a.m. i.e. 7 h 9 m before noon. The approximate time of\\nsunset on that day is therefore 7 h 9 m p.m.\\nLatitude.\\nDate.\\nJan.\\nFeb.\\nMar.\\nApr.\\nMay\\nJune\\nJuly\\nAug.\\nSept.\\nOct.\\nNov.\\nDec.\\n1...\\n11...\\n181..\\n1...\\n11.\\n81...\\n1...\\n11..\\n81...\\n1...\\n11...\\n81...\\n1...\\n11...\\n81...\\n1\\n11...\\n21...\\n1...\\n11...\\n81...\\n1...\\n11...\\n81...\\n1...\\n11...\\n81..\\n1...\\n11...\\n81...\\n1\\n11..\\n81...\\n1..\\n11...\\n81...\\n30\u00c2\u00b0\\nh. m\\n6 56\\n6 57\\n6 50\\n6 50\\n6 44\\n6 34\\n27\\n6 14\\n6 2\\n5 49\\n5 37\\n5 27\\n5 17\\n5 9\\n5 3\\n4 58\\n4 58\\n4 59\\n5 2\\n5 6\\n5 12\\n5 18\\n5 24\\n5 30\\n5 36\\n5 42\\n5 47\\n5 54\\n6\\n6 6\\n6 14\\n6 22\\n6 30\\n6 38\\n6 46\\n6 53\\n38\u00c2\u00b0 34\\nh. m.\\n7\\n7 1\\n7\\n6 54\\n6 47\\n6 36\\n28\\n15\\n2\\n48\\n35\\n24\\n14\\n5\\n58\\n53\\n52\\n54\\n56\\n2\\n7\\n14\\n21\\n28\\n34\\n41\\n47\\n5 54\\n6 1\\n6 9\\n6 17\\n6 25\\n6 34\\n6 43\\n6 51\\n6 58\\nh. m.\\n36\u00c2\u00b0\\nh. m\\n7 5\\n7 5\\n7 3\\n6 57\\n6 50\\n6 29\\n6 16\\n6 1\\n5 47\\n5 34\\n5 22\\n5 11\\n5 1\\n4 58\\n4 48\\n4 47\\n4 48\\n5 32\\n5 40\\n5 47\\n5 55\\n6 2\\n6 11\\n6 21\\n6 29\\n6 38\\n6 47\\n6 56\\n7 3\\n7 10\\n7 10\\n7 7\\n7 1\\n6 52\\n6 41\\n6 31\\n6 16\\n6 1\\n5 46\\n5 33\\n5 20\\n5 7\\n4 57\\n4 49\\n4 43\\n4 41\\n4 42\\n5 55\\n6 3\\n6 13\\n6 42\\n6 52\\n7 1\\n7 8\\n38\u00c2\u00b0 40\\nh. m\\n7 16\\n7 16\\n7 12\\n7 5\\n6 55\\n6 44\\n6 33\\n6 17\\n5 3\\n4 52\\n4 44\\n4 38\\n4 35\\n4 36\\n4 39\\n4 45\\n4 53\\n5 2\\n5 10\\n5 20\\n6 26\\n6 37\\n6 47\\n6 57\\n7 7\\n7 13\\nh. m.\\n7 22\\n7 21\\n7 18\\n7 9\\n6 58\\n6 46\\n6 34\\n6 17\\n6 1\\n5 44\\n5 29\\n5 14\\n4\\n4\\n4\\n4\\n4\\n4\\n4\\n5\\n5\\n5\\n5\\n5 46\\n5 57\\n6 7\\n6 18\\n6 29\\n6 41\\n6 52\\n7 2\\n7 12\\n7 19\\n42 c\\nh. m.\\n7 27\\n7 23\\n7 13\\n7 1\\n6 49\\n6 36\\n6 18\\n6 1\\n5 43\\n5 26\\n5 11\\n4 56\\n4 43\\n4 33\\n4 25\\n4 23\\n4 23\\n4 27\\n4 34\\n4 42\\n4 52\\n5 2\\n5 13\\n5 25\\n5 35\\n5 45\\n5 58\\n6 8\\n6 21\\n6 33\\n6 45\\n6 57\\n7 8\\n7 18\\n7 26\\n44 c\\nh. m.\\n7 36\\n7 33\\n7 28\\n7 18\\n7 5\\n6 51\\n4 IS\\n4 27\\n4 36\\n47\\n57\\n10\\n23\\n34\\n45\\n5 59\\n6 10\\n6 23\\n6 37\\n6 50\\n7 3\\n7 15\\n7 25\\n7 33\\n46\u00c2\u00b0\\nh. m\\n7 43\\n7 40\\n7 34\\n7 23\\n7 9\\n6 53\\n6 40\\n6 20\\n6\\n5 41\\n5 22\\n5 4\\n4 46\\n4 33\\n4 20\\n4 11\\n4 8\\n4 8\\n4 12\\n4 19\\n4 29\\n4 42\\n4 53\\n5 6\\n5 21\\n5 33\\n5 45\\n5 59\\n6 12\\n6 25\\n6 41\\n6 55\\n7 10\\n7 22\\n7 33\\n7 40\\n48=\\nh. m.\\n7 51\\n7 47\\n7 41\\n7 28\\n7 14\\n6 57\\n6 42\\n6 21\\n6\\n4 41\\n4 27\\n4 13\\n4 3\\n3 59\\n3 58\\n4 4\\n4 11\\n4 22\\n4 35\\n4 47\\n5 2\\n5 18\\n5 31\\n5 44\\n6\\n6 14\\n6 28\\n6 45\\n7 1\\n7 17\\n7 29\\n7 41", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0192.jp2"}, "191": {"fulltext": "THE ZODIAC. 169\\nIf the observer is at the Earth s north pole the Sun\\nwould be continuously above his horizon so long as the Sun\\nwas in the northern half of the celestial sphere, that is,\\nfrom March to September; and continuously below his\\nhorizon from September to March. An observer at the\\nsouth pole of the Earth has daylight continuously from\\nSeptember to March and continuous darkness from March\\nto September.\\nFig. 106.\u00e2\u0080\u0094 The Celestial Sphere as it would Appear to an\\nObserver at the North Pole op the Earth.\\nThe Sun would be above the horizon all the time from March 20 to Sep-\\ntember 22. The day would be six months long. The sun would be below\\nthe horizon all the time from September 22 to March 20. The night would\\nalso be six months long.\\nThe Zodiac and the Signs of the Zodiac. The zodiac is\\na belt in the heavens, extending some 8\u00c2\u00b0 on each side of\\nthe ecliptic, and therefore about 16\u00c2\u00b0 wide (see figure 50).\\nThe planets known to the ancients are always seen within\\nthis belt. At a very early day the zodiac was mapped out\\ninto twelve regions known as the signs of the zodiac, the\\nnames of which have been handed down to the present\\ntime. Each of these regions was supposed to be the seat\\nof a constellation or group of stars. Commencing at the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0193.jp2"}, "192": {"fulltext": "170\\nASTRONOMY.\\nvernal equinox, the first thirty degrees of the ecliptic\\nthrough which the Sun passed, or the region among the\\nstars in which it was found during the month following,\\nwas called the sign Aries. The next thirty degrees was\\ncalled the sign Taurus, and so on. The names of the signs\\nin order are\\nSpring\\nsigns.\\nSummer\\nsigns.\\nAutumn\\nsigns.\\nWinter\\nsigns.\\n1. Aries. The sun enters the sign Aries, March 20.\\n2. Taurus.\\n3. n Gemini.\\n4. SB Cancer.\\n5. Leo.\\n6. TT\u00c2\u00a3 Virgo.\\n7. =s= Libra.\\n8. Tr\\\\, Scorpius.\\n9. Sagittarius.\\n10. V3 Capricornus.\\n11. Aquarius.\\n12. Pisces.\\nTaurus, April 20.\\nGemini, May 20.\\nCancer, June 21.\\nLeo, July 22.\\nVirgo, August 22.\\nLibra, September 22.\\nScorpius, October 23.\\nSagittarius, Nov. 23.\\nCapricornus, Dec. 21.\\nAquarius, Jan. 20.\\nPisces, February 19.\\nEach of the signs of the zodiac coincides roughly with a con-\\nstellation in the heavens and thus there are twelve constellations\\ncalled by the names of these signs, but the signs and the constella-\\ntions no longer accurately correspond as they formerly did. Although\\nthe Sun now crosses the equator and enters the sign Aries on the 20th\\nof March, he does not reach the constellation Aries until nearly a\\nmonth later. This arises from the precession of the equinoxes, to be\\nexplained hereafter.\\nWhy are the stars fixed? Are the p]smets fixed Which way\\ndoes the sun move among the stars\u00e2\u0080\u0094 eastwards or westwards? How\\nlong does it take the sun to make a complete circuit of the heavens\\nWhat is the reason that the sun appears to move among the stars\\nWhat is the earth s radius-vector What is the plane of the ecliptic\\nWhat is a sidereal year? Describe the way in which our notions of\\nthe directions east and west have arisen. The stars in their diurnal\\norbits rise in the The earth turns on its axis from to\\nThe sun moves from to among the stars. The earth moves\\nin its real orbit in the same direction that the sun moves in its ap-\\nparent path, from to therefore. What is the geocentric or the\\nheliocentric place of a body? What is the vernal equinox? the\\nautumnal equinox the winter solstice the summer solstice Why\\nare these points called solstices? How long is the sun in the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0194.jp2"}, "193": {"fulltext": "OBLIQUITY OF THE ECLIPTIC. 171\\nnorthern half of the celestial sphere About how far does the sun\\nmove in the sky each day? What is an astronomical year? Why\\nare our winter days shorter than our days in summer How long\\nis a summer day to an observer at the earth s north pole How long\\nis a day to an observer at the earth s equator What is the Zodiac\\nWhat are the signs of the Zodiac\\n22. Obliquity of the Ecliptic. The obliquity of the\\necliptic is the angle between, the plane of the ecliptic and\\nthe plane of the celestial equator. It is the angle between\\nthe planes DOC and AOB in the figure. It is measured\\nAn\\nA kl\\n__\\\\r\\nAd\\nc V-\\n2^=*\\nJ\\nV\\np a\\nFig. 107. Obliquity of the Ecliptic\\nAB is the celestial equator, CD is the ecliptic.\\nby the arc DB or AC. DB is the Sun s greatest northern\\ndeclination; AC is the Snn s greatest southern declination.\\nAs soon as we have measured either of these (with a\\nmeridian-circle, for example) the obliquity is known. It is\\nabout 23^\u00c2\u00b0. It was determined by the ancient astronomers\\nquite accurately by observing the shadow of an obelisk at\\nthe times of the summer and winter solstices. At the\\nsummer solstice the Sun has its greatest north declination,\\nand therefore its meridian altitude on that day is a maxi-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0195.jp2"}, "194": {"fulltext": "172\\nASTRONOMY.\\nmuni. Its meridian altitude on the day of the winter\\nsolstice is a minimum.\\nIf AB is an obelisk and the line Bd is a north and south\\nline, and if the Sun is on the line Ad on December 22 and\\non Aj on June 21, then the shadow of the obelisk will be\\nBj in June (the shortest shadow of the year) and Bd in\\nDecember (the longest meridian shadow of the year) and\\nBm at the equinoxes. The angle dAj can be measured.\\nIt is equal to twice the obliquity and mAB measures the\\nZenith\\nm j B\\nFig. 108. The Obliquity of the Ecliptic\\ndetermined by the shadow of an obelisk at a place whose latitude is 45 N.\\nlatitude of the place, as the student can readily prove for\\nhimself.\\nThe Cause of the Seasons on the Earth. In each and\\nevery year we, who live in the temperate zones of the\\nEarth, witness the coming of spring, of summer, of\\nautumn, of winter. They come and go in a cycle of a\\nyear, and the cause of the change of seasons must therefore\\ndepend on the Earth s annual revolution in its orbit. The", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0196.jp2"}, "195": {"fulltext": "TEE SEASONS.\\n173\\ndifferent seasons are marked by changes in the quantity of\\nheat received from the Sun. In the summer the altitude\\nof the Sun is high and the days are long. In the winter\\nthe altitude of the Sun is not so high and the days are\\nshorter. The difference between the heat of summer and\\nwinter depends chiefly on the differences named. The\\nEarth revolves about the San in an orbit which is very\\nnearly a circle, so that the change of seasons does not\\ndepend on the varying distance of the Earth from the Sun.\\nAs a matter of fact the Earth is somewhat nearer to the\\nSun in January than it is in July.\\nFig. 109. The Ecliptic, CD, and the Celestial Equator,\\nAB, with their poles, Q and P.\\nThe Sun s apparent motion is in the ecliptic CD. The\\nvernal equinox is at E, the summer solstice at D, the\\nautumnal equinox at F, the winter solstice at C. The arc\\nBD AC= 23^\u00c2\u00b0, the obliquity of the ecliptic.\\nThe Sun s North-polar distance at E is 90\u00c2\u00b0;\\nD 66\u00c2\u00a3\u00c2\u00b0;\\nF 90\u00c2\u00b0;\\n113^\u00c2\u00b0,\\nit\\nsi\\na\\na\\n16\\nK", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0197.jp2"}, "196": {"fulltext": "174\\nASTRONOMY.\\nIn Fig. 110 the oval line represents the path of the Earth\\nin its annual revolution about the Sun in the ecliptic. The\\nline NS in each picture of the Earth represents the Earth s\\naxis, JVits north end. The Earth s axis is always directed\\nto a point very near to the star Polaris at all times of the\\nyear, that is, wherever the Earth may be in her orbit.\\nHence the four lines N8 are drawn parallel to each other.\\nFig. 110.\u00e2\u0080\u0094 The Seasons.\\nThe Earth is shown in four positions in its orbit. A the winter sol-\\nstice B the vernal equinox C the summer solstice D the au-\\ntumnal equinox. The orbit of the Earth is nearly a circle. It is much\\nforeshortened in the picture.\\nThe Earth s axis is perpendicular to the celestial equator,\\nbut it is inclined to the ecliptic by an angle of 23^\u00c2\u00b0, and\\nit has been so drawn.\\nIf the student will join the centre of the Sun (S) with\\nthe centre of the Earth in each one of the four positions\\ndrawn he will see that, as has been said,\\nThe Sun s N.P.D. at A (winter solstice) is 113i\u00c2\u00b0;\\nB (vernal equinox) is 90\u00c2\u00b0;\\nC (summer solstice) is 66^\u00c2\u00b0;\\nM D (autumnal equinox) is 90\u00c2\u00b0,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0198.jp2"}, "197": {"fulltext": "THE SEASONS.\\n175\\nHe can also prove that the Sun s altitude to any observer\\nin the northern hemisphere is greater in summer than in\\nwinter by drawing the horizon of an observer on the same\\nparallel of the Earth at A and at C.\\nThe Sun shines on one half of the Earth only namely\\non that half which is turned toward him. This hemi-\\nsphere is left bright in each of the figures ABCD. The\\nother half of the sphere is dark. Consider the picture at\\nFig. 111. A. The Earth at the Winter Solstice.\\nA (winter solstice) and remember that the Earth is turning\\non its axis every 24 hours. Every observer on the Earth,\\nin any latitude, is carried round his parallel of latitude by\\nthe Earth s rotation once in every 24 hours. The parallels\\nFig. 112.\u00e2\u0080\u0094 B. The Earth at the Vernal Equinox.\\nof latitude are drawn in the picture. A person near N\\nwill remain in darkness during the whole 24 hours. A\\nperson anywhere in the northern hemisphere of the Earth", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0199.jp2"}, "198": {"fulltext": "176\\nASTRONOMY.\\nwill be less than half the time in the light, more than half\\nthe time in darkness. The Sun will be less than half the\\ntime above his horizon. The daylight hours will be shorter\\nthan the hours of darkness. This is the time of winter in\\nthe northern hemisphere of the Earth.\\nTake, next, the Earth at the vernal equinox B. Half\\nFig. 113.\u00e2\u0080\u0094 C. The Earth at the Summer Solstice.\\nof the Earth is lighted by the Sun, and the Sun s rays just\\nreach the two poles iVand S. The days and nights are\\nequal. At the summer solstice C an observer at the\\nEarth s north pole has perpetual day; and the Sun is\\nabove the horizon of every person in the northern hemi-\\nPig. 114.\u00e2\u0080\u0094 D. The Earth at the Autumnal Equinox.\\nsphere for more than half the 24 hours. The days are\\nlonger than the nights. At the autumnal equinox, Z), the\\ncircumstances are like those at the vernal equinox,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0200.jp2"}, "199": {"fulltext": "THE SEASONS.\\nm\\nThe foregoing explanation of Fig. 110 illustrates the\\ndependence of the seasons upon the length of time that the\\nSan is above the horizon. The altitude of the Sun above\\nthe horizon also plays an important part in producing the\\nchange of the seasons. (See Fig. 115).\\nIn the figure a beam of\\nsunshine having the cross-\\nsection A BCD strikes the\\nsoil cbDA at an angle h. It\\nis clear that the area cbDA\\nis greater than the area\\nABCD. The amount of\\nheat in the sunbeam is\\nalways the same. This con-\\nstant amonnt of heat is dis-\\ntributed over a larger surface\\naccording as the altitude of\\nthe Sun is less. Hence in a winter s day, when the Sun\\neven at noon is low, each square mile of soil receives less\\nheat than it receives in summer, when the Sun is high.\\nFig. 115.\u00e2\u0080\u0094 The Effect of the\\nSuns Elevation on the\\nAmount of Heat Imparted\\nto the Soil.\\nFig. 116. The Meridian Altitude of the Sun at is\\nEqual to (90\u00c2\u00b0 f 8) HS HQ Q8.\\nAt a place on the Earth whose latitude is 45\u00c2\u00b0 0) the\\nmeridian-altitude of the Sun", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0201.jp2"}, "200": {"fulltext": "178 ASTRONOMY.\\nis 45\u00c2\u00b0 on March 20 (90\u00c2\u00b0 45\u00c2\u00b0 0\u00c2\u00b0);\\n68i\u00c2\u00b0 June 21 (90\u00c2\u00b0 45\u00c2\u00b0 23^\u00c2\u00b0);\\n45\u00c2\u00b0 September 22 (90\u00c2\u00b0 45\u00c2\u00b0 0\u00c2\u00b0);\\n21i\u00c2\u00b0 December 22 (90\u00c2\u00b0 45\u00c2\u00b0 23i\u00c2\u00b0).\\nTherefore the Sun s rays are inclined to the soil at very\\ndifferent angles at different dates, and the amount of heat\\nreceived per square mile varies. Not only is less heat per\\nsquare mile received in December than in June, but it is\\nreceived for a shorter period. In latitude 45\u00c2\u00b0 the Sun is\\nabove the horizon for about 15-J hours on June 21 (see the\\ntable on page 168), while on December 22 it is above the\\nhorizon for a little more than 8-J hours. There are two\\nreasons, then, for the change of seasons: first, the duration\\nof sunshine is longer at some dates than at others, second\\nthe amount of the Sun s heat received per square mile per\\nhour is greater at some dates than at others.\\nThe student should take a pin and put it on the various parallels\\nof latitude in the four diagrams ABCD, Fig. 110. The rotation of\\nthe Earth carries an observer round his own parallel of latitude. The\\npictures show whether the observer is more or less than 1 2 hours in\\nthe light of the Sun whether his days are longer or shorter than his\\nnights. They also show how the altitude of the Sun varies at dif-\\nferent seasons of the year. Notice that an observer on the Earth s\\nequator always has days and nights of equal length, no matter what\\nthe season of the year. Prove that the Sun is always in the zenith\\nto some observer in the Earth s torrid zone.\\nWhat is the obliquity of the ecliptic? How many degrees is it?\\nShow how it can be determined by observing the lengths of the\\nshadow of an obelisk. What are the two causes of the change of\\nseasons on the Earth", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0202.jp2"}, "201": {"fulltext": "CHAPTER IX.\\nTHE APPARENT AND REAL MOTIONS OF THE PLANETS\\n\u00e2\u0080\u0094KEPLER S LAWS.\\n23. The Apparent Motions of the Planets to an Observer\\non the Earth Their Real Motions in Their Orbits. The\\napparent motions of the planets were studied by the ancients\\nby mapping down their positions among the fixed stars from\\nnight to night. The same process can be followed to-day\\nby any one who will give the time to it. The place of the\\nplanet must be fixed by observation each night, with refer-\\nence to stars near it, and then this place mnst be trans-\\nferred to a star-map, like those printed at the end of\\nthis book, for instance. A carved line joining the different\\napparent positions of the planet on different nights will\\nrepresent its apparent path.\\nAstronomers, who are provided with accurate instru-\\nments such as meridian-circles, fix the positions of the\\nplanets by determining their right-ascensions and declina-\\ntions every night. By platting these positions on a map\\nthey obtain a representation of the apparent orbit with\\ngreat accuracy.\\nSomething of the same sort can be done by the student with much\\nsimpler instruments. He needs only a common watch and a straight\\nruler some three feet long, together with a star-map. Suppose that he\\nwishes to determine the place of the planet Mars The first step\\nis to identify the planet in the sky, by its brightness, its place, or by\\nits motion. He then selects two bright stars not very far away from\\nit (let us call them A and B for convenience).\\nHolding up the ruler so that its edge passes through the two\\nstars, he notices that it passes very nearly through the planet, which\\n179", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0203.jp2"}, "202": {"fulltext": "Fig. 117.\u00e2\u0080\u0094 Copernicus.\\nBorn 1473, died 1543.\\n180", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0204.jp2"}, "203": {"fulltext": "APPARENT MOTIONS OF THE PLANETS. 181\\nis, however, let us say, a little to the west of the line. On the star-\\nmap he must find the two stars A and B. Suppose that they are\\na and (5 Auriga (between the numbers 105\u00c2\u00b0 and 120\u00c2\u00b0 at the top of\\nPlate II). A dot must now be put on the map in the proper position\\nFig. 118.\\nFig. 119.\\nto represent the place of the planet and the dot must be numbered\\n(1). In his note-book opposite 1 the observer must write the year,\\nthe month, the day, and the hour of observation thus\\n1. 1899, February 27, 9 b p.m.\\nThe place of the planet is much more accurately fixed if the\\nobserver makes allineations with four stars, thus\\nG might be 8 Auriga on Plate II and D a star given but unnamed\\nthere.\\nOn succeeding nights other positions of the planet can be obtained\\nin the same way, and its apparent path can be had by joining the\\ndifferent positions. The times of each observation are to be noted.\\nThe positions of other planets as Mercury, Venus, Jupiter, and\\nSaturn can also be studied from night to night, and their apparent\\npaths fixed in like manner. Observations of this kind, if continued\\nlong enough, will give the apparent paths of the different planets in\\nthe sky. The courses of the Sun and Moon can be studied in the\\nsame manner, except that observations of the Sun must be made near\\nthe times of sunset and sunrise, because it is only at these times that\\nstars are visible near it.\\nIf such observations are made the student can discover\\nfor himself what the ancients knew Tery well, namely, that", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0205.jp2"}, "204": {"fulltext": "182 ASTRONOMY.\\nthere are heavenly bodies with apparent motions of three\\nvery different kinds. The Sun and Moon have apparent\\nmotions of one kind. If we mark down the positions of\\nthe San day by day upon a star-chart, they will all fall into\\na regular circle which marks out the ecliptic, and its motion\\nis always towards the east. The monthly course of the\\nMoon is found to be of the same nature; and although its\\nmotion is by no means uniform in a month, it is always\\ntowards the east, and always along or very near a certain\\ngreat circle.\\nVenus and Mercury have motions of a different kind.\\nThe apparent motion of these bodies is an oscillating one\\non each side of the Sun. If we watch for the appearance\\nof one of these planets after sunset from evening to even-\\ning, we shall by and by see it appear above the western\\nhorizon. Night after night it will be farther and farther\\nfrom the Sun until it attains a certain maximum distance;\\nthen it will appear to return towards the San again, and for\\na while it will be lost in its rays. A few days later it will\\nreappear to the west of the Sun, and thereafter be visible\\nin the eastern horizon before sunrise. In the case of\\nMercury the time required for one complete oscillation\\nback and forth is about four months; and in the case of\\nVenus it is more than a year and a half.\\nThe third class comprises Mars, Jupiter, and Saturn.\\nThe general or average motion of these planets is towards\\nthe east, a complete revolution around the celestial sphere\\nbeing performed in two years in the case of Mars, 12 years\\nin the case of Jupiter, and 30 years in that of Saturn.\\nBut, instead of moving uniformly forward, they seem to\\nhave a swinging motion first, they move forward or toward\\nthe east through a pretty long arc, then backward or west-\\nward through a short one, then forward through a longer\\none, etc. It is by the excess of the longer arcs over the\\nshorter ones that the circuit of the heavens is made.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0206.jp2"}, "205": {"fulltext": "APPARENT MOTIONS OF THE PLANETS 183\\nObservations of the planets will show that each one of them\\nhas an apparent motion like those just described. The\\nproblem is to discover the real cause of these observed\\nmotions.\\nThe general motion of the Sun, Moon, and planets\\namong the stars being towards the east, observed motions\\nFig. 120.\\nIf S is the Sun, E the Earth, CLM the orbit of an inferior planet, then\\nthe planet is in inferior conjunction at at superior conjunction at C, at\\nits greatest elongation from the Sun at L and M.\\nin this direction are called direct; motions towards the west\\nare called retrograde. During the periods between direct\\nand retrograde motion the planets will for a short time\\nappear stationary.\\nThe planets Venus and Mercury are said to be at greatest\\nelongation when at their greatest angular distance from the\\nSun.\\nAn inferior planet is said to be in conjunction with the\\nSun when both planet and Sun are in the same direction\\nas seen from the Earth. It is in inferior conjunction when\\nit is between the Sun and Earth; in superior conjunction\\nwhen the Sun is between the Earth and the planet. A\\nsuperior planet is said to be in opposition to the Sun when", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0207.jp2"}, "206": {"fulltext": "184 ASTRONOMY.\\nthe planet is directly opposite in direction to the Sun as\\nseen from the Earth.\\nArrangements and Motions of the Planets of the Solar\\nSystem. The Sun is the centre of the solar system and all\\nFig. 121. The Orbits of Mercury, Venus, the Earth, Mars,\\nand Jupiter.\\nThe distance from the Sun to the Earth is 93,000,000 miles from the Sun\\nto Jupiter is 481,000,000 miles the other distances are in proportion.\\nthe planets revolve abont the Sim. Some of the planets\\nhave satellites or moons that revolve about the planet while\\nthe planet itself revolves about the Sun. Our own Moon\\nis such a satellite. The orbits of the planets are all nearly,\\nbut not exactly, in the same plane, namely, in the plane of\\nthe Earth s orbit the ecliptic.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0208.jp2"}, "207": {"fulltext": "ORBITS OF THE PLANETS.\\n185\\nName.\\no\\nPQ\\nS\\nCO\\ns\\n2*.\\nn a\\nD 3\\n3.S\\n5\\nSidereal Period of Revolution.\\nGroup of f Mercury\\nPlanets each i Venus\\nabout the JTnrfh\\nsize of the f *rl/i\\nEarth. 1 Mars\\n9\\n6\\n0.39\\n0.72\\n1.00\\n1.52\\n88 days 3 montlis JL\\n225 7i\\n365\u00c2\u00a3 12\\n687 22|\\nThe Small Planets.\\nAbout\\n2.65\\n3 to 8 years.\\nJupiter..\\nGroups of Saturn.\\np\u00c2\u00a3,e a ts ge granus.\\ni, Neptune\\nU\\n5.20\\n9.54\\n19.18\\n30.05\\n11A years.\\n29|\\n84\\nIMA\\nThe distance of the Earth 1.00 93,000,000 miles.\\nThe planets Mercury and Venus which, as seen from the\\nearth, never appear to recede very far from the Sun, are in\\nreality those which revolve inside the orbit of the Earth.\\nThe planets Mars, Jupiter, and Saturn are more distant\\nfrom the San than the Earth is. Uranus and Neptune\\nare planets generally invisible except in the telescope, and\\ntheir orbits are outside of that of Saturn. On the scale\\nof Fig. 121 the orbit of Neptune, the outermost planet,\\nwould be more than thirty inches in diameter.\\nInferior planets are those whose orbits lie inside that of\\nthe Earth, as Mercury and Venus.\\nSuperior planets are those whose orbits lie outside that\\nof the Earth, as Mars, Jupiter, Saturn, etc. The ancient\\nastronomers gave these names and they have been retained\\nin use, although they now have little significance.\\nThe farther a planet is situated from the Sun the slower\\nis its motion in its orbit. Therefore, as we go outwards\\nfrom the Sun, the periods of revolution are longer, for the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0209.jp2"}, "208": {"fulltext": "186 ASTRONOMY.\\ndouble reason that the planet has a larger orbit to describe\\nand moves more slowly in its orbit. The Earth moves 18\u00c2\u00a3\\nmiles per second in its orbit, while Saturn moves but\\n6 miles per second.\\nAn observer on the Sun at S would see the Earth along\\nthe lines SI, S%, S3, etc. If these lines are prolonged\\n(to the right hand in the figure) the Earth would seem, to\\nan observer on the Sun, to move eastwardly among the\\nFig. 122. The Motion of the Earth in Its Orbit It is\\nDirect Motion.\\nstars (see page 158). The real motion of the Earth seen\\nfrom the Sun is direct. We have proved on page 159 that\\nthe apparent motion of the Sun is always direct also. The\\nplane of the Earth s orbit the ecliptic is the plane in", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0210.jp2"}, "209": {"fulltext": "MOTIONS OF THE PLANETS. 187\\nwhich all the other planets revolve very nearly. It is to\\nthe slower motion of the outer planets that the occasional\\napparent retrograde motion of the planets is due, as may\\nbe seen by studying Fig. 123. The apparent position of\\na planet, as seen from the Earth, is determined by the\\nline joining the Earth and planet. We see the planet\\nalong this line. Supposing this line to be continued so as\\nFig. 123.\\nThe apparent motion of a superior planet, as seen from the Earth, is\\nsometimes direct and sometimes retrograde. The motion is always retro-\\ngrade when the planet is nearest the Earth, always direct when the\\nplanet is farthest from the Earth.\\nto intersect the celestial sphere, the apparent motion of the\\nplanet will be denned by the motion of the point in which\\nthe line meets the celestial sphere. If this motion is\\ntowards the east the motion of the planet is direct; if this\\nmotion is towards the west, the motion of the planet is\\nretrograde.\\nLet us consider the case of one of che superior planets. Its orbit\\nis outside of the Earth s orbit. Its motion in its orbit is slower than", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0211.jp2"}, "210": {"fulltext": "188 ASTRONOMY.\\nthe Earth s motion in its orbit. Let S be the Sun, ABOBEF the\\norbit of the Earth and HIKLMN the orbit of a superior planet\\nMars, for example. The real motion of Mars is direct. It moves\\nround its orbit in the direction of the arrow, just as the Earth moves\\nround its orbit in the direction marked. In both cases the real mo-\\ntion is from west to east.\\nWhen the Earth is at A, Mars is at H\\nB,\\nI\\nC,\\nK\\nt\\nD,\\nL\\nli\\nE,\\nM\\nF,\\nir\\nAs the Earth moves faster than Mars the arcs AB, BO, CD, BE,\\nEF correspond to greater angles at 8 than do the arcs HI, IK, KL,\\nLM, MN.\\nWhen the Earth is at A and Mars at H, an observer on the Earth\\nwill see Mars along the line AH. This line meets the celestial\\nsphere at 0. Mars will then appear to be projected among the stars\\nnear 0. When the Earth is at B and Mars at the planet will be\\nviewed along the line BP and it will be seen on the celestial sphere\\namong the stars near P. While the Earth is moving in its orbit\\nfrom A to B Mars will appear to move (eastwards) among the stars\\nfrom to P. Its apparent motion is in the same direction as the\\nEarth s real motion. When the Earth is at C and Mars at K the\\nplanet will be seen along the line OZf (prolonged). Its apparent place\\namong the stars will be slightly to the west of P it will appear to\\nhave moved backwards its apparent motion is, at this time, retro-\\ngrade.\\nWhen the Earth is at Mars is in opposition to the Sun. The\\nSun and Mars are seen from the Earth in opposite directions. The\\napparent motion of all superior planets at the time of opposition is\\nretrograde.\\nWhile the Earth is moving from to D in its orbit, Mars is mov-\\ning from Kto L m its orbit, and the apparent position of Mars on\\nthe celestial sphere is moving to the west in a retrograde direction.\\nAs the Earth moves from D to E Mars moves from L to M and the\\nplanet is seen along the lines DL and EM prolonged. These lines\\nare parallel. They meet the celestial sphere in the same group of\\nstars. The planet, therefore, seems to stay in the same position\\namong the stars. It appears to be stationary just after opposition,\\nwhile the Earth is moving from D to E.\\nAs the Earth moves from D to F Mars moves in its orbit from L", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0212.jp2"}, "211": {"fulltext": "APPARENT MOTIONS OF THE PLANETS. 189\\nto N. Its apparent place on the celestial sphere among the stars\\nchanges from Q to R. Its apparent motion is again direct towards\\nthe East. It is in this way that a superior planet one whose orbit\\nis outside of the Earth s orbit moves around the celestial sphere.\\nIts general motion is eastwardly through long arcs. Near opposition\\nits apparent motion is retrograde and, for a period, it is stationary.\\nIt does not then change its place with reference to stars near it.\\nThe student can study the apparent motion of a superior planet\\nnear conjunction, or of an inferior planet by constructing suitable\\ndiagrams like the foregoing.\\nThe superior planets (Mars, Jupiter, Saturn, etc.) make\\nthe whole circuit of the sky in long forward arcs with short\\nloops of retrogression. The inferior planets (Mercury and\\nVenus) do not make the circuit of the sky. They oscillate\\non either side of the Sun, never going very far away from\\nit. When they are west of the Sun they rise before him\\nand are morning stars. When they are east of the Sun\\nthey set after the Sun and are evening stars. If Venus is\\nan evening star she will approach the Sun nearer and\\nnearer and set nearer and nearer to the time of sunset.\\nBy and by she approaches so closely as to be lost in his\\nrays (at inferior conjunction ECS in Fig. 123, where K\\nis now the Earth and C Venus). In a few days she has\\npassed the Sun going westwards and rises before him as a\\nmorning star. The apparent motion of all planets is\\nretrograde when they are nearest to the Earth and direct\\nwhen they are farthest from us.\\nThe apparent motions of ail the planets visible to the\\nnaked eye were perfectly familiar to the ancient astronomers,\\nas has been said. The positions of the planets had been\\nobserved by them for centuries. But the reasons for these\\ncomplex movements were not known. It was everywhere\\nbelieved that the Earth was the centre of the Universe and\\nthat the Sun, the Moon, the stars, and all the planets were\\nmade for the sole benefit of mankind. All the explana-\\ntions of the ancient philosophers started with the assump-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0213.jp2"}, "212": {"fulltext": "190\\nASTRONOMY.\\ntion that the Earth was the centre of the Universe and\\nthat the Sun and all the planets revolved around it. No\\none thought of questioning this proposition. It was every-\\nwhere believed.\\nPtolemy of Alexandria in Egypt worked out a theory\\nof the Universe on this scheme about a.d. 140. It was a\\nvery ingenious system and it explained observed appear-\\nances fairly well so long as the observations were not very\\naccurate.\\nFig. 124.\u00e2\u0080\u0094 The System of the World according to Ptolemy.\\nEach planet was supposed to move round the circumfer-\\nence of a small circle called its epicycle (see the cut), while\\nthe centre of the epicycle moved around a larger circle\\ncalled the deferent. By taking the epicycles and the\\ndeferents of suitable sizes a very fair representation of the\\napparent motions of the Sun and planets was made.\\nThe swinging motions of Mercury and Venus on each\\nside of the Sun were explained by their motions around", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0214.jp2"}, "213": {"fulltext": "MOTIONS OF THE PLANETS. 191\\ntheir epicycles, which would make them appear alternately\\neast and west of the Sun if their epicycles moved round\\ntheir deferents at the same rate that the Sun moved (see\\nthe cut). The retrogradations of the superior planets\\nMars, Jupiter, and Saturn were explicable in a similar\\nfashion.\\nIt is not necessary to go into details in this matter\\nbecause Ptolemy s explanation of the Universe is not the\\ncorrect one. Still the student should know something of\\na theory which was believed by every one from the first\\ncenturies of our Christian era until Copernicus proposed\\nthe true explanation. It was not until Copernicus had\\nmade long-continued observations on his own account and\\nhad given his whole life to solving the problem that it was\\nknown that the Sun and not the Earth was the centre of the\\nplanetary motions. He proposed this explanation in 1543,\\nbut it was not generally accepted until the discoveries of\\nGalileo (1610), about three centuries ago.\\nThe theory of Ptolemy accounted pretty well for the\\nfacts known in his time. It represented the apparent\\nmotion of the planets as he observed them. But the\\nobservations of the Arabian astronomers in Spain (a.d. 762\\nto 1492) and of Tycho Brahe (pronounced Tee-ko Bra-hee)\\nin Denmark about 1580, and especially the revelations of\\nGalileo s telescope, made Ptolemy s explanation impossi-\\nble. It was not long before it was found that even the\\nsystem proposed by Copernicus was not entirely satisfac-\\ntory. It was certain that the Sun and not the Earth was\\nthe centre of the planetary motions, as he had said. But\\naccurate observations soon made it equally certain that the\\nplanets did not revolve in circular orbits. They revolved\\nabout the Sun in orbits nearly but not quite circular, in\\ncurves like ovals. They certainly did not revolve in\\ncircles.\\nFrom the time of Copernicus (1543) till that of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0215.jp2"}, "214": {"fulltext": "192 ASTRONOMY.\\nKepler (about 1630) the whole question of the true system\\nof the Universe was in debate. The circular orbits intro-\\nduced by Copernicus also required a complex system of\\nepicycles to account for some of the observed motions of\\nthe planets, and with every increase in accuracy of observa-\\ntion new devices had to be introduced into the system to\\naccount for the new phenomena observed. In short, the\\nsystem of Copernicus accounted for so many facts (as the\\nstations and retrogradations of the planets) that it could\\nnot be rejected, and had so many difficulties that without\\nmodification it could not be accepted.\\nDescribe how the place of a planet may be fixed, among the\\nfixed stars, by simple observations. If such observations are made\\nfor long periods the apparent paths of the Sun and planets become\\nknown. In what apparent paths do the Sun and Moon move?\\nMercury and Venus? The superior planets? Define the inferior\\nconjunction of Venus the superior conjunction of Mercury the\\nopposition of Jupiter. Define the inferior planets the superior\\nplanets. Define direct motion retrograde. What was the theory of\\nthe Universe proposed by Ptolemy in A.D. 140? How long did men\\nhold the belief that the Earth was the centre about which the planets\\nrevolved? Who proposed the heliocentric theory of the solar system?\\nAt what date? What was the shape of the orbits of all the planets\\nin this theory?\\n24. Kepler s Laws of Planetary Motion. Kepler (born\\n1571, died 1630) was a genius of the first order. He had\\na thorough acquaintance with the old systems of astronomy\\nand a thorough belief in the essential accuracy of the\\nCopernican system, whose fundamental theorem was that\\nthe Sun and not the Earth was the centre of our system.\\nHe lived at the same time with Galileo, who was the first\\nperson to observe the heavenly bodies with a telescope of\\nhis own invention, and he had the benefit of accurate\\nobservations of the planets made by Ttcho Brahe. The\\nopportunity for determining the true laws of the motions", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0216.jp2"}, "215": {"fulltext": "MOTIONS OF THE PLANETS\u00e2\u0080\u0094 KEPLER S LAWS. 193\\nof the planets existed then as it never had before; and\\nfortunately he was able, through labors of which it is diffi-\\ncult to form an idea to-day, to reach a true solution.\\nThe Periodic Time of a Planet. The time of revolution\\nof a planet in its orbit round the Sun (its periodic time) k\\nFig. 125.\u00e2\u0080\u0094 John Kepler,\\nBorn 1571, died 1630.\\ndetermined by continuous observations of the planet s\\ncourse among the stars.\\nThe periodic times (the sidereal periods) of the planets\\nwere known to Keplek from the observations of the\\nancient astronomers.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0217.jp2"}, "216": {"fulltext": "194\\nASTRONOMY.\\nMercury revolved about the Sun in about 88 days= 0. 24 yrs.\\nVenus 225 0.62\\nEarth 365 1.00\\nMars 687 1.88\\nJupiter li 4333 11.86 f\\n/Sfl^m f f f 10,759 29.46 f\\nThe Relative Distances of Planets from the Sun.\\nKeplee had no way of determining the absolute distance\\nof each planet from the Sun (its distance iu miles), but if\\nthe distance of the Earth from the San was taken as the\\nunit (1.000) he could determine the distances of the other\\nplanets in terms of this unit in the following way\\nFig. 126\u00e2\u0080\u0094 Method of Determining How Much Greater the\\nDistance of Mars from the Sun is than the Distance of\\nthe Earth from the Sun.\\nIu the figure let She the Sun, EE the orbit of the earth, and MM\\nthe orbit of Mars. When the Earth is at E and Mars at M the planet\\nis in opposition, i.e., it is seen from the Earth in a direction exactly\\nopposite to the Sun. It is on the meridian of the observer exactly at\\nmidnight. After a hundred days, for example, Mars will have", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0218.jp2"}, "217": {"fulltext": "MOTIONS OF THE PLANETS\u00e2\u0080\u0094 KEPLER S LAWS. 195\\nmoved to M and the Earth will have moved to E The observer\\nwill then see the Sun in the direction E to 8 he will see Mars\\nin the direction E to M At this time the angle M E S can be\\nmeasured with a divided circle, and it therefore is a known angle.\\nThe angle ESE is known, because we can calculate through what\\nangle the Earth will move in 100 days, since we know that it\\nmoves through 360\u00c2\u00b0 in 365\u00c2\u00a3 days. The angle MSM is likewise\\nknown, since we can calculate through what angle Mars will move\\nin 100 days, because we know that Mars moves through 360\u00c2\u00b0 in 687\\ndays. The angle M SE is therefore known because ESE MSM\\nM SE Hence in the triangle M SE we know the two angles\\nmarked in the diagram. E SM is measured, M SE is calculated.\\nThe angle SM E 180\u00c2\u00b0 [E SM 1 M E S] because in any plane\\ntriangle the sum of the angles is 180\u00c2\u00b0. Hence in this triangle we\\ncan determine all three angles. We can therefore construct a\\ntriangle of the right shape. If we assume the Earth s distance SE to\\nbe 1.000 we can determine the distance of Mars in terms of that\\nunit. If Kepler had known the distance SE in miles (as it is\\nknown nowadays) then he could have determined the absolute dis-\\ntance, SM of Mars. As it was, he could say that if the Earth s dis-\\ntance, SE was called 1.000 then the distance of Mars, SM must\\nbe 1.52.\\nAt different points of the Earth s orbit the corresponding\\ndistances of Mars were determined. The same thing was\\ndone for the other planets at different points of their\\norbits. Kepler found that if the mean distance of the\\nEarth from the Sun was called 1.000 then the mean dis-\\ntances for all the planets were\\nFor Mercury, a 1 0.3871; for Mars, a K 1.5237;\\nVemis, a q 0.7233; Jupiter, a 6 5.2028;\\nEarth, a 3 1.000; Saturn, a 6 9.5388.\\nThe radius-vector of a planet is the line that joins it to\\nthe Sun.\\nKepler made thousands and thousands of such calcula-\\ntions and determined the radius-vector of Mars from the\\nSun at all points in its orbit, assuming that the Earth s\\naverage (mean) distance was 1.000. He could therefore\\nmake a map of the orbit of Mars as in the following figure.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0219.jp2"}, "218": {"fulltext": "196 ASTRONOMY.\\nIn the figure 8 is the place of the Sun. At some date\\nMars was somewhere along the line SP (Mars was in a\\ncertain known celestial longitude). If the distance of the\\nEarth from the Sun was taken as the unit then the dis-\\nFig 127.\u00e2\u0080\u0094 The Okbit of a Plaket, P, about the Sun, S.\\ntance of Mars was known in terms of that unit. Mars was\\nat the point P. At a later time Mars was somewhere along\\nthe radius-vector 8P iy which was in the right longitude.\\nCalculation showed that Mars was at the point P,. At\\nother times Mars lay somewhere along the radii-vectores\\n8P 9 SP 3 8P A SP b Calculation showed that the planet\\nwas at the points P 3 P 3 P 4 P b The curved line joining\\nall these points was the visible representation of the orbit\\nof Mars. The curve P 1 P 6 was the true shape of\\nthe orbit. Nothing was known of the size of the orbit\\nexcept that it was so and so many times larger than the\\nEarth s; but at any rate its true shape was known. It\\nwas not a circle; it was something like an oval.*\\nKepler s next problem was to determine what kind of\\nThe real orbit of Mars is very nearly a circle and the oval of this\\nfigure has been exaggerated purposely. The curve that Mars\\ndescribes is not exactly circular, but it is much less oval than\\nFig. 127.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0220.jp2"}, "219": {"fulltext": "MOTIONS OF THE PLANETS\u00e2\u0080\u0094 KEPLER S LA WS. 197\\na curve the orbit of Mars really was. It was not a circle\\nat any rate. He tried all kinds of curves and finally dis-\\ncovered that Mars, like every other planet, moved around\\nthe Sun in an ellipse and that the Sun was not at the\\ncentre of the ellipse, but at one of the foci.\\nFig 128.\u00e2\u0080\u0094 An Ellipse.\\nAn ellipse is a curve such that the siwi of the distances\\nof every point of the curve from two fixed points {the foci)\\nis a constant quantity.\\nThe student should draw a number of ellipses for practice. Drive\\ntwo tacks into a board at S and S Tie a string at S and the other\\nend of the string at S. Let the length of the string be SP -f- PS\\nPut a pencil at the point P and move the pencil round the curve,\\nalways keeping the string stretched tight. Wherever the pencil P\\nmay be the length SP plus the length S P is a constant quantity.\\nFor every point of the curve SP -f- S P a constant. Take a string\\nof a different length to start with and tie it to S and S and you will\\nget an ellipse of a different shape. Put the tacks S and S nearer\\ntogether and the ellipse will be of another shape, but it will still be\\nan ellipse.\\nADCP is an ellipse S and 8 are the foci. By the definition of\\nan ellipse SP -f- PS AG, and this is true for every point. S is\\nthe focus occupied by the Sun, the filled focus. AS is the least\\ndistance of the planet from the Sun, its perihelion distance; and A", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0221.jp2"}, "220": {"fulltext": "198 ASTRONOMY.\\nis the perihelion, that point nearest the Sun. C is the aphelion, the\\npoint farthest from the Sun. SA, SB, SO, SB, SP are radii vectores\\nat different parts of the orbit. A C is the major axis of the orbit 2a.\\nThe major axis of the orbit is twice the mean distance of the\\nplanet from the Sun, a. BB is the minor axis, 2b. The ratio of OS\\nto A is called the eccentricity of the ellipse. By the definition of the\\nellipse, again, BS+ BS AC 2a; and BS=BS a. BS* BO*\\nOS*, or OS a* b 1 The eccentricity of the ellipse is\\nOS _ Vgi-W\\n0A~ a\\nAfter years of laborious calculation Kepler discovered\\nthree laws governing the motion of the planets. (The\\nstudent should memorize these laws.)\\nThe first law of Kepler is\\nI. Each planet moves around the Sun in an ellipse,\\nhaving the Sun at one of its foci.\\nSuppose the planet to be at the points P, P 1? P a P\\nP 4 etc., at the times T, T x T T T K etc., in Fig. 129.\\nFig. 129. Kepler s Second Law.\\nSuppose the intervals of time T x T, T s T\u00e2\u0080\u009e T b T K\\nto be equal. Kepler computed the areas of the surfaces\\nP/SP,, P t SP 9 P i SP b and found that these areas were", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0222.jp2"}, "221": {"fulltext": "MOTIONS OF THE PLANETS\u00e2\u0080\u0094 KEPLER S LAWS. 199\\nequal also, and that this was true for each and every planet\\nin every part of its orbit. The second Jaw of Kepler is\\nII. The radius-vector of each planet describes equal areas\\nin equal times.\\nThese two laws are true for each planet moving in its\\nown ellipse about the Sun.\\nFor a long time Kepler sought for some law which\\nshould connect the motion of one planet in its ellipse with\\nthe motion of another planet in its ellipse. Finally he\\nfound such a relation between the mean distances of the\\ndifferent planets and their periodic times.\\nHis third law is:\\n177. The squares of the periodic times of the planets are\\nproportional to the cubes of their mean distances from the\\nSun.\\nThat is, if T 7 T^ T^ etc., are the periodic times of the\\ndifferent planets whose mean distances are 1} a a 3 etc.,\\nthen\\nT? T; a?\\n7V :77\\netc. etc.\\nIf T 3 and a are the periodic time and the mean distance\\nof the Earth and if T 3 1 year) be taken as the unit of\\ntime and a 3 1.000) be taken as the unit of distance,\\nthen for any other planet whose periodic time is T and\\nmean distance a\\nT* (its periodic time) 1 a z (the cube of its mean dist.) 1.\\nBut the periodic time of each planet was already known\\nfrom observation (see page 193); hence its mean distance\\ncan be determined because\\na 3 T or a (T)K\\nIf, in the last equation, we substitute the values of the\\nperiodic time of each planet in succession, expressed in", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0223.jp2"}, "222": {"fulltext": "200 ASTRONOMY.\\nyears and decimals of a year, we shall obtain the value of\\na, its mean distance from the Sun, expressed in terms of\\nthe Earth s mean distance 1.000.\\nFor Mercury,\\nVenus,\\nT t 0.24 years\\nT 0.62\\nand a x 0.39\\na 2 0.72\\nEarth,\\nT s 1.00\\na 1.00\\nMars,\\nT K 1.88\\na K 1.52\\nJupiter,\\ni Saturn,\\nT 6 11.86\\nT 6 29.46\\na b 5.20\\na 9.54\\nKepler s laws are true for the satellites as well as for\\nthe planets. Mars has two satellites, Photos and Deimos,\\nthat revolve in ellipses in periods T and T at mean dis-\\ntances a and a In their ellipses the line joining the\\nsatellite to Mars sweeps over equal areas in equal times\\nand (T Y (T {a (a\\nKepler s three laws give the dimensions of the orbits of\\nevery planet in terms of the Earth s distance 1.00.\\nThey do not explain why it is that the planets follow these\\norbits (this was not known until the time of Newton), but\\nthey enable us to calculate just where any planet will be in\\nits orbit at any time.\\nFor instance, suppose that Mars was at the place P at the time T\\nand we wished to know where it will be at the time T The whole\\narea of the ellipse is swept over by the radius-vector of Mars in 1.88\\nyears. We can calculate how much of an area will be swept over in\\nthe time T T. Then we can calculate what the angle at 8 of the\\nsector PSP must be to give this sector the calculated area. A line\\ndrawn from S to P making the calculated angle with $Pwill inter-\\nsect the orbit at the point P The planet will be at the point P (in\\na known celestial longitude) at the time T\\nElements of a Planet s Orbit. When we know a and b (the major and\\nminor semi-axes) for any orbit, the shape and size of the orbit is\\nknown.\\nKnowing a we also know T, the periodic time in fact a is found\\nfrom T by Kepler s law III.\\nIf we also know the planet s celestial longitude (Z) at a given epoch,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0224.jp2"}, "223": {"fulltext": "MOTIONS OF THE PLANETS\u00e2\u0080\u0094 KEPLER S LAWS. 201\\nsay December 31st, 1850, we Lave all the elements necessary for find-\\ning the place of the planet in its orbit at any time, as has just been\\nexplained.\\nFig. 130. To Calculate the Place of a Planet in its\\nOrbit at any Future Time.\\nThe orbit lies in a certain plane this plane intersects the plane\\nof the ecliptic at a certain angle, which we call the inclination i.\\nKnowing i, the plane of the planet s orbit is fixed. The plane of the\\norbit intersects the plane of the ecliptic in a line, the line of the nodes.\\nHalf of the planet s orbit lies below (south of) the plane of the\\necliptic and half above. As the planet moves in its orbit it must\\npass through the plane of the ecliptic twice for every revolution.\\nThe point where it passes through the ecliptic going from the south\\nhalf to the north half of its orbit is the ascending node; the point\\nwhere it passes through the ecliptic going from north to south is the\\ndescending node of the planet s orbit. If we have only the inclina-\\ntion given, the orbit of the planet may lie anywhere in the plane\\nwhose angle with the ecliptic is i. If we fix the place of the nodes,\\nor of one of them, the orbit is thus fixed in its plane. This we do\\nby giving the (celestial) longitude of the ascending node Q.\\nNow everything is known except the relation of the planet s orbit\\nto the sun. This is fixed by the longitude of the perihelion, or P.\\nThus the elements of a planet s orbit are\\ni, the inclination to the ecliptic, which fixes the plane of the\\nplanet s orbit;\\nQ, the longitude of the node, which fixes the position of the line of\\nintersection of the orbit and the ecliptic;", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0225.jp2"}, "224": {"fulltext": "202 ASTRONOMY.\\nP, the longitude of the perihelion, which fixes the position of the\\nmajor axis of the planet s orbit with relation to the Sun, and hence\\nin space;\\na and e, the mean distance and eccentricity of the orbit, which fix\\nthe shape and size of the orbit (see page 198);\\nT and M, the periodic time and the longitude at the epoch, which\\nenable the place of the planet in its orbit, and hence in space, to be\\nfixed at any future or past time.\\nThe elements of the older planets of the solar system are now\\nknown with great accuracy, and their positions for two or three cen-\\nturies past or future can be predicted with a close approximation to\\nthe accuracy with which these positions can be observed.\\nMoreover it was proved by two great French astronomers (La-\\ngrange and Laplace) about a hundred years ago that all the\\nplanets would always continue to revolve in or near the plane of the\\necliptic; that the eccentricity of each orbit might vary within narrow\\nlimits, but could never depart widely from its present value, and\\nfinally that the mean-distances of the planets would always remain\\nthe same as now. The Earth, for example, will always remain at the\\nsame average distance from the Sun as now, though by a change in\\nthe eccentricity its least and greatest distances from the Sun may be\\nslightly greater or less than at present. Hence there can never be\\nany great changes in the seasons of the Earth due to a change in its\\ndistance from the Sun.\\nIf the mean-distances of the planets remain essentially unchanged\\ntheir periodic times will also remain unchanged, by the 3d law of\\nKepler, so long as we consider the planets as rigid solids.\\nWhat is a planet s periodic-time? How can the relative dis-\\ntances of the planets from the Sun be determined What are the\\nthree laws of planetary motion discovered by Kepler Define an\\nellipse. Do Kepler s laws explain why the planets move in elliptic\\norbits why their radii- vectores describe equal areas in equal times?\\nwhy for any two planets T i TV a 3 a x 3 What are the elements\\nof a planet s orbit", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0226.jp2"}, "225": {"fulltext": "CHAPTEK X.\\nUNIVERSAL GRAVITATION.\\n25. The Discoveries of Sir Isaac Newton.\u00e2\u0080\u0094 Before the\\ntime of Sir Isaac Newton very little was known of the\\nlaws that govern the motion of bodies on the Earth. A\\nstone dropped from the hand falls to the ground. Why\\nNewton s answer was that the Earth attracted the stone\\ndownwards somewhat as a magnet attracts iron to itself.\\nThe Earth itself was made up of stones and soil. Why did\\nnot the stone attract the Earth upwards? Newton s\\nanswer was that the stone did, in fact, attract the Earth.\\nBut as the Earth had a mass of millions of tons and the\\nstone a mass of only a few pounds the motion of the Earth\\nupwards towards the stone was very small compared to the\\nmotion of the stone downwards to the Earth. It was too\\nsmall to be appreciable but the Earth moved nevertheless.\\nThe attraction was in proportion to the attracting mass, he\\nsaid.\\nEach particle of a huge mass, like that of the Earth,\\nwould attract the stone, and the whole of the Earth s\\nattraction would be the sum of all the particular attrac-\\ntions. The stone would also attract each one of the\\nEarth s particles, but as they were all joined together it\\ncould move no one of them without moving them all. If\\nthe Earth attracted a stone near its surface why should it\\nnot attract the Moon in the sky The Moon would be\\nattracted less because it was distant, but it would certainly\\nbe attracted, he said. There were reasons for believing\\n203", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0227.jp2"}, "226": {"fulltext": "204\\nASTRONOMY.\\nthat attractions grew less in proportion to the square of the\\ndistance, not in proportion to the simple distance.\\nHis reasoning was something like this: We see that there\\nis a force acting all over the Earth by which all bodies are\\ndrawn towards its centre. This force is called gravity. It\\nextends to the tops not only of the highest buildings, but\\nof the highest mountains. How much higher does it\\nFig. 131.\\nA stone in a sling is whirled round in the direction of the arrows in the\\ncircle CBA. At A the string breaks and the stone flies away in the\\ntangent AD. It would move away in that direction forever if the Earth\\ndid not attract it downwards.\\nextend Why should it not extend to the Moon If it\\ndoes, the Moon would tend to drop towards the Earth, just\\nas a stone thrown from the hand drops. As the Moon\\nmoves round the Earth in her monthly course, there must\\nbe some force drawing her towards the Earth; else she\\nwould fly entirely away in a straight line just as a stone\\nthrown from a sling would fly away in a straight line if the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0228.jp2"}, "227": {"fulltext": "Fig. 132.\u00e2\u0080\u0094 Sir Isaac Newton.\\nBorn 1642 died 1727.\\n205", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0229.jp2"}, "228": {"fulltext": "206 ASTRONOMY.\\nEarth did not attract it. Why should not the force which\\nmakes the stone fall be the same force which keeps the\\nMoon in her orbit\\nTo answer this question, it was necessary to calculate\\nthe intensity of the force which would keep the Moon her-\\nself in her orbit, and to compare it with the intensity of\\ngravity at the Earth s surface. It had long been known\\nthat the distance of the Moon was about sixty radii of the\\nEarth. If this force diminished as the inverse square of\\nthe distance, then at the Moon it would be only ^g^o as\\ngreat as at the Earth s surface.\\nExperiments at the Earth s surface had proved that a\\nbody fell 16 feet in a second of time. The Moon in her\\norbit ought then to fall towards the Earth (that is, ought to\\nbend away from a straight line) by -^-gVo P ar of 16 feet in\\neach and every second, or the Moon should bend away from\\na straight line (a tangent to her orbit) by about part of\\nan inch every second. Now the size of the Moon s orbit\\nwas known and its curvature was known. It was found\\nthat the orbit of the Moon did, in fact, deflect from the\\ntangent to the orbit by -fa part of an inch per second.\\nNewton proved this point by calculation, and from that\\ntime forward he felt sure that the force that kept the Moon\\nin its orbit about the Earth was a force of the same kind\\nas the gravity that made a stone fall to the Earth, and that\\nit was this very same force that kept all the planets in their\\norbits about the Sun.\\nTo prove that his idea was right it was necessary to prove\\nthat if the Sun attracted the planets just as the Earth\\nattracted the Moon the laws of Kepler would be a neces-\\nsary consequence. Newton made such a proof. He\\nproved strictly and mathematically that any two bodies\\nwhich attracted each other in proportion to their masses\\nand inversely as the square of their distances apart would\\nobey laws like those of Kepler. If one of the bodies was", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0230.jp2"}, "229": {"fulltext": "UNIVERSAL GBAYITATION. 207\\nvery large (like the San) and the other much smaller (like\\none of the planets) then it necessarily followed from the\\nsingle law of gravitation that\\nI. The planet would revolve about the Sun in an ellipse\\n(or in one of a set of curves of the same sort). II. The\\nradius-vector of the planet would describe equal areas in\\nequal times. And he further proved that if there were\\ntwo planets in the system the following law would be very\\nnearly true III. The squares of their periodic times would\\nbe proportional to the cubes of their mean distances from\\nthe Sun. These are the three laws which Kepler deduced\\nfrom observation. All the planets in the solar system obey\\nthese laws. All the planets obey the law of gravitation\\ntherefore.\\nKepler s laws were proved to be true by observation. Newton\\nshowed that if any planet moved about the sun so that its radius-\\nvector described equal areas in equal times then the planet obeyed a\\nforce that was directed always to the sun as a centre of force. Ift\\\\\\\\e\\npath of any planet was an ellipse (or if it were a parabola or hyper-\\nbola) then the central force must vary inversely as the square of the\\ndistance, and could vary in no other way. If all the planets were\\nbound together (as they are) by Kepler s third law, then all the plan-\\nets are acted on by one and the same kind of force. The amount of\\nforce acting on any planet depends on its distance from the Sun and\\non the mass of the Sun. Observations fixed the length of each plan-\\net s year and its distance from the Sun.\\nFrom these data the mass of the Sun could be calculated in terms\\nof the Earth s mass. Not only were these things true for all the\\nplanets they governed the motions of satellites about their primary\\nplanet. The Moon revolves about its primary, the Earth, in obe-\\ndience to its attraction but it is likewise attracted by the Sun and\\nhence its orbit is perturbed. Newton calculated perturbations of\\nthe Moon s motion that had been known as facts of observation since\\nthe time of Hipparchus, and others that had been observed by Tycho\\nBrahe and Flamsteed, and he accounted for all these observed facts\\nby his theory. He also calculated some of the perturbations of the\\npath of one planet by the attraction of other planets.\\nUp to Newton s day the motions of comets had been simply mys-\\nterious. He showed that they moved according to Kepler s laws,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0231.jp2"}, "230": {"fulltext": "208 ASTBONOMY.\\nusually in parabolas, not in ellipses. He calculated the shape that a\\nrotating fluid mass should assume and from this deduced the figure\\nof the Earth. He showed that it was a spheroid, not a sphere, and\\nproved that the precession of the equinoxes, observed as a fact by\\nHipparchus, and unexplained since his time, was a mere result of\\nthe spheroidal shape of the Earth. The Tides another mystery\\nwere explained by Newton as a result of the Moon s attraction of\\nthe waters of the Ocean.\\nHis discoveries in pure mathematics are only second in importance\\nto his discoveries in celestial mechanics. The binomial theorem was\\ndiscovered by him (it is engraved on his tomb in Westminster\\nAbbey). The Differential Calculus is his invention. He made most\\nimportant discoveries in optics also.\\nThe epigram of the English poet Pope expresses the feeling of\\nawed amazement with which the men of his own time regarded this\\nmighty genius\\nNature and Nature s laws lay hid in Night\\nGod said let Newton be and all was Light.\\nLet us see what Newton thought of himself. Towards the end\\nof his life he said, I know not what the world will think of my\\nlabors, but to myself it seems that I have been but as a child playing\\non the seashore now finding some pebble rather more polished and\\nnow some shell rather more agreeably variegated than another, while\\nthe immense ocean of Truth extended itself, unexplored, beyond me.\\nIn science his name is venerated and honored by all those who can\\nappreciate his marvellous genius. His greatest effect on Mankind\\nhas been to set before them a new path for their thoughts to follow.\\nSince his day men have a new view-of-the-world, and his discoveries\\nhave influenced the thoughts, beliefs, and ideals of men and nations\\nas powerfully and as effectively as those of Plato, Aristotle, Co-\\npernicus, and Galileo. We should not now. think as we all do if\\nour thoughts did not run in channels first opened by him.\\nAll the motions of all the bodies in the solar system were\\ndeduced by Newton from one single law the law of\\nUniversal Gravitation. The discoveries of Ptolemy, of\\nCopekntcus, of Keplee, and of all other astronomers were\\nnothing but special cases of one universal law. Ptolemy\\nand other great astronomers before his time had mapped\\nout the apparent courses of the planets in the sky with", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0232.jp2"}, "231": {"fulltext": "UNIVERSAL GRAVITATION. 209\\ndiligence and with accuracy. Copernicus had shown\\nthat these apparent paths were described because the\\nreal centre of the motion was the Sun. Kepler had\\nproved that the paths of the planets about the Sun were not\\ncircles as Copernicus supposed, but ellipses; and he gave\\nthe laws according to which the planets moved in their real\\norbits.\\nNewton started with the simple fact of gravity (Latin\\ngravitas heaviness). He said a body is heavy because\\nthe Earth attracts it. The Earth (like every mass) at-\\ntracts all other bodies in the Universe, the nearer bodies\\nmore, the distant bodies less. The attraction is directly\\nproportional to the mass; it is inversely proportional to the\\nsquare of the distance. If this law is true everywhere (as\\nexperiment proves it to be true on the Earth) then all\\nKepler s laws are a necessary consequence of it. One\\nsingle law accounts for every motion in the solar system.\\nProbably this law accounts for all the motions of the stars\\nalso.\\nThe student should memorize the law of universal gravi-\\ntation in the form that Newton gave to it as follows\\nEvery particle of matter in the universe attracts every\\nother particle with a force directly as the masses of the two\\nparticles and inversely as the square of the distance between\\nthem.\\nTo thoroughly understand the discoveries of Newton it is neces-\\nsary to study Mechanics or the science that treats of the action of\\nforces on bodies. This science requires a mathematical treatment\\ntoo difficult and too long to be given here. After the Mechanics of\\nterrestrial bodies is understood it must be applied to the special case\\nof the heavenly bodies Celestial Mechanics. Only the barest out-\\nline of Newton s achievements can be given in this place. The fol-\\nlowing paragraphs may help the student to understand the nature\\nof the questions involved.\\nIf we represent by m and m the masses of two attracting bodies,\\nwe may conceive the body m to be composed of m particles, and the\\nother body to be composed of rri particles. Let us conceive that", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0233.jp2"}, "232": {"fulltext": "210 ASTRONOMY.\\neach particle of one body attracts each particle of the other with a\\nforce that varies as Then every particle of m will be attracted\\nby each of the m particles of the other, and therefore the attractive\\nm!\\nforce on each of the m particles will vary as Each of the m\\nparticles being equally subject to this attraction, the total attractive\\nforce between the two bodies will vary as\\nEach of the two masses attracts the other by a force varying\\nmm\\nas\\nr l\\nIf a straight stiff rod whose length was r could be slipped in\\nbetween the two masses m and m the pressure on either end of\\n-m\\nFig. 133.\\nthe rod would be the same. It would be a pressure proportional\\nmm\\nto ~Za-\\nWhen a given force acts upon a body, it will produce less motion\\nthe larger the body is, the accelerating force being proportional to\\nthe total attracting force divided by the mass of the body moved.\\nTherefore the accelerating force which acts on the body m and\\nm\\nwhich determines the amount of motion, will be and conversely\\nthe accelerating force acting on the body m will be represented by\\nm!\\nthe fraction -j. If m is very large (as in the case of the Sun) and\\nif m is relatively small (as in the case of a planet), the motion of the\\nplanet will be determined by the Sun s accelerating force while the\\nSun will be but little affected by the accelerating force of the planet.\\nIt makes no difference at all of what substances m and m are\\nmade up. A mass of gas (as a comet) attracts in proportion to its\\nquantity of matter, just as a mass of lead attracts in proportion to\\nits quantity of matter.\\nIt is in this respect, especially, that the force of gravitation differs\\nfrom a force like magnetism. A magnet will attract iron but not\\nwood. But both wood and iron are heavy.\\nThe attraction of a spherical body on a particle outside of itself\\nis the same as if the whole mass of the spherical body were con-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0234.jp2"}, "233": {"fulltext": "UNIVERSAL GRAVITATION. 211\\ncentrated at its centre. We may treat the problems of Celestial\\nMechanics as if the Sun and all the planets were mere points, the\\nwhole mass of each body being- concentrated at their centres. The\\nattraction of the Earth for bodies on its surface is the same as if the\\nearth were a mere point, its whole mass being concentrated at\\nits centre.\\nA word may be said on the variation of forces inversely as the\\nsquare of the distance. Suppose we take the force of gravitation.\\nAt a distance of one radius of the Earth from the Earth s centre (at\\nthe Earth s surface) let us call its intensity one at a distance of two\\nradii (some 4000 miles above the Earth s surface therefore) it will\\nbe at a distance of 3 radii it will be and so on.\\nDistances =1,2,3,4,5, 6 100 1000\\nForces =1 i i, A A\u00c2\u00bb A nmnr tot W\\nAn excellent practical example of a quantity that varies inversely\\nas the square of the distance may be had by watching the headlight\\nof a tram-car as it approaches you. When it is five blocks off the\\nintensity of the light is ^jth, four blocks off yjth, three blocks\\ntwo blocks of the intensity at a distance of a block. Gravitation\\nvaries according to a similar law.\\nGravitating force seems to go out from every particle of matter in\\nthe Universe in all directions somewhat as rays of light stream out\\nin all directions from a lamp. It streams out in straight lines. What-\\never is in its way is attracted. If a planet is there it attracts the\\nplanet. If nothing is there no attraction is exerted on empty space.\\nThe rays of gravitation (so to speak) pass directly through a body and\\na second body beyond it is attracted just as if the first body were not\\nthere. There is no gravitational shadow, as it were.\\nA B C\\nIf A were a lamp and B and G two screens, the screen B would be\\nlighted and the screen C would be in shadow. But if A is a heavy\\nbody it will attract a body at B and another body G beyond it just as\\nif B were not there.\\nMoreover, the storehouse of gravitational attraction in a heavy body\\nis never exhausted. The sun attracts a planet at a certain distance\\njust as much in July as in the preceding January, just as much in\\n1907 as in 1620.\\nIt requires time for the light of the Sun to travel across the space\\nthat separates it from the Earth. A beam of light leaves the Sun\\nand does not arrive at the .Earth for 8 m 19 s it does not arrive at\\nJupiter for 43 m 15 s It takes these times to pass over the intervening", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0235.jp2"}, "234": {"fulltext": "212 ASTRONOMY.\\nspaces. But the gravitating effect of the Sun traverses these spaces\\ninstantaneously, so far as we now know. When gravitation is con-\\nsidered in this way, as a force inherent in a body, as sourness is in-\\nherent in a fruit, a recital of its properties sounds like a fairy-tale.\\nThe explanation of gravitation is not yet known. This force, like\\nthe force of magnetism and other forces, is a mystery. When its ex-\\nplanation comes to be known it will probably be found that a heavy\\nbody must not be considered to be in empty space, but in a space\\nfilled with some substance like the ether which transmits light. The\\nbody influences the ether and sets up strains and stresses within it.\\nThese stresses are transmitted in all directions with immense (prob-\\nably not infinitely great) velocities. When these stresses meet a\\nsecond body they act upon it to produce the phenomena of gravita-\\ntion.\\nA word may also be said as to the intensity of the force of gravi-\\ntation. The popular notion is that gravitation is a very powerful\\nforce. This is because we live on an earth which is very large in\\ncomparison to our own size, and to the sizes of objects that we use in\\nour daily life. In reality gravitation may be called a feeble force\\ncompared to such a force as the expansion of water when it freezes\\nand bursts the stout pipes in which it is contained. Two masses M\\nand M each weighing 415,000 tons, a mile apart, attract each other\\nwith a force of one pound. Imagine two huge cubes of iron, each\\nweighing 415,000 tons. If at a mile s distance they only exert a force\\nof one pound we must decide that the force of gravitation is feeble\\nrather than powerful. If M and M were two miles apart their mu-\\ntual attraction would be only four ounces. If M was doubled in size,\\ntheir attraction at one mile s distance would be two pounds if both\\nM and M were doubled their attraction would be four pounds, and\\nso on. These effects one would call small rather than large.\\nThe discoveries of Newton in relation to the force of gravitation\\nthat binds the planets together and that determines every circum-\\nstance of every motion of everything on the Earth lead to conclu-\\nsions like those just set down. What the true nature of this force\\nis we do not know any more than we know the true nature of the\\nforces of chemical affinity and the like. No doubt a complete under-\\nstanding of it will some day be reached, and what now seems mar-\\nvellous will then be simple. There is no doubt that the motions of\\nevery particle on the Earth and of every planet in the solar system\\nare obedient to this law. The simple proof is that the motions of\\nplanets, comets, and of many stars have been calculated beforehand\\non this theory and that observation has subsequently verified the\\npredictions. The pages of the Nautical Almanac (see page 150) are", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0236.jp2"}, "235": {"fulltext": "UNIVERSAL GRAVITATION.\\n213\\nnothing but a series of such predictions that are afterwards verified\\nover and over again in the minutest particular. The place that a\\nplanet will occupy in the sky a century hence can be predicted\\nnearly as accurately as the planet can then be observed. Not only\\nthis, but the paths of thousands of projectiles to be fired from can-\\nnon have been calculated beforehand, and these predictions have been\\nsubsequently verified by experiment. Every swing of a pendulum\\nand every fall of a heavy body is obedient to this law, and in thou-\\nsands and thousands of similar cases the law has been accurately\\nverified by experiment.\\nMutual Actions of the Planets Perturbations. Kepler s laws\\nwould be accurately followed in any system of only two heavy\\nbodies, as the Sun and any one planet, Mars for example. If a third\\nbody exists, the Earth for instance, it will attract the Sun and also\\nMars. The Sun and Mars will likewise attract the Earth. The\\nmotion of Mars about the Sun will not be exactly the same in a sys-\\ntem of three bodies as in a system of two.\\nThe mass of the Sun is so very much\\ngreater than the mass of the Earth that\\nMars will travel in an orbit almost the\\nsame as its undisturbed orbit almost, but\\nnot quite. The Earth will produce slight\\ndisturbances\u00e2\u0080\u0094 pertui bations they are called\\nin the orbit of Mars, and these perturba-\\ntions can be exactly calculated from New-\\nton s law. The orbit of the Earth will\\nalso be perturbed by Mars.\\nEach of the planets will act on every\\none of the other planets to alter its motion.\\nThese disturbances in the solar system are\\nsmall, because the Sun s mass is so very\\nlarge compared to the mass of the dis-\\nturbing body. Even Jupiter, the largest\\nof the planets, has a mass less than T oVo\\nthe Sun s mass.\\nThe Vertical Line.\u00e2\u0080\u0094 The direction FlG 134 _ A Pendttlum\\nup and down, the vertical direction, is at Rest Hangs Ver-\\ndefined for any observer by the line TICALLY\\nin which a pendulum at rest hangs. The pendulum is at-\\ntracted by the whole Earth and if the Earth were a sphere\\nit would always point to the Earth s centre. As the Earth", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0237.jp2"}, "236": {"fulltext": "214\\nASTRONOMY.\\nis a spheroid (its meridians being ellipses and not circles)\\na pendulum at rest at any point of the Earth s surface does\\nnot point exactly to the centre, although its direction is\\nFig. 135. A Pendulum at Rest on a Spherical Earth\\nPoints nearly to the Centre of the Earth.\\nnever far from that of the Earth s radius. (The radius of\\nthe Earth and the pendulum never make an angle of more\\nthan 12 of arc a fifth of a degree with each other.)\\nThe zenith of an observer may now be defined as that\\npoint over his head where a pendulum at rest at his station\\nwould meet the celestial sphere if the pendulum were in-\\ndefinitely long. A pendulum at rest always lies in the line\\nof joining an observer s zenith and nadir.\\nRemarks on the Theory of Gravitation.\\nThe real nature of the discovery of Newton is frequently\\nmisunderstood. Gravitation is sometimes spoken of as if\\nit were a theory of Newton s, now very generally received,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0238.jp2"}, "237": {"fulltext": "UNIVERSAL GRAVITATION. 21 5\\nbat still liable to be ultimately rejected as a great many\\nother theories have been.\\nNewton did not discover any new force, but only showed\\nthat the motions of the heavenly bodies conld be accounted\\nfor by a force which we all know to exist. Gravitation is\\nthe force which makes all bodies here at the surface of the\\nEarth tend to fall downward; and if any one wishes to\\nsubvert the theory of gravitation, he must begin by proving\\nthat this force does not exist. This no one would think of\\ndoing. What Newton did was to show that this force,\\nwhich, before his time, had been recognized only as acting\\non the surface of the Earth, really extended to the\\nheavens, and that it resided not only in the Earth itself,\\nbut in the heavenly bodies also, and in each particle of\\nmatter, wherever situated. To put the matter in a terse\\nform, what Newton discovered was not gravitation, but\\nthe universality of gravitation.\\nWhat was tlie principal work of Ptolemy and bis predeces-\\nsors What was the discovery of Copernicus What was Kep-\\nler s discovery What was the greatest discovery of Newton\\nGive Newton s law of universal gravitation in his own words. Did\\nNewton discover gravitation What, in fine, was his discovery\\nDefine the zenith of an observer his nadir.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0239.jp2"}, "238": {"fulltext": "CHAPTER XL\\nTHE MOTIONS AND PHASES OF THE MOON.\\n26. The Moon makes the circuit of the heavens once in\\neach (lunar) month. She revolves in a nearly circular\\norbit around the Earth (not the Sun) at a mean distance\\nof 240,000 miles. At certain times the new Moon, a\\nslender crescent, is seen in the west near the setting Sun.\\nOn each succeeding evening we see her further to the east,\\nso that in two weeks she is exactly opposite the Sun, rising\\nin the east as he sets in the west. Continuing her course\\ntwo weeks more, she has approached the Sun from the west,\\nand is once more lost in his rays. At the end of twenty-\\nnine or thirty days, we see her again emerging as new\\nMoon, and her circuit is complete. The Sun has been\\napparently moving towards the east among the stars during\\nthe whole month at the rate of 1\u00c2\u00b0 daily (see page 165), so\\nthat during the interval from one new Moon to the next\\nthe Moon has to make a complete circuit relatively to the\\nstars, and to move forward some 30\u00c2\u00b0 further to overtake\\nthe Sun. The revolution of the Moon among the stars is\\nperformed in about 27\u00c2\u00a3 days, so that if the Moon is very\\nnear some star on March 1, for example, we shall find her\\nin the same position relative to the star on March 28.\\nThe Moon s revolution relative to the stars is performed\\nin 27| days; relative to the Sun in 29\u00c2\u00a3 days. Her periodic\\ntime in her orbit about the Earth is 27^ days therefore.\\nPhases of the Moon. The Moon is an opaque body and\\nis formed of materials something like the rocks and soils of\\n216", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0240.jp2"}, "239": {"fulltext": "THE MOTIONS AND PHASES OF THE MOON. 217\\nthe Earth. Like the planets, she does not shine by her own\\nlight, but by the light of the Sun, which is reflected from\\nher surface much as sunlight would be reflected from a\\nrough mirror. As the Moon, like the Earth, is a sphere,\\nonly half of her globe can be illuminated at a time namely,\\nthat half turned towards the Sun.\\nFig. 136.\u00e2\u0080\u0094 The ^!oon (M) in her Orbit Round the Earth (E).\\nHalf of each body is illuminated by the Sun. The Sun is not shown in\\nthe drawing. If it were to be inserted it would have to be on the right-\\nhand side of the picture about thirty-five feet distant from E.\\nWe can see only half of the Moon namely, that half that\\nis turned toward us. An eye at 8 (on the left-hand side\\nof the page) could see half of the Moon if it were illumi-\\nnated. But as the dark side is turned toward S an eye\\nplaced there would see nothing. No light would come to\\nit. An eye at V would see the Moon as a bright circle.\\nThe half turned toward V is fully illuminated.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0241.jp2"}, "240": {"fulltext": "218\\nASTRONOMY.\\nIn this figure the central globe is the Earth; the circle\\naround it represents the orbit of the Moon. The rays of\\nthe Sun fall on both Earth and Moon from the right, the\\nFig. 137.\u00e2\u0080\u0094 The Phases op the Moon Explained.\\nSun being some thirty feet away (on the scale of the draw-\\ning) in the line BA. For the present purpose we suppose\\nboth Earth and Sun to be at rest and the Moon to move\\nround her orbit in the direction of the arrows. Eight\\npositions of the Moon are shown around the orbit at A, E,\\n0, etc., and the right-hand hemisphere of the Moon is\\nilluminated in each position. Outside of these eight posi-\\ntions are eight pictures showing how the Moon looks as\\nseen from the Earth in each position.\\nAt A it is new Moon, the Moon being nearly between\\nthe Earth and the Sun. Its dark hemisphere is then", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0242.jp2"}, "241": {"fulltext": "THE MOTIONS AND PHASES OF THE MOON. 219\\nturned towards the Earth, so that it is entirely invisible.\\nThe Sun and Moon then rise and set together. They are\\nin the same direction in space.\\nAt E the observer on the Earth sees about a fourth of\\nthe illuminated hemisphere, which looks like a crescent, as\\nshown in the outside figure. In this position a great deal\\nof light is reflected from the Earth to the Moon and back\\nagain from the Moon to the Earth, so that the part of the\\nMoon s face not illuminated by the Sun shines with a\\ngrayish light. At C the Moon is in her first quarter. The\\nMoon is on the meridian about 6 p.m. She is about 90\u00c2\u00b0\\n(6 hours) east of the Sun. When the Sun is setting the\\nMoon is therefore near the meridian. At G three-fourths\\nof the hemisphere that is illuminated by the Sun is visible\\nto the observer; and at B the whole of it is visible. The\\nMoon at B is exactly opposite to the Sun and it is then\\nfull Moon. The full Moon rises at sunset. As the\\nMoon moves to H, D, F, the phases change in a reverse\\norder to those of the first half of the month.\\nThe Tides. \u00e2\u0080\u00a2The phenomena of the tides are familiar 1o those who\\nlive near the seashore. Twice a day the waters of the ocean rise\\nhigh on the beach. Twice a day they recede outwards. The first\\nhigh tide occurs at any place (speaking generally) about the time\\nwhen the Moon is on the meridian of that place. About six hours\\nlater comes low tide about twelve hours after the first high\\ntide comes a second hi^h tide, and finally, about six hours after\\nthis a second low tide. The Moon revolves about the Earth once\\nin about 25 h (not 24 h for it is moving eastwards among the stars\\nnearly 15\u00c2\u00b0 daily.\\nIn figure 138 suppose to be the centre of the Earth and m a\\nplace on its surface. Suppose, for simplicity, that the whole Earth\\nis surrounded by a shallow shell of water. There is a high tide at\\nm when the Moon (M) is on the meridian of m. Let us see why this\\nis so. The Earth is attracting the Moon, and by its attraction the\\nMoon is kept in her orbit.\\nThe Moon moves towards the Earth a little every second.\\nThe Moon likewise attracts every particle of the earth, solid and\\nfluid alike. The fluid particles nearest M (at m) are perfectly free", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0243.jp2"}, "242": {"fulltext": "220\\nASTRONOMY.\\nto more, and tliey are therefore headed up into a kind of a wave\\nwhose crest is at m. The particles of water near rri and m! are\\ndrawn towards m. The Moon at M also attracts the solid body of\\nHI\\n\u00e2\u0096\u00a0^V K\\nFig. 138. The Tides of the Ocean are Produced by the\\nMoon s Attraction.\\nthe Earth with a force that is inversely proportional to the square of\\nthe distance MO to and the Earth moves towards the Moon\\n{MOy\\na little every second. The Moon also attracts the fluid particles\\nnear m in the further hemisphere of the Earth, with a force pro-\\nportional to If they were a solid separate from the main\\nbody of the Earth, they would move less than the rest of the Earth,\\nbecause they are less attracted, being more distant.\\nThe Moon at M attracts the solid Earth as a whole, more than it\\nattracts the waters of the distant hemisphere m m m The solid\\nEarth, which must move as a whole, moves towards M in consequence\\nof its attraction more than the waters of the distant hemisphere,\\nwhich are therefore left behind as it were, heaped up into a kind of\\nwave whose crest is at m opposite to the moon M. The shape of\\nthe tidal ellipsoid is shown by the shaded area in the figure.\\nWhen the moon is at M on the meridian of a place at m, the tidal\\nellipsoid is as drawn. There is high tide at m, low tide at a place\\n90\u00c2\u00b0 distant (ra high tide at m low tide again at m Whenever", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0244.jp2"}, "243": {"fulltext": "THE MOTIONS AND PHASES OF THE MOON. 221\\nthe moon is on the meridian of any place such an ellipsoid is formed.\\nAs the Moon moves round the earth each day from rising to setting,\\nthis ellipsoid moves with it.\\nIn an hour the moon will have moved to V and the crest of the\\nwave to 1. The tide will be high at 1 and falling at ra. As the\\nmoon moves by the diurnal motion to 2 3 M M the crest will\\nmove with it. When the moon is at M it is low water at m and\\nm When the moon is at M it is again high water at ra and\\nso on.\\nIf we suppose M to be the sun, a similar set of solar tides will be\\nproduced every 24 hours. The actual tide is produced by the super-\\nposition of the solar and lunar tides.\\nThe foregoing explanation relates to an Earth covered by an ocean\\nof uniform depth. To fit it to the facts as they are a thousand cir-\\ncumstances must be taken into account which depend on the modify-\\ning effects of continents and islands, of deep and shallow seas, of\\ncurrents and winds. Practically the time of high tide at any station\\nis predicted in the Tide-Tables by adding to the time of the\\nMoon s transit over the meridian a quantity that is determined from\\nobservation and not from theory.\\nDescribe the changes of the shape of the Moon s disk from\\nnew moon to the next new moon. Does the Moon shine by her own\\nlight? What part of the globe of the Moon is illuminated by the\\nSun About what time does the new moon rise the full moon", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0245.jp2"}, "244": {"fulltext": "CHAPTER XII.\\nECLIPSES OF THE SUN AND MOON\\n27. The Earth s Shadow the Moon s Shadow Lunar\\nEclipses Solar Eclipses Occultations of Stars by the\\nMoon. A point of light L sends out rays in every direc-\\ntion. If an opaque disk VO is interposed in the path of\\nsome of these rays it will form a shadow on the side\\nfurthest from the light. All the space between the lines\\nFig. 139.\\n-The Shadow of a Disk VO Formed by a Point op\\nLight L and Projected on a Screen TS.\\nLV, LO, and other lines drawn from the point L to the\\nborders of VO will be dark. The regiou VOSTis dark and\\nit is called the shadow of VO, If the source of light is\\nnot a mere point the shadow is not so simple. The candle-\\nflame AB shines on the sphere DC and illuminates one-\\nhalf of it. The region to the right of the sphere and\\nbetween the lines BDS and ACS receives no light at all.\\nIf a screen is interposed the shadow is shown quite black\\nat S S. None of the region to the right of the sphere\\nbetween the lines AP and BP is fully illuminated. Some\\nof ihe candle-flame is cut off from every part of this region\\n222", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0246.jp2"}, "245": {"fulltext": "ECLIPSES OF THE SUN AND MOON.\\n223\\nby the sphere. Let the student mark a point half way\\nfrom S to P (call it a). From a draw a line tangent to\\nthe sphere near C and prolong it till it meets the candle-\\nFig. 140.\u00e2\u0080\u0094 The Shadow\u00e2\u0080\u0094 Umbra and Penumbra\u00e2\u0080\u0094 of a Sphere\\nFormed by a Candle.\\nflame (at a point that we may call b). Draw also the lme\\na A. The point a is illuminated by part of the flame (the\\npart between b and A) and it receives no light from the\\npart of the fl me between b and B. It is impossible to\\ndraw a straight line through a that will meet the flame\\nbetween b and B unless such a line passes through the\\nsphere DC The region DS SC is the umbra of the\\nshadow; the region DP S CSP, etc., is the penumbra.\\nIf the shadow is received on a screen the circle SS is often\\ncalled the umbra and the riug PSP S the penumbra.\\nThe Shadow of the Earth. In figure 141 S is the Sun,\\nE the Earth. The cone BVB is the umbra; that part of\\nthe cone BPB P which is not umbra is the penumbra.\\nDimensions of the Earth s Shadow. Let us investigate the distance\\nEV from the centre of the Earth to the vertex of the shadow. The\\ntriangles VEB and VSD are similar, having a right angle at B and\\nat D. Hence\\nVE: EB VS: SD ES: (SD EB).", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0247.jp2"}, "246": {"fulltext": "224\\nASTRONOMY.\\nSo if we put\\nI VE, the length of the shadow measured from the centre of\\nthe Earth,\\nr ES t the radius -vector of the Earth, 92,900,000 miles,\\n11= SB, the radius of the Sun, 483,000\\nft EB the radius of the Earth, 4000\\nwe have\\nES X EB rft\\nl VE\\nSB-EB B- p\\nFig. 141.\u00e2\u0080\u0094 Dimensions of the Shadow of the Earth.\\nThat is, I is expressed in terms of known quantities, and thus is\\nknown.\\nIts length is about 866,000 miles.\\nFig. 142.-^45 is the Ecliptic OB is the Moon s Orbit.\\nThe three dark circles on AB are three positions of the Earth s shadow.\\nSometimes the Moon is totally eclipsed as at G, sometimes partially\\neclipsed as at F, sometimes she just escapes eclipse as at E.\\nEclipses of the Moon. The mean distance of the Moon\\nfrom the Earth is about 238,000 miles and the Moon often\\npasses through the Earth s shadow-cone {EV). While", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0248.jp2"}, "247": {"fulltext": "ECLIPSES OF TEE SUN AND MOON. 225\\nthe Moon is within that cone none of the light of the Sun\\ncan reach her surface and she is said to be eclipsed.\\nIf the Moon moved exactly in the plane of the ecliptic\\nshe would pass through the Earth s shadow-cone at every\\nfull Moon (for it is at full Moon that the Sun and Moon\\nare oh opposite sides of the Earth) and would be totally\\neclipsed once every lunar month. The Moon s orbit is,\\nhowever, inclined to the ecliptic at an angle of about 5\u00c2\u00b0,\\nand therefore she often escapes eclipse, as is shown by the\\ndiagram. As a matter of fact it is very seldom that more\\nthan two lunar eclipses occur in any calendar year.\\nEclipses of the Moon are calculated beforehand and the phases are\\nprinted in the almanac. Supposing the Moon to be moving around\\nthe Earth from below upward in figure 141, its advancing edge first\\nmeets the boundary B P of the penumbra. The time of this ocur-\\nrence is given in the almanac as that of Moon entering penumbra.\\nA small portion of the sunlight is then cut off from the advancing\\nedge of the Moon, and this amount constantly increases until the\\nedge reaches the boundary B V of the shadow. Tbe eye can\\nscarcely detect any diminution in the brilliancy of the Moon until\\nshe has almost touched the boundary of the true shadow. The\\nobserver must not, therefore, expect to detect the coming eclipse\\nuntil very nearly the time given in the almanac as that of Moon\\nentering shadow. As the Moon enters the true shadow the advancing\\nportion of the lunar disk will be entirely lost to view. It takes the\\nMoon about an hour to move over a distance equal to her own diam-\\neter, so that if the eclipse is nearly central the whole Moon will be\\nimmersed in the shadow about an hour after she first strikes it.\\nThis is the time of beginning of total eclipse. So long as only a\\nmoderate portion of the Moon s disk is in the shadow, that portion\\nwill be entirely invisible, but if the eclipse becomes total the whole\\ndisk of the Moon will nearly always be visible, shining with a red\\ncoppery light,\\nThis is owing to the refraction of the Sun s rays by the lower\\nstrata of the Earth s atmosphere. We shall see hereafter that if a\\nray of light DB (see Fig. 141) passes from the Sun to the Earth, so\\nas just to graze the latter, it is bent by refraction more than a degree\\nout of its course. At the distance of the Moon the whole shadow of\\nthe Earth is filled with this refracted light. Some of it is reflected\\nback to the Earth, and as it has passed twice through the Earth s", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0249.jp2"}, "248": {"fulltext": "226 ASTBONOMY.\\natmosphere the light is red for the same reason that the light of the\\nsetting Sun is red.\\nThe Moon may remain enveloped in the shadow of the Earth\\nduring a period ranging from a few minutes to nearly two hours,\\naccording to the distance at which she passes from the axis of the\\nshadow and the velocity of her angular motion. When she leaves\\nthe shadow, the phases which we have described occur in reverse\\norder.\\nIt very often happens that the Moon passes through the penumbra\\nof the Earth s shadow without touching the shadow at all. The\\ndiminution of light in such cases is scarcely perceptible unless the\\nMoon at least grazes the edge of the shadow.\\nEclipses of the Sun. The shadow of the Earth falling\\nupon the Moon cuts off the Sun s light from it and causes\\na lunar eclipse. The shadow of the Moon falling on a part\\nof the Earth cuts off the light of the San from all observers\\nin that region of the Earth and causes a solar eclipse.\\nFig. 143. Dimensions of the Shadow of the Moon.\\nIn this figure let 8 represent the Sun, as before, and let\\nE represent the Moon. The cone B VB is now the umbra\\nof the Moon s shadow. We wish to know the length of\\nthe Moon s shadow VE. By a method similar to that\\ngiven on page 224, using accurate values of the different\\nquantities, it is found that VE at new Moon is about\\n232,000 miles. The average distance of the centre of the\\nMoon from the centre of the Earth is about 239,000 miles\\n(or from the centre of the Moon to the surface of the Earth", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0250.jp2"}, "249": {"fulltext": "ECLIPSES OF TEE SUN AND MOON. 227\\nabont 235,000 miles), and hence generally the Moon s\\nshadow will not quite reach to the Earth s surface and\\ngenerally there will be no solar eclipse at new Moon. If\\nthe Moon s orbit were a circle with a radius of 239,000\\nmiles we should have no solar eclipses at all. It is, how-\\never, an ellipse, and at favorable times (that is when the\\nMoon s shadow is long enough and when it points at the\\nEarth) the Moon s shadow may reach the Earth and even\\nbeyond it. At such times the Sun s light will be cut off\\nfrom all observers on the Earth within the shadow and a\\nsolar eclipse will occur. The conditions at such favorable\\ntimes are illustrated by the figure.\\nFig. 144.\\nThe Sun is eclipsed to all observers on the Earth within the shadow of\\nthe new moon (A). The full Moon is eclipsed whenever it passes through\\nthe Earth s shadow (B).\\nIt is clear that all observers on the Earth within the\\numbra of the Moon s shadow at A cannot see the Sun at\\nall. To them the Sun will be totally eclipsed. Observers\\non the Earth within the penumbra of the Moon s shadow\\n(see the figure) will see a part of the Sun only. To such\\nobservers the Sun will be partially eclipsed.\\nThe diameter of the Moon s umbra at the surface of the\\nEarth is seldom more than 160 miles. It is usually much\\nless. Observers within this umbra see a total solar eclipse.\\nAs the Moon moves in its orbit at the rate of over 2000\\nmiles per hour (which is about twice the velocity of a\\ncannon-ball) the shadow moves correspondingly. It sweeps", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0251.jp2"}, "250": {"fulltext": "228 ASTRONOMY.\\nover the surface of the Earth in a curved line or belt. The\\nobservers within this belt see the total eclipse one after\\nanother. At any one place the totality cannot last more\\nthan 8 minutes and it usually lasts much less than this.\\nAt the total solar eclipse of July, 1878, for example, the shadow of\\nthe Moon travelled diagonally across North America from Behring s\\nStraits through Alaska west of the Rocky Mountains of British Co-\\nlumbia and entered the United States not far east of Vancouver.\\nFrom thence the shadow crossed Washington, Idaho, the south-\\nwestern part of Wyoming, the State of Colorado (near Denver), the\\nState of Texas, and, curving across the Gulf of Mexico, traversed\\nCuba. The duration of totality was about 3 minutes near Van-\\ncouver, about 2| minutes near Galveston. The shadow-path of the\\ntotal solar eclipse of May 28, 1900, is described in Chapter XVI.\\nIn order to see a total eclipse an observer must station\\nhimself beforehand at some point of the Earth s surface\\nover which the shadow is to pass. These points are gen-\\nerally calculated some years in advance, in the astronomical\\nephemerides.\\nEclipses of the Sun are useful to astronomy because\\nduring an eclipse the Sun s light is cut off from the Earth s\\natmosphere and we have a short period of darkness during\\nwhich the surroundings of the Sun can be examined with\\nthe spectroscope or with the photographic camera. Great\\ndiscoveries have been made at these times, as we shall see.\\nEclipses are useful to history and to chronology because they\\nafford a precise means of fixing dates. Total solar eclipses\\nare so impressive (see Chapter XVI for a description of the\\nphenomena) that they are often recorded in ancient annals.\\nCalculation can fix the date at which such an event was\\nvisible, and thus render a service to chronology. Lunar\\neclipses are often serviceable in the same way.\\nThere is another way of looking at the problem of solar\\neclipses which is worth attention. An observer on the\\nEarth sees the Sun as a bright circle in the sky. The\\napparent angular diameter of the Sun (the angle between", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0252.jp2"}, "251": {"fulltext": "ECLIPSES OF THE SUN AND MOON.\\n229\\ntwo lines drawn from the observer s eye to the upper edge\\nand to the lower edge of the San, respectively) is greatest\\nwhen the Earth is nearest to the Sun, least when the Earth\\nis farthest away. In the same way the apparent angular\\ndiameter of the Moon to an observer on the Earth is\\ngreatest when the Moon is nearest, least when the Moon is\\nfurthest away.\\nThese apparent angular diameters have been measured\\nand the results of observation are given in the following\\nlittle table:\\nAverage.\\nApparent diameters of the Moon\\nApparent diameters of the Sun.\\nGreatest.\\nLeast.\\n33 33\\n32 33\\n29 24\\n31 28\\n31 08\\n32 00\\nIf at any new Moon the centres of the Sun, Moon, and\\nEarth are in a straight line, an eclipse will occur. If the\\nFig. 145.\\nangular diameter of the Moon is less than that of the Sun\\nwe shall have an annular eclipse of the Sun. When the\\ncentre of the Moon just covers the centre of the Sun the\\nappearance will be like figure 146. As the Sun at this time\\nhas a larger angular diameter it will appear, at the moment\\nof central eclipse, like a bright ring round the dark\\n(unilluminated) body of the Moon. The Moon will move\\nacross the disk of the Sun from west towards east and the\\nring will only endure for a short time.\\nIf the centres of the Earth, Sun, and Moon are in a\\nstraight line at any new Moon, and if at that time the\\napparent angular diameter of the Moon is greater than that\\nof the Sun there will be a total eclipse of the Sun.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0253.jp2"}, "252": {"fulltext": "230\\nASTRONOMY.\\nIf at the time of new Moon the Moon does not pass\\ncentrally across the Sun s disk, but above the centre or\\nbelow it, there may be a total eclipse (or an annular eclipse),\\nbut usually there will only be a partial eclipse. Only a\\npart of the Sun s disk will be covered in such a case.\\nThere are more eclipses of the Sun than of the Moon.\\nA year never passes without at least two of the former, and\\nsometimes five or six, while there are rarely more than two\\neclipses of the Moon, and in many years none at all. But\\nat any one place on the Earth more eclipses of the Moon\\nthan of the Sun will be seen. The reason of this is that an\\neclipse of the Moon is\\nvisible over the entire\\nhemisphere of the Earth\\non which the Moon is\\nshining, and as it lasts\\nseveral hours, observers\\nwho are not in this hemis-\\nphere at the beginning of\\nthe eclipse may, by the\\nEarth s rotation, be\\nbrought into it before it\\nends. Thus the eclipse\\nFig. 146.\u00e2\u0080\u0094 The Dark Body of w usually be seen over\\nthe Moon Projected on the *_\u00e2\u0080\u00a2,*, -n j_i\\nDisk of the Sun at the Mid- more than half the Earth s\\ndle of an Annular Eclipse, surface. But each eclipse\\nof the Sun can be seen over only so small a part of the\\nEarth s surface, and while there are many more solar\\neclipses than lunar for the whole Earth taken together,\\nfewer are visible at any one station.\\nOccultation of Stars by the Moon. Since all the bodies of the solar\\nsystem are nearer than the fixed stars, it is evident that they must\\nfrom time to time pass between us and the stars. The planets are,\\nhowever, so small that such a passage is of very rare occurrence.\\nBut the Moon is so large and her angular motion so rapid that she", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0254.jp2"}, "253": {"fulltext": "ECLIPSES OF THE SUN AND MOON. 231\\npasses over some star visible to the naked eye every few days.\\nSuch phenomena are termed occultations of stars by the Moon.\\nThe Nautical Almanac contains predictions of all occultations,\\nThese predictions are obtained by calculating the Moon s path on\\nthe celestial sphere and by noticing what bright stars (or planets)\\nher disk will cover to observers at different stations on the Earth.\\nWhat is a shadow its umbra penumbra Draw a diagram\\nshowing the shadow of the Earth cast by the Sun. Point out the\\numbra and the penumbra of this shadow. What is the cause of a\\nlunar eclipse Why do we not have lunar eclipses at every full\\nmoon once a month What is che color of the totally eclipsed\\nmoon Why does it have this color What is the cause of a solar\\neclipse Why do we not have solar eclipses at every new moon\\n(Answer: because in the first place the Moon s shadow is often too\\nshort to reach the surface of the Earth and also because it often\\ndoes not at new Moon point at the Earth, but above the Earth or\\nbelow it.)\\nFig. 147. A Schoolroom Experiment to Illustrate a\\nSolar Eclipse.\\nThe room must be darkened. The lamp should have a ground glass or\\nan opal globe to represent the circle of the Sun s disk. An orange (B)\\nfastened to a pincushion by a knitting-needle may stand for the Earth.\\nA golf -ball suspended by a string (C) may stand for the Moon. By placing\\nC on the other side of B the circumstances of a lunar eclipse may be illus-\\ntrated.\\nWhat is a partial eclipse of tbe Moon of the Sun a total\\neclipse of the Moon? of the Sun? an annular eclipse of the Sun?\\nWhy can there never be an annular eclipse of the Moon What is\\nan occultation f Longfellow has a poem, The Occultation of\\nOrion. Could the Moon cover a whole constellation", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0255.jp2"}, "254": {"fulltext": "CHAPTER XIII.\\nthe earth.\\n28. Astronomy has to do with the Earth as a planet.\\nPhysical Geography treats of the Earth without considering\\nits relation to the other bodies of the solar system. But\\nour only means of understanding the conditions on other\\nplanets is to be found in a comparison of these conditions\\nwith circumstances on the Earth. For this reason it is\\nconvenient to recall some of the facts taught by Physical\\nGeography and to group them with others derived from\\nAstronomy.\\nThe Earth s average distance from the Sun is about 92,800,000\\nmiles. Its least distance (in December) is 91,250,000 miles; its\\ngreatest distance (in June) is 94,500,000 miles. The seasons on the\\nEarth depend chiefly on the north-polar distance of the Sun and not\\non the Earth s proximity to it. The Earth revolves on its axis once\\nin 24 (sidereal) hours. By its rotation an observer at the equator is\\ncarried round at the rate of more than 1000 miles per hour. It was\\na favorite argument of the men of the Middle Ages against the\\ntheory that the earth was in rotation that so great a velocity as this\\ncould not possibly fail to be remarked. If the rotation were not\\nuniform and regular the argument would be convincing.\\nThe Earth travels around the circumference of its orbit once in\\n365 days, at the rate of about 66,000 miles per hour, at the rate of\\nabout 18| miles per second.\\nFigure of the Earth. Ptolemy taught in the Almagest\\n(a.d. 140) that the Earth was a sphere. Five hundred\\nyears before his time Aristotle had proved the same\\nthing, and before Aristotle there were philosophers who\\nheld the same opinion. Ptolemy maintained that the\\n232", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0256.jp2"}, "255": {"fulltext": "THE EARTH.\\n233\\nEarth was rounded in an east-to-west direction because the\\nSun, Moon, and stars do not rise and set at the same\\nmoment to all observers, but at different moments. The\\nEarth was rounded in a north and south direction because\\nnew stars appeared above the southern horizon as men\\ntravelled southwards, or above the northern horizon as they\\ntravelled northwards.\\nIt was well known in his time that a journey of a few\\nhundred miles to the north or south would change the\\nhorizon of an observer so that new stars became visible.\\nSuch short journeys could not produce such results on a\\nglobe of very large size. The voyage of Magellan at the\\nbeginning of the sixteenth century first established in all\\nmen s minds the fact that the Earth was a spherical body.\\nFig. 148.\u00e2\u0080\u0094 An Ellipse.\\nAC 2a is the major axis BD 2b is the minor axis.\\nThe popular opinion for many centuries was that the Earth\\nwas a flat disk everywhere surrounded by water.\\nThe Earth is not a sphere, but a spheroid. If it is cut\\nby meridian planes (through the poles) the curves cut out\\nof its surface are ellipses, not circles.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0257.jp2"}, "256": {"fulltext": "234\\nASTRONOMY.\\nIf an ellipse is revolved about the axis BD the resulting\\nsolid is a spheroid. The Earth s meridian is very little\\ndifferent from a circle. The minor axis, the line joining\\nthe two poles, is the axis of rotation.\\nNORtH POLE\\nSOUTH POLE\\nFig. 149\u00e2\u0080\u0094 The Earth\u00e2\u0080\u0094 its Axis, its Poles, its Equator.\\nIts equatorial semi-diameter a 20,926,202 feet,\\n3963.296 miles,\\n6,378,190 metres.\\nb 20,854,895 feet,\\n3949.790 miles,\\n6,356,456 metres.\\n2a 7926 6; miles,\\n26 7899 6\\nabout 500,000,000 inches.\\nThe circumference of the equator 24.899 miles,\\na meridian =24,856\\n40,000,000 metres.\\nA railway train travelling a mile a minute would require 17 days\\nand nights of continuous travel to go once around the Earth.\\nThe area of the whole Earth is about 197,000,000 square miles.\\ndry land 50,000,000\\nIts polar semi-diameter\\nThe equatorial diameter\\npolar", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0258.jp2"}, "257": {"fulltext": "THE EARTH.\\n235\\nSo that the area of the Earth is more than fifty times that of the\\nUnited States. We shall see that the planets Jupiter, Saturn,\\nUranus, and Neptune are, each one of them, far larger than the\\nEarth and the Sun is immensely larger. Its diameter is 866,400\\nmiles.\\nGeodetic Surveys. Since it is practically impossible to\\nmeasure around or through the Earth, the figure and the\\nsize of our planet has to be found by combining measure-\\nments on its surface with astronomical observations. Even\\na measurement on the Earth s surface made in the usual\\nway of surveyors would be impracticable, owing to the in-\\ntervention of mountains, rivers, forests, and other natural\\nobstacles. The method of triangulation is therefore uni-\\nversally adopted for measurements extending over large\\nareas.\\nFig. 150.\u00e2\u0080\u0094 A Part of the French Triangulation near Paris.\\nTriangulation is executed in the following way: Two points, a and\\nb, a few miles apart, are selected as the extremities of a base-line.\\nThey must be so chosen that their distance apart can be accurately\\nmeasured; the intervening ground should therefore be as level and\\nfree from obstruction as possible. One or more elevated points, EF,\\netc., must be visible from one or both ends of the base-line. The\\ndirections of these points relative to the meridian are accurately\\nobserved from each end of the base, as is also tbe direction ab of the\\nbase-line itself. Suppose F 10 be a point visible from each end of\\nthe base, then in the triangle abF we have the length ab determined", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0259.jp2"}, "258": {"fulltext": "236 ASTRONOMY.\\nby actual measurement, and the angles at a and 6 determined by\\nobservations. With, these data the lengths of the sides aJ^and bF\\nare determined by a simple computation.\\nThe observer then transports his instruments to F, and determines\\nin succession the direction of the elevated points or hills DEGHJ,\\netc. He next goes in succession to each of these hills, and deter-\\nmines the direction of all the others which are visible from it.\\nThus a network of triangles is formed, of which all the angles are\\nobserved, while the sides are successively calculated from the first\\nbase. For instance, we have just shown how the side aFis calcu-\\nlated; this forms a base for the triangle EFa, the two remaining\\nsides of which are computed. The side EF forms the base of the\\ntriangle GEF, the sides of which are calculated, etc.\\nChains of triangles have thus been measured in Russia and Sweden\\nfrom the Danube to the Arctic Ocean, in England and France from\\nthe Hebrides to the Sahara, in this country down nearly our entire\\nAtlantic coast and along the great lakes, and through shorter dis-\\ntances in many other countries. An east and west line has been\\nmeasured by the Coast Survey from the Atlantic to the Pacific\\nOcean.\\nSuppose that we take two stations, a and^*, Fig. 150, situated north\\nand south of each other, determine the latitude of each, and calcu-\\nlate the distance between them by means of triangles, as in the\\nfigure. It is evident that by dividing the distance between them by\\nthe difference of latitude in degrees we shall have the length of one\\ndegree of latitude. Then if the Earth were a sphere, we should at\\nonce have its circumference by multiplying the length of one degree\\nby 360. It is thus found that the length of 1 degree is a little more\\nthan 111 kilometres, or between 69 and 70 English statute miles. Its\\ncircumference is therefore about 40,000 kilometres, and its diameter\\nbetween 12,000 and 13,000.* (25,000 and 8000 miles.)\\nThe general surface of the Earth is found, to be rather smooth.\\nThe highest mountain is about 5^ miles high; the deepest ocean is\\nabout 5| miles deep. Eleven miles covers the range of height and\\ndepth. The average elevation of the continents above the sea-level\\nis about 2000 feet. The average depth of the ocean is about\\n12,000 feet.\\nWhen the metric system was originally designed by the French, it was in-\\ntended that the kilometre should be i^Tsrs of the distance from the pole of the\\nEarth to the equator. This would make a degree of the meridian equal, on the\\naverage, to 11 1\u00c2\u00a3 kilometres.. But the metre actually adopted is nearly T n of an\\ninch too short.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0260.jp2"}, "259": {"fulltext": "THE EARTH. 237\\nMass and Density of the Earth.\\nThe mass of a body is the quantity of matter it contains. It is\\nmeasured by the product of its volume V) by its density (D)\\nM V D. For another body M -V .D\\nFor equal volumes V \u00e2\u0080\u0094V and M M D D\\nThat is, the densities of equal volumes of two substances are pro-\\nportional to the masses of the substances, to the quantity of matter\\nin them. For example, copper is of greater density than water\\nbecause a cubic foot of copper contains more matter than a cubic\\nfoot of water. The density of pure water at about 39\u00c2\u00b0 Fahr. is\\ntaken as the unit-density. The unit-volume may be taken as a\\ncubic foot. The unit-mass will then be that of a cubic foot of pure\\nwater at 39\u00c2\u00b0 Fahr.\\nThe weight of a body is the force with which it is attracted to the\\ncentre of the Earth. A body of mass m is attracted by the Earth s\\nmass M by where r is the distance Mm. (See page 210.) The\\n3fm\\nweight w of m is then The weight w of any other body m\\nJkfm\\nis w 7T If the bodies are at the same place on the Earth r r\\nr\\nand w\\\\ w m\\\\ m! or the weights of bodies at the same place on the\\nEarth are proportional to their masses. It is easy to measure the\\nrelative weights of two bodies by balancing them in scales against\\ncertain pieces of metal. Hence by weighing two bodies of weights\\nw and w we can determine the ratio of their masses m and m If\\nm is a cubic foot of water, m! is the absolute mass of the other\\nsubstance.\\nThe weight of a body m due to the Earth s attraction is\\nIf the body is at the pole of the Earth r 7899.6\\nmiles. If it is at the equator r 7926.6 miles. Its\\nweight will therefore be greater at the pole than at the\\nequator. If we wish to weigh out a certain quantity of\\ngunpowder in Greenland we may balance it against a piece\\nof metal that we call an ounce. If we take the gunpowder\\nto Peru it will weigh less because it will there be", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0261.jp2"}, "260": {"fulltext": "238 ASTRONOMY.\\nfarther from the Earth s centre. Bat it will still balance\\nthe oance in Peru, because that also is less attracted by the\\nEarth in precisely the same proportion. A piece of iron a\\ncubic foot in volume weighs less in a balloon than at\\nthe Earth s surface. In practical life no note need be\\ntaken of the differences of the Earth s attraction at differ-\\nent latitudes. But in Astronomy these differences of at-\\ntraction due to differences of distance must be taken into\\naccount. The attraction of the Earth for the Moon is\\ndifferent at different times because the Moon is sometimes\\nnear the Earth, sometimes further away.\\nThe density of pure water at about 39\u00c2\u00b0 Fahr. is taken as the unit-\\ndensity. For equal volumes of any two substances M M D D\\nor, their densities are proportional to their masses. At the same\\nplace on the Earth W W M M or, their weights are also pro-\\nportional to their masses, hence\\nW:W D:D\\nIf one of these substances is pure water (W, D we have\\nW. D\\nD w and we can determine D, the density of any substance,\\nas copper, by weighing it against an equal volume of water. In\\nthis way the densities of all substances on the Earth have been\\ndetermined.\\nThe surface-rocks of the Earth are about 2\u00c2\u00a3 times as dense as\\nwater, and volcanic lavas deep down in the Earth are about 3 times\\nas dense. The deeper the origin of the rocks the denser they are,\\nbecause they are subject to greater pressures. We can determine\\nthe density of any single specimen of rock that can be brought to\\nthe surface. We can get no specimens of rock from depths greater\\nthan a few miles. How then shall we determine the average density\\nof the whole Earth\\nTo determine the density of the Earth we must find hoic much matter\\nit must contain in order to attract bodies on its surface with forces\\nequal to their observed weights, that is, with such intensity that at the\\nequator a body shall fall nearly five metres {about 16 feet) in a second.\\nTo find this we must know the relation between the mass of a body and\\nits attractive force. This relation can be found by measuring the\\nattraction of a body of known mass.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0262.jp2"}, "261": {"fulltext": "THE EARTH. 239\\nWe may measure the attraction of a body of known mass in the\\nfollowing ingenious way. In Fig. 151 H1KL is a cube of lead 1 metre\\non each edge. Two holes are bored\\nthrough the cube at DF and EG-\\nA pair of scales ABC bas its scale-\\npans HE connected by fine wires\\nto other scale-pans EG, below the\\nblock. Suppose the pans empty\\nand everything at rest.\\nI. Put a weight W in H, and\\nbalance the scales by weights in G.\\nAt H the total attraction on W is\\nthe attraction of the Earth plus the Fig. 151 Experiment to\\nattraction of the block, while at Determine the Density op\\nG we have the attraction of the THE Earth\\nEarth (downwards) minus the attraction of the block (upwards);\\nhence\\nThe weight in H 4- (attraction of the block) The weights in G\\n(attraction of the block), whence\\n(1) Weights in G weight in H -f- 2 (attraction of block).\\nII. Put the weight W in F, and balance the scales by weights in\\nE. At F the total attraction is earth minus block, and at E it is\\nearth plus block.\\nThe weight in F (attraction of the block) The weights in E\\n(attraction of the block), whence\\n(2) Weights in E weight in F 2 (attraction of block).\\nSubtract equation (2) from (1), remembering that the (weight in\\nH) (weight in F).\\nWeights in G weights in E 4 (attraction of block),\\nafter small corrections have been applied for the difference of height\\nof H, E, F, G, etc.\\nThe attraction of this block, which has a known mass in kilo-\\ngrammes (or pounds), is thus known, and hence the general relation\\nbetween mass in kilogrammes (or pounds) and attractions.\\nThe attraction of the Earth is known. It is such as to cause\\nbodies to have their observed weights. Hence the mass of the Earth\\nbecomes known. The volume of the Earth is known from geodetic", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0263.jp2"}, "262": {"fulltext": "240\\nASTRONOMY.\\nsurveys. The density of the whole Earth is therefore known from\\nM\\nthe equation D\\nThe density of the Earth is about 5$ times that of water,\\ncopper 8^\\nC ft y 2\\nThe mass of the Earth is 6, 000, 000, 000, 000, 000, 000, 000 tons.\\nFig. 152.\\nDetermination of the Mass of the Earth in Terms of the Mass of the\\nSun. The mass of the Earth expressed in tons or pounds is known\\nThe mass of the Earth in fractions of the Sun s mass 1.0) can be\\ndetermined by calculating how far the Earth is deflected by the Sun s\\nattraction each second, as she travels in her orbit. Her motion\\nalong her orbit is 18$ miles per second (because the circumference of\\nher orbit is 584,600,000 miles and because it is traversed in a sidereal\\nyear of 365 d 9 h 9\u00e2\u0084\u00a2 9 s (Fig. 152.) Her deflection from a straight\\nline each second is\\n100\\nof an inch, as may be proved from the\\nforegoing diagram, in which E is the place of the Earth at the\\nbeginning of a second, E its place at the end of the second, EE the\\norbit of the Earth, 8 the place of the Sun, X another point of the\\nEarth s orbit, Ee the Earth s fall towards the Sun in a second.\\nIn the two right triangles XE E and EE e we have EX EE\\nEE Ee, or (twice 93,000,000) 18$ 18| Ee, whence Ee 0.01\\nof a foot, approximately.\\nThe mass of the Sun at 93,000,000 miles causes the Earth to move\\ntowards his centre 0.01 foot. If the Sun were 4000 miles from\\nthe Earth his attraction would be greater in the proportion of\\n[93, 000, 000] to [4000] 2 or as 8,650,000,000,000,000 to 16,000,000 or\\nas 540,500,000 to 1. If the Sun were at a distance of only 4000 miles\\nfrom the Earth (or from any heavy body) the body would fall in a\\nsecond 540,500,000 times T fo of a foot or 5,405,000 feet. The Earth", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0264.jp2"}, "263": {"fulltext": "THE EARTH. 241\\nmakes a heavy body at its surface (4000 miles from its centre) fall\\n16 T X (5 feet in a second. Hence\\nMass of Sun Mass of Earth 5,405,000 feet 16.1 feet,\\nor as 335,000 to 1. If the exact values of all the quantities are\\nemployed instead of the approximate ones used above the value of\\nthe Earth s Mass (Sun s Mass 1.0) is 33^70-\\nConstitution of the Earth. The body of the Earth is\\nmade up of layers of rocks of different density arranged in\\nshells like the coats of an onion. The outer layers are the\\nleast dense; the inner layers (those subject to the greatest\\npressures) are the most dense. The Earth is composed of\\nvarious substances, some simple (elements) like iron, some\\ncompound like clay. There are about 70 or 80 elementary\\nsubstances (gold, iron, carbon, oxygen, hydrogen, etc.), and\\nit is noteworthy that nearly all of these elements are known\\nto exist in the Sun, and that many of them are known to\\nexist in the stars. It is probable that the Sun, the Earth,\\nand all the planets are made out of the same elements and\\nthat the amazing differences between them are chiefly due\\nto differences in their temperature.\\nThe temperature of the solid crust of the Earth increases as we go\\ndownwards at the rate of about 1\u00c2\u00b0 Fahr. for every 55 or 60 feet, or\\nabout 90\u00c2\u00b0 per mile. At the depth of 10 miles the temperature is\\nabout 900\u00c2\u00b0; at the depth of 30 miles about 2700\u00c2\u00b0, and so on. Iron\\nmelts at the surface of the Earth (where it is free from great pressure)\\nat about 3000\u00c2\u00b0. If the substances in the Earth s interior were free\\nfrom pressure the interior would be a fluid mass, and there would be\\ngreat tides in this interior ocean. Astronomical observations show\\nthat there are no such tides, whence it follows that the interior of\\nthe Earth is, on the whole, solid. There are many reservoirs of\\nmelted rocks (lavas) no doubt in the neighborhood of volcanoes, but\\non the whole the Earth is solid and about as stiff as a globe of steel.\\nThe spheroidal shape of the Earth seems to show that it once was in\\na fluid condition, for a rotating mass of fluid will take the form of a\\nspheroid. It will be flattened at the poles. Its meridians will be\\nellipses. This is the shape, not only of the Earth, but of all the\\nplanets.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0265.jp2"}, "264": {"fulltext": "242 ASTRONOMY.\\nAll the heat of the Earth comes to it from the Sun. The Sun\\nsends its heat out in all directions along every possible line that can\\nbe drawn from the San outwards. The Sun would warm the whole\\ninterior surface of a sphere 93,000,000 miles in diameter just as\\nmuch as it now warms the Earth which occupies one small point\\nof such a sphere. So far as mankind is concerned all the heat that\\ndoes not fall on the Earth is lost. The Earth receives only the\\nminutest fraction of it (not more than s^^VuFffo)-\\nAtmosphere of the Earth. The Earth is surrounded by an ocean of\\nwater in which the attractions of the Sun and Moon produce tides.\\nIt is likewise surrounded by an ocean of air, and in this atmosphere\\nslight tides are also observed. The effect of the atmosphere on the\\nclimates of the Earth is most important, and it is treated in works\\non Meteorology.\\nAstronomy is chiefly concerned with the effects of the Earth s\\natmosphere in producing a refraction (a bending) of the rays of light\\nthat reach us from the stars so that we do not see them quite in\\ntheir true directions. The atmosphere of the Earth surrounds it to a\\nheight of a hundred miles or more. Its heavier layers are nearest\\nthe Earth s surface. Even at a height of 3 or 4 miles there is\\nscarcely enough air for breathing.\\nRefraction of Light by the Atmosphere. In figure 153\\nO is the centre of the Earth and A the station of an\\nobserver on its surface. S is a star. If there were no\\natmosphere the observer would see the star along the line\\nAS. But the atmosphere acts like a lens and bends\\n(refracts) the light from the star along the curved line\\ne, d, c, a, and the light from the star comes to the\\nobserver along the line AS He sees the star projected on\\nthe celestial sphere at S\\\\ therefore, and not in its true\\nplace S. The star is (apparently) thrown nearer to his\\nzenith by refraction. It will rise sooner and set later,\\ntherefore, on this account.\\nAt the zenith the refraction is 0, at 45\u00c2\u00b0 zenith distance the refrac-\\ntion is 1 and at 90\u00c2\u00b0 it is 34 30 The ravs of light traverse greater\\nthicknesses of air at large zenith distances and are more refracted\\ntherefore. Stars at the zenith distances of 45\u00c2\u00b0 and 90\u00c2\u00b0 appear ele-\\nvated above their true places by 1 and 344 respectively. If the sun\\nhas just risen\u00e2\u0080\u0094 that is, if its lower edge is just in apparent contact", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0266.jp2"}, "265": {"fulltext": "THE EARTH. 243\\nwith the horizon it is in fact entirely below the true horizon, for\\nthe refraction (35 has elevated its centre by moie than its whole\\napparent diameter (32\\nThe moon is full when it is exactly opposite the sun, and therefore,\\nwere there no atmosphere, moon-rise of a full moon and sunset\\nwould be simultaneous. In fact, both bodies being elevated by\\nrefraction, we see the full moon risen before the sun has set.\\nFig. 153. Refraction of the Light of a Star by the\\nEarth s Atmosphere.\\nTwilight. It is plain that one effect of refraction is to\\nlengthen the duration of daylight by causing the Sun to\\nappear above the horizon before the time of his geometrical\\nrising and after the time of true sunset.\\nDaylight is also prolonged by the reflection of the Sun s\\nrays (after sunset and before sunrise) from the small\\nparticles of matter suspended in the atmosphere. This\\nproduces a general though faint illumination of the atmos\\nphere, just as the light scattered from the floating particles\\nof dust illuminated by a sunbeam let in through a crack in\\na shutter may brighten the whole of a darkened room.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0267.jp2"}, "266": {"fulltext": "244\\nASTRONOMY.\\nThe Sun s direct rays do not reach an observer on the\\nEarth after the instant of sunset, since the solid body of\\nthe Earth intercepts them. But the Sun s direct rays\\nilluminate the clouds of the upper air, and are reflected\\ndownwards so as to produce a general illumination of the\\natmosphere, which is called twilight.\\nIn the figure let ABCD be the Earth and A an observer\\non its surface, to whom the Sun 8 is just setting. Aa is\\nFig. 154.\u00e2\u0080\u0094 The Phenomena of Twilight.\\nthe horizon of A Bb of B; Cc of C; Dd of D. Let the\\ncircle PQR represent the upper layer of the atmosphere.\\nBetween ABCD and PQR the air is filled with suspended\\nparticles that reflect light. The lowest ray of the Sun,\\nSAM, just grazes the Earth at A; the higher rays /S^and\\nSO strike the atmosphere above A and leave it at the points\\nQ and R.\\nEach of the lines SAPM, SQNis bent from a straight\\ncourse by refraction, but SRO is not bent since it just", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0268.jp2"}, "267": {"fulltext": "THE EAUTR. 245\\ntouches the upper limits of the atmosphere. The space\\nMABCDE is the Earth s shadow. An observer at A\\nreceives the (last) direct rays from the San, and also has\\nhis sky illuminated by the reflection from all the particles\\nlying in the space PQRT which is all above his horizon Aa.\\nAn observer at B receives no direct rays from the San.\\nIt is after his sunset. Nor does he receive any light from\\nthat portion of the atmosphere below APM; but the por-\\ntion PRx, which lies above his horizon Bb is lighted by the\\nSun s rays, and reflects some light to B. The twilight is\\nstrongest at R, and fades away gradually towards P. The\\naltitude of the twilight at B is id.\\nTo an observer at C the twilight is derived from the\\nillumination of the portion PQz which lies above Ins\\nhorizon Cc. The altitude of the twilight at C is cd.\\nTo an observer at D it is night. All of the illuminated\\natmosphere is below his horizon Dd.\\nThe twilight arch is more marked in summer than in winter in\\nhigh latitudes than in low ones. There is no true night in Scotland\\nat midsummer, for example, the morning twilight beginning before\\nthe evening twilight has ended and in the torrid zone there is no\\nperceptible twilight. Twilight ends when the Sun reaches a point\\nabout 20\u00c2\u00b0 below the horizon. The student should observe the\\nphenomena of twilight for himself. It is best seen in the country,\\nshortly after sunset, as far away from city lights as may be.\\nAstronomical Measures of Time to the Inhabitants of the\\nEarth. The simplest unit of time is the sidereal day, that\\nis the interval of time required for the Earth to turn once\\non its axis. It is measured by the interval between two\\nsuccessive transits of the same star over the observer s\\nmeridian; and it is divided into 24 sidereal hours.\\nThe most obvious unit of time is the (apparent) solar\\nday, that is the interval of time between two successive\\ntransits of the true Sun over the observer s meridian. As\\napparent solar days are not equal in length, a more con-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0269.jp2"}, "268": {"fulltext": "246 ASTRONOMY.\\nvenient unit has been devised, that is the mean solar day,\\nwhich is the interval of time between two successive\\ntransits of the mean San (see page 90) over the observer s\\nmeridian. The relation between the sidereal and mean\\nsolar day has been previously given (page 95) and is as\\nbelow\\n366.24222 sidereal days 365.24222 mean solar days,\\n1 sidereal day 0.997 mean solar day,\\n24 sidereal hours 23 h 56 m 4 9 .091 mean solar time,\\n1 mean solar day 1.03 sidereal days,\\n24 mean solar hours 24 h 3 m 56 s 555 sidereal time.\\nThe quantity to be added to (or subtracted from) ap-\\nparent solar time to obtain mean solar time is calculated\\nbeforehand and printed in the Nautical Almanac under\\nthe heading Equation of Time. (See page 151.)\\nThe months now or heretofore in use among the peoples\\nof the globe may for the most part be divided into two\\nclasses\\n(1) The lunar month pure and simple, or the mean\\ninterval between successive new Moons.\\n(2) An approximation to the twelfth part of a year,\\nwithout respect to the motion of the Moon.\\nThe mean interval between consecutive new Moons being\\nnearly 29^ days, it was common in the use of the pure lunar\\nmonth to have months of 29 and 30 days alternately.\\nThe interval between two successive returns of the Sun\\nto the same star is called the sidereal year. Its length is\\nfound by observation to be\\n365 (mean solar) days 6 hours 9 minutes 9 seconds 365 d 25636.\\nThe interval between two successive returns of the Sun to\\nthe same equinox is called the equinoctial year. Its length\\nis found by observation to be\\n365 (mean solar) days 5 hours 48 minutes 46 seconds 365 d 24220.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0270.jp2"}, "269": {"fulltext": "THE EARTH. 2^7\\nThe sidereal year measures the time of the revolution of\\nthe Earth in her orbit. The equinoctial year governs the\\nrecurrence of the seasons, because the seasons depend on\\nthe Sun s declination (see page 175) and the declination\\nchanges from south to north at the vernal equinox at the\\npassage of the Sun across the celestial equator.\\nThe solar year of 365\u00c2\u00a3 days has been a unit of time-\\nreckoning from very early times. Four such years are\\nequal to 1461 days. The cycle of four years, three of them\\nof 365 days and the fourth of 360, which we use, was\\nadopted in China in the remotest historic times.\\nThe Julian Calendar.\u00e2\u0080\u0094 The civil calendar now in use\\nthroughout Christendom had its origin among the Romans,\\nand its foundation was laid by Julius C^sar. Before his\\ntime, Rome can hardly be said to have had a chronological\\nsystem. The length of the year was not prescribed by any\\ninvariable rule, and it was changed from time to time to\\nsuit the caprice or to compass the ends of the rulers.\\nInstances of this tampering disposition are familiar to the histori-\\ncal student. It is said, for instance, that the Gauls having to pay a\\ncertain monthly tribute to the Romans, one of the governors ordered\\nthe year to be divided into 14 months, in order that the pay-days\\nmight recur more rapidly. Caesar fixed the year at 365 days, with\\nthe addition of one day to every fourth year. The old Roman months\\nwere afterwards adjusted to the Julian year in such a way as to give\\nrise to the somewhat irregular arrangement of months which we now\\nhave. The names of our days are partly from Roman, partly from\\nScandinavian mythology. The student should consult a dictionary\\nfor the derivations of their names.\\nOld and New Styles.\u00e2\u0080\u0094 The mean length of the Julian year is about\\n11\u00c2\u00a3 minutes greater than that of the equinoctial year, which measures\\nthe recurrence of the seasons. This difference is of little practical\\nimportance, as it only amounts to a week in a thousand years, and a\\nchange of this amount in that period can cause no inconvenience.\\nBut, in order to have the year as correct as possible, two changes\\nwere introduced into the calendar by Pope Gregory XIII. with this\\nobject. It was decreed that", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0271.jp2"}, "270": {"fulltext": "248 ASTRONOMY.\\n(1) The day following October 4, 1582, was to be called tbe 15th\\ninstead of the 5th, thus advancing the count 10 days.\\n(2) The closing year of each century, 1600, 1700, etc., instead of\\nbeing always a leap-year, as in the Julian calendar, was to be such\\nonly when the number of the century is divisible, by 4. Thus while\\n1600 remained a leap-year, as before, 1700, 1800, and 1900 were to be\\ncommon years.\\nThis change in the calendar was speedily adopted by all Catholic\\ncountries, and more slowly by Protestant ones, England holding out\\nuntil 1752. In Russia, the Julian calendar is still continued without\\nchange. The Russian reckoning is therefore 12 days behind ours,\\nthe ten days dropped in 1582 being increased by the days dropped\\nfrom the years 1700 and 1800 in the new reckoning.* The modified\\ncalendar is called the Gregorian Calendar, or New Style, while the\\nold system is called the Julian Calendar, or Old Style.\\nIt is to be remarked that the practice of commencing the year on\\nJanuary 1st was not universal until comparatively recent times. The\\nmost common times of commencing were, perhaps, March 1st and\\nMarch 22d, the latter being the time of the vernal equinox. But\\nJanuary 1st gradually made its way, and became universal after its\\nadoption by England in 1752.\\nPrecession of the Equinoxes. It has just been said that\\nobservation proves the sidereal year to have a length of\\n365.25636 mean solar days, and the eqainoctial year to\\nhave a length of 365.24220 days. The San in his annual\\ncircuit of the heavens moves from a star to the same star\\nagain in the sidereal year, from an equinox to the same\\nequinox again in the equinoctial year.\\nAs the stars are fixed, the Sun s revolution around the\\necliptic from star back to the same star again must be a\\nrevolution through exactly 360\u00c2\u00b0 0 0 of right-ascension.\\nAs the equinoctial year is shorter than the sidereal year,\\nthe Sun s revolution from equinox t\u00c2\u00a9 equinox must be a\\nrevolution through an angle slightly less than 360\u00c2\u00b0.\\nj 365 d 25636 j 365 d 24220 OCAO 0Kft0 Kn 1A\\ni J- j f 360 359 59 10 approx.\\nsidereal year equinoctial year rtr\\nThe equinox must therefore be moving in space so that\\nRussia will adopt the New Style in A.D. 1901", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0272.jp2"}, "271": {"fulltext": "THE EARTH. 249\\nwhen it is met a second time the San has made one revolu-\\ntion less 50 The Sun s annual circuit is performed\\namong the stars from west to east. The equinox therefore\\nmoves (to meet the Sun) westward in right-ascension at the\\nrate of about 50 per year.\\nFig. 155. The Celestial Equator (AB) and the Ecliptic\\n{CD) E, is the Vernal Equinox.\\nThe equinox (E in the figure) is nothing but the point\\nwhere the ecliptic (CD) intersects the celestial equator\\n(AB). If their point of intersection changes it must be\\nbecause one or both of these circles is moving. If the plane\\nof the celestial equator is moving the declinations of all the\\nstars will change from year to year. Observation shows\\nthat the declinations do change slightly from year to year.*\\nIf the plane of the ecliptic is fixed the celestial latitudes of\\nall the stars (their angular distances from the ecliptic) will\\nnot change from year to year. Observation shows that\\nThe right-ascensions also change slightly because the equinox,\\nwhich is the origin of R A., is moving. The effect of annual pre-\\ncession on the places of stars is given in the fourth and sixth columns\\nof Table V at the end of this book.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0273.jp2"}, "272": {"fulltext": "250 ASTRONOMY.\\nwhile the declinations of all the stars do change annually\\nby small amounts their celestial latitudes do not change.\\nHence the plane of the ecliptic is fixed; and hence the\\nwestward motion of the equinox is entirely due to a motion\\nof the plane of the celestial equator.\\nIf the plane of a circle of the celestial sphere is fixed the\\nplace of the pole of that circle on the celestial sphere is\\nstationary. The ecliptic (CD) is fixed (see the figure), and\\nhence the place of its pole (Q) among the stars is station-\\nary. If the pole of the ecliptic is 10\u00c2\u00b0 from a star in 1800\\nit will be 10\u00c2\u00b0 from that star in 1900. On the other hand,\\nif the plane of the celestial equator (AB) is moving, as it\\nis, the place of its pole (P) among the stars must be\\nmoving. The north pole of the heavens is now near to\\nPolaris, but it will in time move away from it. At the\\ntime when the pyramids were built, about B.C. 2700,\\nPolaris was not the north-star, but the star Alpha\\nDraconis (see star-map No. VI).\\nThe pole of the ecliptic (Q) is fixed; the pole of the\\ncelestial equator (P) is moving. The angle between the\\nplane of the ecliptic and the plane of the celestial equator\\n(POQ 23%\u00c2\u00b0) does not change. Therefore the pole P\\nmust revolve about the fixed pole Q in a circle. The in-\\nclination of the two planes CD and AB will not be changed\\nby such a revolution, but their line of intersection (EF)\\nwill move slowly round the celestial sphere. Their line of\\nintersection is the line joining the two equinoxes. The\\nannual motion of the equinox is, as we have seen, 50 of\\narc, so that in about 25,920 years the equinox (E) will\\nmove completely around the circle of the ecliptic and will\\nreturn to its starting-point. In the same period the pole\\nof the celestial equator (P) will move in a circle completely\\naround the pole of the ecliptic (Q).\\n25,920 X 50 1,296,000 360\u00c2\u00b0.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0274.jp2"}, "273": {"fulltext": "THE EARTH.\\n251\\nThe student can trace the path of the north pole of the heavens\\namong the stars on Star-map No. IV, following. Turning this map\\nupside-down let him find the constellations Draco, Ursa minor,\\nCepheu?, Cygnus, and Lyra.\\nAbout 3000 years ago the pole was near a in Draco,\\nAt the present time the pole is near a in Ursa minor,\\nAbout 2000 years hence the pole will be very near to a in Ursa minor,\\n4000 near y in Cephevs,\\n7500 a in\\n11500\\n14000\\n8 in Cygnus,\\na in Lyra.\\nIf he has a celestial globe at hand he will find the path of the\\nnorth pole of the heavens about the north pole of the ecliptic marked\\ndown among the stars.\\nFig. 156.\u00e2\u0080\u0094 The Seasons on the Earth.\\nThe effects of the motion of the pole of the heavens on our sea-\\nsons may be studied in the figure. The figure represents the Earth in\\nfour positions during its annual revolution. Its axis inclines to the\\nright in each of these positions. In Chapter VIII it was said that\\nthe Earth s axis always remained parallel to itself. The phenomena\\nof precession show that this is not absolutely true, but that, in real-\\nity, the direction of the axis is changing with extreme slowness.\\nAfter the lapse of some 6400 years, the north pole of the Earth, as\\nrepresented in the figure, will not incline to the right, but towards", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0275.jp2"}, "274": {"fulltext": "252\\nASTRONOMY.\\nthe reader, the amount of the inclination remaining nearly the\\nsame. The result will evidently be a shifting of the seasons. At D\\nwe shall have the winter solstice, because the north pole will be in-\\nclined towards the reader and therefore from the Sun, while at A\\nwe shall have the vernal equinox instead of the winter solstice, and\\nso on.\\nIn 6400 years more the north pole will be inclined towards the left,\\nand the seasons will be reversed. Another interval of the same\\nlength, and the north pole will be inclined from the reader, the\\nseasons being shifted through another quadrant. Finally, at the\\nend of about 25,900 years, the axis will have resumed its original\\ndirection.\\nFig. 157.\u00e2\u0080\u0094 The Earth s Axis and Equator.\\nThe north pole of the heavens is the point where the\\ncelestial sphere is met by the axis of the Earth prolonged.\\nThe celestial equator is the plane of the terrestrial equator\\nproduced. The axis of the Earth does not move relatively\\nto the Earth s crust. The Earth s equator always passes\\nthrough the same countries Ecuador, Brazil, Africa,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0276.jp2"}, "275": {"fulltext": "THE EARTH. 253\\nSumatra. The latitudes of places on the Earth do not\\nchange. Precession is not due to a motion of the Earth s\\naxis simply, but to a motion of the whole Earth that carries\\nthe axis with it.\\nFig. 158.\u00e2\u0080\u0094 Diagram to Illustrate the Cause op Precession.\\nThe Cause of Precession.\\nThe cause of precession, etc., is illustrated in the figure, which\\nshows a spherical Earth surrounded by a ring of matter at the equa-\\ntor. If the Earth were really spherical there would be no precession.\\nIt is, however, ellipsoidal with a protuberance at the equaior. The\\neffect of this protuberance is to be examined. Consider the action\\nbetween the Sun and Earth alone. If the ring of matter were absent,\\nthe Earth would revolve about the Sun as is shown in Fig. 156\\n(Seasons).\\nThe Sun s North Polar Distance is 90\u00c2\u00b0 at the equinoxes, and 66^\u00c2\u00b0\\nand 113^\u00c2\u00b0 at the solstices. At the equinoxes the Sun is in the direc-\\ntion Cm that is, NCm is 90\u00c2\u00b0. At the winter solstice the Sun is in\\nthe direction Cc NCc 113|\u00c2\u00b0. It is clear that in the latter case the\\neffect of the Sun on the ring of matter will be to pull the Earth\\ndownwards so that the direction Cm tends to become the direction Cc.\\nAn opposite effect will be produced by the Sun when its polar dis-\\ntance is 66^\u00c2\u00b0.\\nThe Moon also revolves round the Earth in an orbit inclined to the\\nequator, and in every position of the Moon it has a different action\\non the ring of matter. The Earth is all the time rotating on its axis,\\nand these varying attractions of Sun and Moon are equalized and\\ndistributed since different parts of the Earth are successively pre-\\nsented to the attracting body. The result of all the complex motions", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0277.jp2"}, "276": {"fulltext": "254 ASTRONOMY,\\nwe have described is a conical motion of the Earth s axis NG about\\nthe line GE.\\nThe Earth s shape is of course not that given in the figure, but an\\nellipsoid of revolution. The ring of matter is not confined to the\\nequator, but extends away from it in both directions. The effects of\\nthe forces acting on the Earth as it is are, however, similar to the\\neffects just described. The motion of precession is not uniform, but\\nis subject to several small inequalities which are called nutation.\\nThe fact of precession was discovered by Hipparchus\\nmore than 2000 years ago. He observed (1) That the Sun\\nmade a revolution from equinox to equinox in a shorter\\ntime than that required for its revolution from star to star.\\n(2) As the stars were fixed the equinox mast be moving.\\n(3) The equinox is the intersection of the ecliptic and the\\ncelestial equator, and hence one or both of these planes\\nmust be moving. (4) The ecliptic was not moving because\\nthe celestial latitudes of stars did not change. (5) The\\ncelestial equator was in motion because the declinations of\\nall the stars (and their right-ascensions also) did change.\\nThis was a mighty discovery, and it required a genius of\\nthe first order to make it.\\nCopernicus, in 1543, declared that precession was due\\nto a conical motion of the Earth s axis of rotation about\\nthe line joining the Earth s centre with the pole of the\\necliptic.\\nNewton, in 1687, worked out the complete explanation.\\nThis could not possibly have been done until the theory of\\ngravitation was thoroughly understood nor until the science\\nof mathematics had been developed (by Newton s own\\nresearches) to a high point. Three of the greatest names\\nof science are associated in this discovery.\\nThe Progressive Motion of Light. Galileo made ex-\\nperiments to determine whether light required time to pass\\nfrom one place to another. His methods were not suffi-\\nciently refined to decide the question, but the subject was\\nnot lost sight of. In the year 1675, Olaus Homer, a", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0278.jp2"}, "277": {"fulltext": "THE EARTH. 255\\nDanish astronomer (to whom we owe the invention of the\\ntransit instrument, among other things), was engaged in\\nmaking tables of the times of the eclipses of the satellites\\nof Jupiter.\\nFig. 159. The Eclipses of Jupiter s Satellites and the\\nProgressive Motion of Light.\\nS, is the Sun T, is the Earth in its orbit J, is Jupiter in opposition with\\nthe Sun J is Jupiter in conjunction with the Sun.\\nThe figure shows the Earth at T. When Jupiter is at J\\nit is nearest to the Earth; when Jupiter is at J (and the\\nEarth at T) the two bodies are as far apart as possible.\\nTJ is larger than TJ by the diameter of the Earth s\\norbit; by about 186,000,000 miles therefore. Jupiter\\ncasts a long shadow (see the cut) and one of its satellites\\n(its orbit is the small circle about J and about J is\\neclipsed at every revolution. Eomee calculated the times\\nat which an observer on the Earth would see such eclipses.\\nHe found that his tables could be reconciled with observa-\\ntion only by supposing that the light from the satellite\\nrequired time to pass from Jupiter to the Earth, When\\nJupiter is at J its light has to pass over the line J I to\\nreach the Earth. When Jupiter is at J its light has to\\npass over the longer line J T. Accurate observations show\\nthat eclipses of the satellites are seen 16 minutes 38 seconds\\nearlier when the planet is at than when it is at J\\nLight requires 16 m 38 s to pass over the diameter of the\\nEarth s orbit, therefore, or 8 m 19 8 to pass over the radius of\\nthe orbit.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0279.jp2"}, "278": {"fulltext": "Sunlight\\nis 3 m\\n(t\\n6 m\\na\\ngm\\n(l\\ni -J^m\\na\\ni 43m\\na\\n^10\u00e2\u0084\u00a2\\na\\n2 h 38 ra\\nit\\n4h gm\\n256 ASTRONOMY.\\nIn 499 s light travels 92,900,000 miles, or at the rate of\\n186,200 miles in one second of time.* The sunlight is\\n499 seconds old when it reaches the Earth. As the velocity\\nof light is uniform it follows that (approximately)\\nold when it reaches Mercury,\\nVenus,\\nEarth,\\nMars,\\nJupiter,\\nSaturn,\\nJJranus,\\nNeptune.\\nThe time required for the light of a planet to reach the Earth is\\ncalled the planet s aberration-time. For instance, the aberration-t me\\nof Neptune is about 4 h 8 m This means that the light by which\\nwe see Neptune now this instant is 4 h 8 m old when it reaches us.\\nNeptune may have vanished 4 U 7 m ago for all we know. We can only\\nfind out by waiting. The stars are very much further away than\\nNeptune. We shall see, later on, that the light of even the nearest\\nstar is more than 4 years on its passage to the Earth. The light from\\nPolaris takes more than 40 years to reach us. Polaris may have\\nvanished 40 years ago for all we know now we can only find out\\nby waiting. Only a very few of the stars are so near as this. Most\\nof them are immensely further away.\\nThe theory of Romer was not fully accepted by his contemporaries.\\nThe velocity of light was so much greater than any known terres-\\ntrial velocity that it seemed difficult to accept it. Even the motion\\nof the Earth in its orbit was only 18 miles per second. The velocity\\nof light was 10,000 times as large. In the year 1729 James Bradley,\\nafterwards Astronomer Royal of England, observed a phenomenon of\\na different character that entirely confirmed Romer s conclusions.\\nBradley discovered that the stars are not seen in their true places,\\nbut that each star is displaced by a small angle (never more than\\n21 This displacement occurs because the Earth is moving among\\nthe stars with a velocity that is comparable with the velocity of light.\\nIn the figure suppose AB to be the axis of a telescope, S a\\nstar, and SAB a ray of light which emanates from the star. The\\nThe most accurate determinations give 186,330 miles.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0280.jp2"}, "279": {"fulltext": "ABERRATION. 257\\nstudent may imagine AB to be a rod which an observer at B seeks\\nto point at the star 8. It is evident that he must point this rod in\\nsuch a way that the ray of light shall run\\naccurately along its length. If the observer\\n(and the Earth) were at rest at B he will\\npoint the rod along the line SB.\\nSuppose now that the observer (and the\\nEarth) are moving from B toward B with\\nsuch a velocity that he moves from B to\\nB during the time required for a ray of\\nlight to move from A to B Suppose,\\nalso, that the ray of light from 8 (SA)\\nreaches A at the same time that the end of\\nhis rod does. Then it is clear that while\\nthe rod is moving from the position AB\\nto the position A B the ray of light will Fig. 156.\\nmove from A to B, and will therefore run accurately along the\\nlength of the rod.\\nFor instance, if 6 is one third of the way from B to B then the\\nlight, at the instant when the rod takes the position ba, will be one\\nthird of the way from A to B and will therefore be accurately on\\nthe rod. Consequently, to the observer, the rod will appear to be\\npointed at the star. In reality, however, the pointing will not be in\\nthe true direction of the star, but will deviate from it by a certain\\nangle depending upon the ratio of the velocity with which the\\nobserver is carried along to the velocity of light.\\nIf the Earth stood still there would be no aberration. If the\\nvelocity of light were 10,000 times greater than it is the aberration\\nwould be vanishingly small.\\nEffects of Aberration. The velocities of light and of the Earth\\nbeing what they are, the apparent displacement of a star s position,\\ndue to aberration is always less than 21\\nAberration phenomena can be observed on the Earth\\non any rainy day with no wind. (See fig. 157.) The\\nrain-drops descend vertically. If the observer stands still\\nhe must hold his umbrella straight above his head. Now\\nlet him walk briskly towards the south. He will find that\\nhe must incline his umbrella southwards (in the direction\\nof his motion) to protect himself. If he walks towards the\\nwest he most incline his umbrella westwards (in the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0281.jp2"}, "280": {"fulltext": "258 ASTRONOMY.\\ndirection of his motion). If instead of walking he runs,\\nthe same effects are produced, only he will find that he\\nmust incline his umbrella still more. All this while each\\nrain-drop is falling vertically yet every change of his\\ndirection of motion and of his velocity (relative to the\\nvelocity of the falling drops of rain) requires him to alter\\nthe inclination of his protecting umbrella. The experiment\\nis so easy that the student should not fail to try it.\\nWhat is the Earth s distance from the Sun? (Answer, about\\n93,000,000 miles.) Do the seasons depend on the Earth s proximity\\nto the Sun What is the shape of the Earth? How is the figure of\\nthe Earth determined? Define the mass, the density, the volume\\nof a body. Define the weight of a body. Does a body say a cubic\\ninch of copper weigh the same in Brazil and in Iceland If you\\ncould take it 10,000 miles above the Earth would it weigh less or\\nmore than in New York How is the density of specimens of rocks\\ndetermined? How is the density of the whole Earth, considered as\\none mass, determined Is the temperature of the Earth greater 10\\nmiles below the surface than 5 miles deep Does the Earth receive\\nall the Sun s heat? What is the refraction of light by the Earth s\\natmosphere Does refraction increase or diminish the apparent zenith\\ndistance of the Sun? Describe the phenomena of twilight? Did\\nyou ever see it yourself? Define the different kinds of day. What\\nis a month? Define the sidereal and the equinoctial year. Which\\nis the longer? What does that prove? Will Polaris always be our\\npole-star? What is the cause of precession? What three great\\nnames are connected with the discoveries regarding precession and\\nat what dates How was it first proved that light required time to\\npass from place to place About how long does it take sunlight to\\nreach the Earth? to reach Neptune? About how long does it require\\nfor the light of the nearest star to reach the Earth (Answer, 4\\nvears.)", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0282.jp2"}, "281": {"fulltext": "ABERRATION.\\n259\\nFig 157.\u00e2\u0080\u0094 Effects of Aberration.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0283.jp2"}, "282": {"fulltext": "CHAPTER XIV.\\nCELESTIAL MEASUREMENTS OF MASS AND DISTANCE.\\n29. The Celestial Scale of Measurement. The units of\\nlength and mass employed in Astronomy are necessarily\\ndifferent from those used in daily life. The distances and\\nmagnitudes of the heavenly bodies are never reckoned in\\nmiles or other terrestrial measures for astronomical pur-\\nposes; when so expressed it is only for the purpose of\\nmaking the subject clearer to the general reader. The\\nmass of a body may be expressed in terms of that of the\\nSan or of the Earth, but never in kilogrammes or tons,\\nunless in popular language.\\nThere are two reasons for this course. One is that in\\nmost cases celestial distances have first to be determined in\\nterms of some celestial unit the Earth s distance from the\\nSun, for instance and it is more convenient to retain this\\nunit than to adopt a new one. The other is that the\\nvalues of celestial distances in terms of ordinary terrestrial\\nunits are more or less uncertain, while the corresponding\\nvalues in astronomical units are known with great accuracy.\\nAn example of this practice is afforded when we deter-\\nmine the dimensions of the solar system. By a series of\\nobservations of their positions on different dates, investi-\\ngated by means of Keplek s laws and the theory of\\ngravitation, it is possible to determine the forms of the\\nplanetary orbits, the positions of their planes, and their\\nrelative dimensions, with great precision.\\nKepler s third law enables us to determine the mean\\ndistance of a planet from the Sun when we know its\\n260", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0284.jp2"}, "283": {"fulltext": "CELESTIAL MEASUREMENTS. 261\\nperiod of revolution (see page 200). All the major planets,\\nas far out as Saturn, have been observed through so many\\nrevolutions that their periodic times can be determined with\\ngreat exactness in fact within a fraction of a millionth\\npart of their whole amount. The more recently discovered\\nplanets, Uranus and Neptune, will, in the course of time,\\nhave their periods determined with equal precision. Then,\\nif we square the periods expressed in years and decimals of\\na year, and extract the cube root of this square, we have\\nthe mean distance of the planet with the same order of\\nprecision.\\nAgain, the eccentricities of the orbits are exactly deter-\\nmined by careful observations of the positions of the planets\\nduring successive revolutions. Thus we can draw a map\\nof the planetary orbits so exact that its errors will entirely\\nelude the most careful scrutiny, though the map itself\\nmight be many yards in extent.\\nOn such a map we can lay down the magnitudes of the\\nplanets as accurately as our micrometers can measure\\ntheir angular diameters. Thus we know that the mean\\ndiameter of the Sun, as seen from the earth, subtends\\nan angle of 32 We can therefore, on such a map of the\\nsolar system, lay down the Sun in its true size, on the scale\\nof the map. This can be done in the same way for each\\nof the planets, the Earth and Moon excepted. There is no\\nimmediate and direct way of finding how large the Earth\\nor Moon would look from the Sun or from a planet;\\nwhence the exception.\\nBut without further research we shall know nothing\\nabout the scale of our map. That is, we shall have no\\nmeans of knowing how many miles in space correspond to\\nan inch on the map. If we can learn either the distance\\nor magnitude of any one of the planets laid down on the\\nmap, in miles or in semi diameters of the Earth, we shall\\nbe able at once to find the scale.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0285.jp2"}, "284": {"fulltext": "262 ASTRONOMY.\\nThe general custom of astronomers is not to attempt to\\nuse a scale of miles at all, but to employ the mean distance\\nof the Sun from the Earth as the unit in celestial measure-\\nments. Thus, in astronomical language, we say that the\\ndistance of Mercury from the Sun is 0.387, that of Venus\\n0.723, that of Mars 1.523, that of Saturn 9.539, and so\\non in terms of the Earth s distance 1.000. But this\\ngives us no information respecting the distances in terms\\nof terrestrial measures.\\nThe distance of the Earth in miles is not the only\\nunknown quantity on our map. We know nothing respect-\\ning the distance of the Moon from the Earth, because\\nKepler s laws apply directly to bodies moving around the\\nSun. We must therefore determine the distance of the\\nMoon as well as that of the San. When these two things\\nare done a map of the solar system can be made in which\\nevery measurement can be expressed in miles.\\nThe Solar and Lunar Parallax.\\nThe problem of distances in the solar system is thus\\nreduced to measuring the distances of the Sun and Moon\\nin miles or in terms of the Earth s radius 4000 miles).\\nThe most direct methods of doing this are as follows:\\nfmSn\\nFig. 158. Determination of the Distance of the Moon.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0286.jp2"}, "285": {"fulltext": "SOLAR AND LUNAR PARALLAX 263\\nDistance of the Moon. In the figure C is the centre of\\nthe Earth, and S and S are the positions of two\\nobservers on its surface. P is the Moon in space and PC\\nis the distance to be determined (in miles). The observer\\nat S sees the Moon on the celestial sphere at P and he\\nmeasures its zenith distance Z P At the same instant\\nthe observer at 8 sees the Moon on the celestial sphere at\\nP and he measures its zenith distance Z P (The student\\nmust imagine the circle Z P to be completed, and Z a\\npoint in the circle.) Z P and Z P are then known arcs\\nand Z S P and Z S P are known angles, since they are\\nmeasured by known arcs. The angle PS Cm known; it\\nis 180\u00c2\u00b0 PS Z The angle PS C is known; it is\\n180\u00c2\u00b0 PS Z As the two observers are at stations whose\\nlatitudes are known, the angle S CS is known. It is the\\ndifference of their latitudes (for if H H i is the plane of the\\nEarth s equator, S CH X is the latitude of S and S Cff l\\nis the latitude of S and therefore S CS is known).\\nIn the quadrilateral S CS P the three angles whose\\nvertices are at C, at S\\\\ and at S are known, and therefore\\nthe fourth angle whose vertex is at P is known. In this\\nquadrilateral two of the sides are known because they are\\nradii of the Earth. Hence the distances S P and S P\\nare known. From either of the triangles S CP or S CP\\nthe distance of the Moon, CP, can be calculated.\\nThis is one method of determining the distance of the\\nMoon. Knowing the actual dimensions of the Earth in\\nmiles, observations of the Moon made at stations widely\\nseparated in latitude, as Paris and the Cape of Good Hope,\\ncan be combined so as to give the Moon s distance in miles.\\nOn precisely the same principles the distances of Venus or\\nMars have been determined in miles.\\nThe Distance of the Sun from Transits of Venus. When\\nVenus is at inferior conjunction she is between the Sun\\nand the Earth. If her orbit lay in the ecliptic, she would", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0287.jp2"}, "286": {"fulltext": "264\\nASTRONOMY.\\nbe projected on the Sun s disk at every inferior conjunc-\\ntion. The inclination of her orbit is, in fact, about 3\u00c2\u00a3\u00c2\u00b0,\\nand thus transits of Venus occur only when she is near the\\nnode of her orbit at the time of inferior conjunction. In\\nFig. 159 let E, V, S be the Earth, Venus, and the Sun.\\nDC is a part of Venus 1 orbit. An observer at B will see\\nVenus impinge on the Sun s disk at be just internally\\ntangent at II, move across the disk to III, and off at IV.\\nFro. 159. Determination of the Distance of the Sun.\\nSimilar phenomena will occur for A at 1, 2, 3, 4. When\\nA sees Venus at a, B will see her at b. ab AB Va VA\\nbut VA Va as 1 2-J nearly, ab therefore occupies on\\nthe Sun s disk a space 2-j- times as large as the Earth s\\ndiameter. If we measure the angular dimension ab in any\\nE\\nFig. 160.\\nway, and divide the resulting angle by 2|, we shall have\\nthe angle subtended at the Sun by the Earth s diameter;\\nor if we divide it by 5, the angle subtended at the Sun\\nby the Earth s radius. (Fig. 160.) Having this angle", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0288.jp2"}, "287": {"fulltext": "MASSES OF THE PLANETS. 265\\nwe can calculate the Earth s distance from the Sun in\\nmiles from the right triangle 8EG, where S is the Sun,\\nthe Earth s centre, and E the end of the Earth s radius.\\nThe angular space ab can be calculated at a transit of\\nVenus, when we know the length of the chords II, III,\\nand 2, 3. The length of each chord is known by observing\\nthe interval of time elapsed from phase to phase III.\\nOther Methods of Determining Solar Parallax. The\\nmost accurate method of measuring the Sun s distance\\ndepends upon a knowledge of the velocity of light. The\\ntime required for light to pass from the Sun to the Earth\\nis known with considerable exactness, being very nearly\\n498 seconds.* If we can determine experimentally how\\nmany miles light moves in a second, we shall at once have\\nthe distance of the Sun in miles by multiplying that\\nquantity by 498. The velocity of light is about 186,330\\nmiles per second. Multiplying this by 498, we obtain\\n92,800,000 miles for the distance of the Sun. The time\\nrequired for light to pass from the Sun to the Earth is\\nstill somewhat uncertain, but this value of the Sun s\\ndistance is probably the best yet obtained.\\nRelative Masses of the Sun and Planets.\\nIn estimating the masses as well as the distances of celestial bodies\\nit is necessary to use what we may call celestial units; that is, to take\\nthe mass of some celestial body as a unit, instead of any multiple of\\nthe pound or kilogram. The reason of this is that the ratios be-\\ntween the masses of the planetary system, or, what is the same\\nthing, the mass of each body in terms of that of some one body as\\nthe unit, can be determined without knowing the mass of any one\\nof them in pounds or tons. To express a mass in kilograms or\\nother terrestrial units, it is first necessary to find the mass of the\\nEarth in such units, as already explained. This, however, is entirely\\nunnecessary for astronomical purposes. In estimating the masses of\\nSee page 256.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0289.jp2"}, "288": {"fulltext": "266 ASTRONOMY.\\nthe individual planets, that of the Sun is generally taken as a unit.\\nThe planetary masses are then all very small fractions.\\nThe ma s of the Sun being 1.00, the mass of Mercury is t^-^o^u^;\\nVenus\\n1S sofVott;\\nEarth\\ni S 733TT1P\\nMars\\n1S ToTOTTff\\nJupiter\\n1S iVls!\\nSaturn\\ni s T502\\nUranus\\n18 2 r rm\\nNeptune is T g^ 7\\nThe mass of the Earth being 1, the mass of the Moon is -g T\\nThe masses of the planets that have satellites are determined by\\nmeasuring the attraction that is exerted by the planet on the satellite.\\nIf the distance of the planet from the Sun (M) is R and its peri-\\nodic time is T, and if the planet whose mass is m has a satellite re-\\nvolving in a circular orbit whose radius is r in a time t, it is proved\\nin Mechanics that\\nby which expression we can determine m, the mass of the planet, in\\nfractions of the Sun s mass M, because R, T, r, t are known. In this\\nway the masses of all the planets that are attended by satellites are\\ncalculated, after making suitable allowances for the fact that the\\norbits of the satellites are ellipses and not circles.\\nMercury and Venus have no satellites, and their masses are calcu-\\nlated by determining the perturbations that they cause in the motions\\nof other planets (and of comets) in their vicinity.\\nThe angular diameters of the planets are measured with a microm-\\neter attached to a telescope. The result is expressed in seconds of\\narc. Knowing the distance of the planet in miles, the diameter can\\nalso be expressed in miles. (See page 144.)\\nThe surface of a planet is proportional to the square, and its vol-\\nume to the cube of its diameter. The mass of a planet is deduced as\\nabove described. Its density is obtained by dividing its mass by its\\nvolume.\\nIn what has gone before the methods of determining the\\nmass, the distance, the diameter, the orbit, etc., of each\\nplanet have been described with more or less fnllness. The\\nfundamental principles of the methods are all that can be", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0290.jp2"}, "289": {"fulltext": "CELESTIAL MEASUREMENTS. 267\\ngiven here. The details of the processes of observation\\nare explained in works on Practical Astronomy. The\\nmathematical forms involved are treated in works on\\nCelestial Mechanics. It will at least be obvious to the\\nstudent that the masses, the distances, and the orbits of\\nthe planets can be determined in the ways that have been\\nexplained, though he will not know the details of the\\nprocesses actually employed.\\nThe results that have been obtained are given in two\\ntables printed in Chapter XV. These two tables contain\\ndata sufficient to enable us to construct the map of the\\nsolar system that was spoken of on page 261. With the\\nnumbers there set down a plan of the solar system can be\\nmade with great exactness. The orbit of each planet can\\nbe drawn in its true shape and situation. The place of\\neach planet at any past or future time can be assigned.\\nThe diameter of each planet can be marked on the map to\\nthe proper scale. The mass, the force of gravity at the\\nsurface, the volume, the density of each planet is known.\\nThe problems that were attacked by Ptolemy, Coper-\\nnicus, Kepler, and Newton are solved. The solar\\nsystem considered as a collection of heavy bodies revolving\\nin space under the law of gravitation is explained.\\nIn order to complete the description of the solar system\\nsomething more is necessary. We desire to know the\\ntopography, the meteorology, the physical condition of\\neach planet just as we know the topography, the climates,\\nthe physics of our own Earth. We wish to know the\\ngeology and the chemistry of the stars just as we know\\nthe geology and the chemistry of the Earth. The science\\nthat treats of the physical condition of the Sun, Moon,\\nplanets, stars, and other celestial bodies is called Astro-\\nnomical Physics or Astro-physics. The methods of this\\nscience are the methods of terrestrial physics extended so\\nas to deal -with all celestial bodies. A description of the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0291.jp2"}, "290": {"fulltext": "268 ASTRONOMY.\\nphysical condition of the heavenly bodies is called Descrip-\\ntive Astronomy. The second part of this book is chiefly\\ndevoted to such a description, which it is necessary to give\\nin a very abbreviated form. The student can supplement\\nwhat is printed here by consulting articles on the Sun,\\nMoon, and planets, etc., in any good encyclopedia, or by\\nreading some of the excellent books on Popular Astronomy\\nwritten by Sir Kobekt Ball, Flammakion, Proctor, and\\nothers.\\nWhat is the most convenient unit of length in the description\\nof the solar system How is the periodic-time of a planet deter-\\nmined Knowing the periodic-time, how do you obtain its mean-\\ndistance How is the angular diameter of a planet found Explain\\nthe principle by which the distance of the Moon from the Earth may\\nbe determined. Explain how the distance of the Sun from the Earth\\nis found from Transits of Venus over the Sun s disk. Explain how\\nthe Sun s distance can be found when we know the velocity of light.\\nWhat is the most convenient unit of mass in the description of the\\nsolar system On what principle are the masses of Mars, Jupiter,\\netc., determined? the masses of Mercury and Venus f When the\\nmass and the volume of a planet is known, how is its density\\nobtained", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0292.jp2"}, "291": {"fulltext": "PART II.\\nTHE SOLAR SYSTEM.\\nCHAPTER XV.\\nTHE SOLAR SYSTEM.\\n30. The solar system consists of the sun as a central\\nbody, around which revolve the major and minor planets,\\nwith their satellites, a few periodic comets, and a number\\nof meteor swarms. These are permanent members of the\\nsystem. Other comets appear from time to time and make\\npart of a revolution around the sun, and then depart into\\nspace again, thus visiting the system without being per-\\nmanent members of it.\\nThe bodies of the system may be classified as follows:\\nI. The central body the Sun.\\nII. The four inner planets Mercury, Venus, the Earth,\\nMars.\\nIII. A group of small planets, called Asteroids, re-\\nvolving outside of the orbit of Mars.\\nIV. A group of four outer planets Jupiter, Saturn,\\nUranus, and Neptune.\\nV. The satellites, or secondary bodies, revolving about\\nthe planets, their primaries.\\nVI. A number of comets and meteor swarms revolving\\nin very eccentric orbits about the Sun.\\nThe eight planets of Groups II and IV are classed\\n269", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0293.jp2"}, "292": {"fulltext": "270 ASTRONOMY.\\ntogether as the major planets, to distinguish them from the\\nfive hundred or more minor planets of Group III. Mercury\\nand Venus are inferior planets the other major planets\\nare called superior planets.\\nFig 1(51. Plan of the Solar System.\\nDimensions of the Solar System. The figure gives a plan\\nof part of the solar system as it would appear to a spectator\\nimmediately above or below the plane of the ecliptic. It is\\ndrawn approximately to scale, the mean distance of the\\nEarth (=1) being half an inch. On this scale the mean\\ndistance of Saturn would be 4.77 inches, of Uranus 9.59\\ninches, of Neptune 15.03 inches. On the same scale the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0294.jp2"}, "293": {"fulltext": "TEE SOLAR SYSTEM. 271\\ndistance of the nearest fixed star would be over one and\\none half miles. The student should remember that the\\nimmense spaces between the planets and between the stars\\nare empty except for a few comets and for swarms of\\nmeteors. The striking fact is how few are the bodies that\\ncirculate in these immense regions of space.\\nThe arrangement of the planets and satellites is, then\\nThe Inner Group. Asteroids. The Outer Group.\\nMercury. n r Jupiter and 5 satellites.\\ntt 500 mmor planets, Lf\\nV enus. I J Saturn and 9 satellites.\\nEarth and Moon. f 1 Uranus and 4 satellites.\\nMars and 2 satellites. J I Neptune and 1 satellite.\\nThe different planets are at very different distances from\\nthe Sun. To the nearest planets, Mercury and Ve?ius, the\\nSun appears as a very large disk. To the earth the Sun\\nappears as a disk about half a degree in diameter. The\\namount of light and heat received from the San by any\\nplanet varies as where r is the planets distance. The\\nsurface of the circles in figure 162 also vary as 2 and hence\\nthe surfaces show the relative amounts of light and heat\\nreceived by the planets (Flora and Mnemosyne are two of\\nthe asteroids). The distance of Neptune from the Sun is\\neighty times that of Mercury and it receives only ^jVo P art\\nas much light and heat.\\nTo avoid repetitions, the elements of the major planets\\nand other data are collected into the following tables, to\\nwhich the student should constantly refer in his reading.\\nThe units in terms of which the various quantities are\\ngiven are those familiar to us, as miles, days, etc., yet some\\nof the distances, etc., are so immensely greater than any\\nknown to our daily experience that we must have recourse\\nto illustrations to obtain any idea of them at all.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0295.jp2"}, "294": {"fulltext": "272\\nASTRONONY.\\nFor example, the distance of the sun is said to be about 93 million\\nmiles. It is of importance that some idea should be had of this dis-\\ntance, as it is the unit, in terms of which not only the distances in\\nthe solar system are expressed, but also distances in the stellar\\nFig. 162.\\n-The Apparent Size of the Sun as Viewed from\\nthe Different Planets.\\nuniverse. Thus when we say that the distance of the nearest star is\\nover 200,000 times the mean distance of the sun, it becomes\\nnecessary to see if some conception can be obtained of one factor in\\nthis.\\nOf the abstract number, 93,000,000, we have no idea. It is far too", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0296.jp2"}, "295": {"fulltext": "THE SOLAR SYSTEM. 273\\ngreat for us to have counted. We have never taken in at one view\\neven a million similar discrete objects. The largest tree has less\\nthan 500,000 leaves. To count from 1 to 200 requires, with very\\nrapid counting, 60 seconds. Suppose this kept up for a day without\\nintermission at the end we should have counted 288,000, which is\\nabout 3I3 of 93,000,000. Hence over 10 months uninterrupted\\ncounting by night and day would be required simply to enumerate\\nthe number, and long before the expiration of the task all idea of it\\nwould have vanished.\\nWe may take other and perhaps more striking examples. We\\nknow, for instance, that the time of the fastest express-trains between\\nNew York and Chicago, which average 40 miles per hour, is about a\\nday. Suppose such a train to start for the sun and to continue run-\\nning at this rapid rate. It would take 363 years for the journey.\\nThree hundred and sixty-three years ago there was not a European\\nsettlement in America.\\nA cannon-ball moving continuously across the intervening space\\nat its highest speed would require about eight years to reach the sun.\\nIn a little less than a day it would go once round the earth if its\\ncourse was properly curved. To reach the sun it would have to\\ntravel for eight years at this velocity. The report of the cannon, if\\nit could be conveyed to the sun with the velocity of sound in air,\\nwould arrive there four years after the projectile. Such a distance\\nis entirely inconceivable, and yet it is only a small fraction of those\\nwith which astronomy has to deal, even in our own system. The\\ndistance of Neptune is 30 times as great.\\nIf we examine the dimensions of the various orbs, we meet almost\\nequally inconceivable numbers. The diameter of the sun is 866,400\\nmiles its radius is but 433,200, and yet this is nearly twice the\\nmean distance of the moon from the earth. Try to conceive, in\\nlooking at the moon in a clear sky, that if the centre of the sun could\\nbe placed at the centre of the earth, the moon would be far within\\nthe sun s surface.\\nOr again, conceive of the force of gravity at the surface of the\\nvarious bodies of the system. At the sun it is nearly 28 times that\\nknown to us. A pendulum beating seconds here would, if transported\\nto the sun, vibrate with a motion more rapid than that of a watch-\\nbalance. The muscles of the strongest man would not support him\\nerect on the surface of a planet of the mass of the sun even lying\\ndown he would be crushed to death under his own weight of more\\nthan two tons. At the moon s surface the weight of a man would be\\nabout one-sixth of his weight on the earth (since the Moon s mass is", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0297.jp2"}, "296": {"fulltext": "274 ASTRONOMY.\\nabout one-sixth of the Earth s), and his muscular force, on such a\\nplanet, would enable hirn to bound along with leaps of 30 feet or\\nmore. There are, of course, no human beings on the sun or on the\\nmoon. One of these bodies is too hot, the other too cold, to support\\nhuman life. We may by these illustrations get some rough idea of\\nthe meaning of the numbers in these tables, and of the incapability\\nof our limited powers to comprehend the true dimensions of even the\\nsolar system. When we come to a description of the stellar uni-\\nverse we shall meet with distances and dimensions almost infinitely\\nlarger.\\nIt is important that the student should realize, so far as\\nhe can, the data given in these tables; and there is no\\nbetter way to do this than to make drawings to scale from\\nthe numbers there set down. For instance, let the student\\ndraw lines to represent the apparent angular diameters of\\nthe different planets as seen from the Earth (Table II) on\\na scale of one inch 30 and then draw the circles corre-\\nsponding to these diameters. None of the circles for the\\nplanets will be more than two and a half inches in diam-\\neter; but if he wishes to draw a circle to represent the\\napparent disk of the Sun on this scale it will have to be\\nover five feet in diameter. If a diagram of this sort is\\nactually constructed it will impress the student s mind far\\nmore than a mere reading of the figures of the table. If\\nhe makes a drawing, to scale, of the system of Jupiter s\\nsatellites putting in the data of Table III and whatever\\nelse he can find in Chapter XVIII, a definite idea of the\\narrangement and sizes of these satellites will be acquired\\nand it will not soon be forgotten. The distances of the\\nperiodic comets given in Table IV should be platted to\\nscale along with the major axes of the Earth, Mars,\\nJupiter, Saturn, Uranus, and Neptune.\\nSimilar diagrams of the inclinations of the planetary\\norbits, of their periodic times, volumes, masses, densities,\\netc., will serve to impress the mind with the resemblances\\nand with the differences in the different bodies of the\\nsystem.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0298.jp2"}, "297": {"fulltext": "THE SOLAR SYSTEM. 275\\nThe mass of the sun is far greater than that of any single planet in\\nthe system, or indeed than the combined mass of all of them. If the\\nmass of the earth is represented by a single grain of wheat the mass\\nof the Sun will be represented by about four bushels of such grains.\\nIt is a remarkable fact that the mass of any given planet exceeds the\\nsum of the masses of all the planets of less mass than itself.\\nThe total mass of the asteroids, like their number, is unknown, but\\nit is probably less than one-thousandth that of our Earth. The Sun s\\nmass is over 700 times greater than that of all the other bodies, and the\\nfact of its central position in the solar system is thus explained. In\\nfact, the centre of gravity of the whole solar system is very little out-\\nside the body of the Sun, and will be inside of it when Jupiter and\\nSaturn are in opposite directions from it (when their celestial longi-\\ntudes differ by 180\u00c2\u00b0).\\nThere are very few persons who realize in any vivid way\\nthe distances and dimensions of the planets of the solar\\nsystem. No very keen realization is to be had by merely\\nreading the figures of the tables. If it is practicable the\\nstudent should, once in his life, make a plan of the solar\\nsystem in the following way. If a whole class can make\\nthe experiment in company it will be an advantage.\\nFrom the Tables I and II it should first be proved that if the Sun\\nwere two feet in diameter instead of 866,400 miles the different\\nplanets would be fairly well represented in bulk as follows\\nMercury by a grain of mustard-seed, Venus by a very small green\\npea, The Earth by a common sized green pea, Mars by the head of a\\nrather large pin, Jupiter by a ball of the size of an orange, Saturn\\nby a golf-ball, Uranus by a common marble, Neptune by a rather\\nlarger marble.\\nThe scale of the plan of the solar system is to be two feet 870,-\\n000 miles. To make the plan a level road about 2$ miles long is\\nneeded. A stake should be driven into the ground to represent the\\nplace of the Sun, and if the length of the stake above ground is two\\nfeet the Sun s diameter will be represented by it.\\nThe distances of the planets must be laid off on the same scale of\\ntwo feet 870,000 miles. Steps of two feet long will serve to\\nmeasure the distances. The student should first verify the following\\nfrom the numbers given in Table I. On the adopted scale the dis-\\ntance from the Sun to Mercury is 82 steps from Mercury to Venus\\nis 60 steps from Venus to the Earth is 73 steps from the Earth to", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0299.jp2"}, "298": {"fulltext": "276\\nASTRONOMY.\\nMars is 108 steps from Mars to Jupiter is 785 steps from Jupiter\\nto Saturn is 934 steps Saturn to Uranus is 2,086 steps and from\\nUranus to Neptune is 2,322 steps.\\nWith these distances let the student set oat from the stake that\\nrepresents the Sun and deposit the models of the different planets at\\ntheir proper distances Mercury at 82 steps from the stake, Venus at\\n142 steps, and so on to Neptune, which will be 6,450 steps away\\nnearly 2\\\\ miles.\\nA marble 2\u00c2\u00a3 miles away from a globe 2 feet in diameter represents\\nthe relation in distance and in size between Neptune and the Sun. A\\nfew other globes, all very small, at large intervals, represent the\\nmajor planets A few grains of sand represent the asteroids. The\\nspaces of the solar system between the planets are empty except for\\na few comets and meteor-swarms.\\nOn the scale of the model the distance of the nearest fixed star\\nfrom the stake that represents the Sun is 8,000 miles. A globe\\nabout three or four feet in diameter at Peking might stand for this\\nstar if the model of the solar system were made in New York. A\\nmorning spent in actually making such a model of the solar system\\nwill not be wasted. There is no better way of realizing the dimen-\\nsions of the bodies of the solar system and the immense extent of\\nempty space between them.\\nTABLE I.\\n(Approximate) Elements of the Orbits of the Eight\\nMajor Planets.\\nMean Distance\\nfrom Sun.\\no\\no a\\no\\no\\nDo\\n3\\n2\\na\\n31\\n3-~ _\\nName.\\nMil-\\nAstronom-\\nlions\\n11\\nSc aS\\n_C\u00e2\u0080\u0094\\nEcu\\np 2\\n0) s-, \u00c2\u00ab3\\nical Units.\\nof\\nBo\\n^H\\no J\\nV O\\n0.387099\\nMiles.\\nw\\nJ\\na\\nJ\\n323\u00c2\u00b0\\nH\\nMercury..\\n36.0\\n0.21\\n75\u00c2\u00b0\\n7\u00c2\u00b0 0\\n47\u00c2\u00b0\\nh 3 m\\nVenus..\\n0.723332\\n67.2\\n0.01\\n129\\n3 24\\n75\\n244\\n6!\\nEarth...\\n1.000000\\n92.9\\n0.02\\n100\\n100\\n8\\nMars.\\n1.523691\\n141\\n0.09\\n333\\n1 51\\n48\\n83\\n13\\nJupiter\\n5.202800\\n483\\n0.05\\n12\\n1 19\\n99\\n160\\n43\\nSaturn.\\n9.538861\\n886\\n0.06\\n90\\n2 29\\n112\\n15\\n1 19\\nUranus\\n19.18329\\n1782\\n0.05\\n171\\n46\\n73\\n29\\n2 38\\nNeptune..\\n30.05508\\n2791\\n0.01\\n46\\n1 47\\n130\\n335\\n4 8", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0300.jp2"}, "299": {"fulltext": "THE SOLAR SYSTEM.\\n277\\nr-H H\\nO\\n(-3 23\\nEh\\nO\\n10\\nOrbita\\nVeloc-\\nity in\\nMiles\\nper\\nSecond\\nCO\\nco\\nOS\\nGO\\nO lH\\niO 00\\nO\\nco\\nCO\\n1\\nCO\\n00 -co\\nCO\\n0?\\nGO\\nj,o\u00c2\u00ab\\n03\\nOS\\ni\\nC3\\nOS \u00c2\u00bbso\\nt^\\nIO\\nCO fr-,\\nOS\\nTtf\\ntH\\na\\n00\\nOJ\\nCO\\ng\\nOJ\\nGO\\nCO\\n03\\nCO\\n1-1\\n-s\u00c2\u00abS ii\\n03 \u00c2\u00b0S u\\n00\\nc i\\n05\\nOS\\nO\\nos\\nO 0Q\\nW\\na\\nos\\nt-\\n00\\na\\na\\na\\n.2\\nCO\\n03\\nt-\\nS\\nCO\\n10\\nS S\\ni 1.0\\nco 10\\ne\\na\\na\\np\\nP\\nCO\\nCO\\nOS\\nP\\nP\\n3^\\nCO\\nOS\\nt- O?\\nrH\\nCQ\\n01\\n11\\nO\\nO\\nrH\\nc\\nO\\nw\\nDO\\nOS\\nCO\\nc\\nCO\\ni\\nOJ\\n,_\\nn\\n1-1\\nCO\\nrji\\n^T\\nO\\n1-1\\nu\\nO\\nO\\n00\\nc\\nO\\n5 S\\nO\\nCO\\nO\\noc\\nO\\nr\\nOS\\nc\\n10\\nO\\nOS\\n00\\nSg-S\\nCO\\nCO\\nt-\\nt-\\nT\\nCO\\n\u00c2\u00a7.g s\\n00\\ni\\nCO\\nCO\\ncv-\\nc\\nCO\\n07\\nOl\\nr-t\\n10\\nC\\nOI\\nIQ\\nCO\\nO\\nc\\nO\\n\u00c2\u00ab4\\nS3\\nv\\n5\\n5\\n1-1\\nos\\n43\\nco\\nt\\nO\\na o3 9 u s-\\ng\\nco\\n10\\nCM\\ntf\\nCQ\\nt-*g V cs sj\\nc\\nO\\ng,bBg fl H O\\nH -1\\nCW-. S3 0) -h\\nO\\nCO\\n10\\n01\\nCO\\n00\\nco\\n05\\nb\\nto\\n|o\\nlo\\nfo\\nl\\nJ:\\nI p\\na\\n|M\\nlo\\nrite\\nto\\nto\\nlo\\nen\\nDQ\\n=3\\n5\\nJ?\\n100\\n10\\nlea\\nto\\n1\\n1\\n1\\nIO\\nto\\nto\\n03\\na\\nc\\nCO\\n3\\nG\\nt-\\n13\\n(1\\nC\\nC\\na,\\n3\\n03\\nas.\\nw\\nDC\\nS\\n-5\\n0Q\\nP\\nJ3\\nH o\\nC3", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0301.jp2"}, "300": {"fulltext": "278\\nASTRONOMY.\\n0 m\\nii a\\nCO O\\n\u00c2\u00a33\\nB\\nCD co\\n.-no O\\nH CG\\nco c3\\n\u00c2\u00a3S\\nco\\n\u00e2\u0096\u00a0Si 1\\neS 3 cS\\nj= 2^ 8\\no .o\\nI M*\\nx cS 0) w\\n*i-* a co\\nCE-r\u00c2\u00a3cr-t\\nCD 13\\no\\na\\n.B c3\\nCO\\ns\\n.8\\nCD\\ns u\\n8 w\\nbe\\n\u00c2\u00a3j w\\ncu\\noS\\nw\\nCD *j\\ncd,b\\n;S,B\\nO\\nCD CO\\nOiO-IIOO*\\ntDQO -N/^lOlOO .lOTTOcjO\\nos s\\nojoj\\n1-1 Tl\\nIO m\\n_ w B\\n\u00c2\u00b0.2\\nCD-rJ+J\\n\u00c2\u00a72.2.2\\n2\u00c2\u00a32\\nCCCl, cd\\niO i \u00c2\u00bb0 i-i rr Oi NOIOMCWO riWinrfi\\ni- oi so oi co i-i mtDifloii-icoi^t^ t^ oico\\nno o* co tj co r* ot oi et i\\nt- i t- co o? o* nooiHini-(\\n\u00c2\u00abD i~l 00 CO CO O CV00i-it-0*C?tOt\\n\u00c2\u00abni\\nOHMt\\nO* Wi-i i\\nCN tj-qocC\\nQ\u00c2\u00a3 a\\nBCD -5\\nc to\\noo ooooo\\nj m ooooo)\\n)to pooop\\n3 W rH 1(5 Tf l-\\nooo\\nooo\\nSO IO 00 CO CO l~ CO OJ\\n-h \u00e2\u0080\u00a2Hni-iNmt C\u00c2\u00bb\u00c2\u00ab\\ni-l OJ\\noooo^^\\noooo\\n\u00e2\u0096\u00a0t-toooTj-itinf.ONWirj\\nIO CO CC M N CO 7) OiCOt^CD\\nooooo\\n2S5cdo\\nc^: s\\n1 CO GO QC\\ni CO 00 C- 5\u00c2\u00b0 CO i-.-CO iflinrlr\\ncX S\\n03 ^^2 CC 50\\n.5 S2.S-SO S\\nc3 t -CQ c6\\nO SO- Hi\\na\\nre\\n2\\nII II ii li II II ii ii II n H il ll li ll II II ii\\nf\u00c2\u00bb HM 2f i-iOJCOTf\u00c2\u00bbOOt-G005i-iOI OiJ\\n\u00e2\u0080\u00a221 2 e s", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0302.jp2"}, "301": {"fulltext": "TEE SOLAR SYSTEM.\\n279\\nTABLE IV.\\nThe Comets op the Solar System (Periodic Comets).\\nNo.\\n1\\n2\\n3\\n4\\n5\\n6\\n7\\n8\\n9\\n10\\n11\\n12\\n13\\n14\\n15\\n16\\n17\\nName.\\nEncke\\nTempel\\nBrorsen\\nTeinpel- Swift\\nWinnecke\\nDa Vico-Swift\\nTempel\\nBiela\\nFinlay\\nD Arrest\\nWolf\\nBrooks\\nFaye\\nTuttle\\nPons-Brooks..\\nOlbers\\nHalley\\nTime of Peri-\\nc\\na _\\nc X\\no u X\\nhelion Passage.\\n-f\\nD O\\ns a o\\nZ Cj t;\\nP-\\nJC C\\nfe\\nP..5S g.\\nPn\\n\u00c2\u00a3cS\\nQS\\n1895 Feb. 4\\n3.30\\n0.34\\n4.10\\n1894 April 23\\n5.22\\n1.35\\n4.67\\n1890 Feb. 24\\n5.46\\n0.59\\n5.61\\n1891 Nov. 14\\n5.53\\n1.09\\n5.17\\n1892 June 30\\n5.82\\n0.89\\n5.58\\n1894 Oct. 12\\n5.86\\n1.39\\n5.11\\n1885 Sept. 25\\n6.51\\n2.07\\n4.90\\n1852 Sept.\\n6.6-\\n0.86\\n6.2-\\n1893 July 12\\n6.62\\n0.99\\n6.06\\n1890 Sept. 17\\n6.69\\n1.32\\n5.78\\n1891 Sept. 3\\n6.82\\n1.59\\n5.60\\n1896 Nov. 4\\n7.10\\n1.96\\n5.43\\n1881 Jan. 22\\n7.57\\n1.74\\n5.97\\n1885 Sept. 11\\n13.76\\n1.02\\n10.46\\n1884 Jan. 25\\n71.48\\n0.76\\n33.67\\n1887 Oct. 8\\n72.63\\n1.20\\n33.62\\n1835 Nov. 15\\n76.37\\n0.59\\n35.41\\n13\u00c2\u00b0\\n13\u00c2\u00b0\\n29\u00c2\u00b0\\n5\u00c2\u00b0\\n15\u00c2\u00b0\\n3\u00c2\u00b0\\n11\u00c2\u00b0\\n13\u00c2\u00b0\\n3\u00c2\u00b0\\n16\u00c2\u00b0\\n25\u00c2\u00b0\\n6\u00c2\u00b0\\n12\u00c2\u00b0\\n55\u00c2\u00b0\\n74\u00c2\u00b0\\n45\u00c2\u00b0\\n162\u00c2\u00b0", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0303.jp2"}, "302": {"fulltext": "CHAPTER XVI.\\nTHE SUN.\\n31. The Sun is a huge globe 866,400 miles in diameter.\\nIts mass is 333,470 times that of the Earth, its volume is\\n1,310,000 times the Earth s volume, its density one fourth\\nof the Earth s density. The force of gravity on its surface\\nis nearly 28 times the force of gravity on the Earth. On\\nthe Earth a heavy body falls 16 feet daring the first second\\nof its descent; at the San it woald fall 444 feet. Some\\nidea of its enormous size can be had by remembering that\\nthe Earth and Moon are but 238,000 miles apart while the\\nSun s radius is 433,200 miles. If the San were hollow\\nand the Earth was at its centre the Moon would revolve far\\nwithin the outer shell of the Sun s surface. The motions\\nof all the planets are controlled by its attraction.\\nThe San is a star. It is a sphere of incandescent gases\\nand metallic vapors. It shines by its own light and gives\\noat enormoas quantities of heat unceasingly. Only the\\nsmallest fraction of the Sun s heat reaches the Earth. Yet\\nthat small fraction (about -owoV o o o o part) supports all the\\nlife on the Earth, both of animals and plants. It main-\\ntains the circulation of winds, of ocean currents, the flow\\nof glaciers and of rivers; it is the cause of the rains, the\\nclouds, the dews that support vegetation; it controls the\\nseasons and the climates of all the regions of our globe and\\nof all the planets in the solar system. In the strictest sense\\nall the life, energy, and activity on the Earth are main-\\ntained by the Sun and principally and chiefly by the Sun s\\n280", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0304.jp2"}, "303": {"fulltext": "THE SUN. 281\\nheat. If the Sun s heat were cut off all life on the Earth\\nwould quickly cease.\\nWhile it is true that the Sun is as different as possible\\nfrom the Earth in its present state, it is to be especially\\nnoted that the difference is chiefly due to a difference of\\ntemperature. The spectroscope detects the presence of\\n(the vapors of) metals and earths in the Sun and it is likely\\nthat there is no element on the Earth that is not found\\non the Sun. Calcium, carbon, copper, hydrogen, iron,\\nmagnesium, nickel, silver, sodium, zinc, among others,\\nhave been detected, some of them in great abundance.\\nThere is every reason to believe that if the Earth were to\\nbe suddenly raised to the temperature of the Sun it would\\nbecome at once, and in virtue of temperature alone, a Sun\\nthat is a star.\\nPhotosphere. The visible shining surface of the Sun is\\ncalled the photosphere, to distinguish it from the body of\\nthe Sun as a whole. The apparently flat surface presented\\nby a view of the photosphere is called the Sun s disk.\\nSpots. When the photosphere is examined with a tele-\\nscope, dark patches of varied and irregular outline are fre-\\nquently found upon it. These are called the solar spots.\\nRotation. When the spots are observed from day to\\nday, they are found to move over the Sun s disk from east\\nto west in such a way as to show that the Sun rotates on\\nits axis in a period of 25 or 26 days. The Sun, therefore,\\nhas axis, poles, and equator, like the Earth, the axis being\\nthe line around which it rotates. It turns on its axis from\\nwest to east in 25 days, 7 hours, 48 minutes.\\nFaculae. Groups of minute specks brighter than the\\ngeneral surface of the Sun are often seen in the neighbor-\\nhood of spots or elsewhere. They are clouds of the vapors\\nof metals and are called faculm.\\nChromosphere. Just above the solar photosphere there\\nis a layer of glowing vapors and gases from 5000 to 10,000", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0305.jp2"}, "304": {"fulltext": "282 ASTRONOMY.\\nmiles in depth. At the bottom of it lie the vapors of many-\\nmetals, magnesium, sodiam, iron, etc., volatilized by the\\nintense heat, while the upper portions are composed prin-\\ncipally of hydrogen gas. The vaporous atmosphere is called\\nthe chromosphere. It is entirely invisible to direct vision,\\nwhether with the telescope or naked eye, except for a few\\nseconds about the beginning or end of a total eclipse, but it\\nmay be seen on any clear day through the spectroscope.\\nProminences, Protuberances, or Red Flames. The gases\\nof the chromosphere are frequently thrown up in irregular\\nmasses to vast heights above the photosphere, it may be\\n50,000, 100,000, or even 200,000 kilometres (120,000\\nmiles). These masses can never be directly viewed except\\nwhen the sunlight is cut off by the intervention of the\\nMoon during a total eclipse. They are then seen as rose-\\ncolored flames, or piles of bright red clouds of irregular and\\nfantastic shapes rising from the edge of the Sun. The\\nspectroscope shows that they are chiefly composed of in-\\ncandescent calcium, helium, and* hydrogen.\\nCorona. During total eclipses the Sun is seen to be\\nenveloped by a mass of soft\\nwhite light, much fainter than\\nthe chromosphere, and extend-\\ning out on all sides far beyond\\nthe highest prominences. It\\nis brightest around the edge\\nof the Sun, and fades off\\ntoward its outer boundary by\\nFig. 163. -A Method of Ob- U1 i,.\\nserving the Sun with a insensible gradations. This\\nTelescope. halo of light is called the\\ncorona, and is a very striking object during a total eclipse.\\n(Fig. 163.)\\nMethods of Observing the Sun. The light and heat of the Sun con-\\ncentrated at the focus of a telescope are very intense. An experi-\\nment with a burning-glass will illustrate this obvious fact. Special", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0306.jp2"}, "305": {"fulltext": "THE SUN 283\\neye-pieces are made so that the Sun can be looked at directly with\\nthe telescope, but the method of projecting the Sun s image on a\\nsheet of cardboard (as in the figure) is very convenient, especially\\nbecause several observers can examine the image at the same time.\\nA sheet of white cardboard is fastened to the telescope (accurately\\nperpendicular to its axis) by a light wooden or metal frame. The\\nimage of the Sun is projected on the cardboard and must be made\\nas sharp and neatly defined as possible by moving the eye piece to\\nand fro till the right focus is found. It is desirable to fasten another\\nsheet of cardboard over the tube of the telescope to shut off a part\\nof the daylight, as in the figure.\\nFig. 164.\u00e2\u0080\u0094 Copy of a Photograph of the Sun showing the\\nCentre of the Disk to be Brighter than the Edges.\\nOne of the best ways to study the Sun is to photograph it with a\\ncamera of long focus the longer the better. The exposures must\\nbe very short indeed a few thousandths of a second in most cases.\\nThe surroundings of the Sun its red flames, its corona\u00e2\u0080\u0094 can be seen\\nwith the naked eye at a total solar eclipse, and they can then be\\nphotographed. The spectroscope is used for the study of the Sun s\\nsurroundings and of its surface, as explained in the Appendix on\\nSpectrum Analysis. If the student is not already familiar with the\\nsubject through his study of physics, he should interrupt his read-\\ning of this chapter and master the principles explained in the Ap-\\npendix, as they are necessary to an understanding of what follows.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0307.jp2"}, "306": {"fulltext": "284\\nASTRONOMY.\\nThe Photosphere. The disk of the Sun is circular in\\nshape, no matter what side of the Sun s globe is turned\\ntowards the Earth, whence it follows that the Sun is a\\nsphere. The disk of the Sun is not equally bright over all\\nthe circle of the surface. The centre of the disk is most\\nbrilliant and the edges are shaded off so as to appear much\\nless brilliant, as in Fig. 164. The deficiency of brightness\\nat the edges is due to the fact that the rays that reach us\\nfrom the centre of the disk traverse a smaller depth of the\\nSun s atmosphere than those from the edges and are less\\nabsorbed by the Sun s atmosphere therefore.\\nFig\\n105.\u00e2\u0080\u0094 The Absorption of the Sun s Rays is Greater at\\nthe Edges of the Disk than at the Centre.\\nIn figure 165 let SE be the Sun s radius and SM the radius of his\\natmosphere. A person stationed beyond M (to the left hand of the\\nfigure) looking at the Sun along the lines ME and M E would see\\nthe centre of the disk by rays that had traversed the distance ME\\nonly; while the edge of the disk would be seen by rays that bad\\ntraversed the much greater distance M E", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0308.jp2"}, "307": {"fulltext": "THE SUN. 285\\nThe ray which leaves the centre of the Sun s disk in passing to\\nthe Earth traverses the smallest possible thickness of the solar\\natmosphere, while the rays from points of the Sun s body which\\nappear to us near the limbs pass, on the contrary, through the maxi-\\nmum thickness of atmosphere, and are thus longest subjected to its\\nabsorptive action.\\nThe Solar Spots. When the Sun s disk is examined with\\nthe telescope several Sun-spots can usually be seen. The\\nsmallest are mere black dots in the shining surface 500\\nmiles or so in diameter. The largest solar spots are\\nthousands of miles in diameter (100,000 miles or more).\\nFig. 166. A Large Sun-spot seen in the Telescope.\\nSolar spots generally have a black central nucleus or\\numbra, surrounded by a border or penumbra, intermediate\\nin shade between the central blackness and the bright\\nphotosphere.\\nThe first printed account of solar spots was given by\\nFabrititjs in 1611, and Galileo in the same year (May,\\n1611) also described them, Galileo s observations showed", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0309.jp2"}, "308": {"fulltext": "286 ASTRONOMY.\\nthem to belong to the Sun itself, and to move uniformly\\nacross the solar disk from east to west. A spot just visible\\nat the east limb of the Sun on any one day travelled slowly\\nacross the disk for 12 or 13 days, when it reached the west\\nlimb, behind which it disappeared. After about the same\\nperiod, it reappeared at the eastern limb.\\nThe spots are not permanent in their nature, but dis-\\nappear after a few days, weeks, or months somewhat as\\ncyclonic storms in the Earth s atmosphere persist for hours\\nor days and then are dissipated. But so long as the spots\\nlast they move regularly from east to west on the Sun s ap-\\nparent disk, making one complete rotation in about 25 days.\\nThis period of 25 days is therefore approximately the rota-\\ntion period of the Sun itself.\\nSpotted Region. It is found that the spots are chiefly confined to\\ntwo zones, one in each hemisphere, extending from about 10\u00c2\u00b0 to 35\u00c2\u00b0\\nor 40\u00c2\u00b0 of heliographic latitude. In the polar regions spots are\\nscarcely ever seen, and on the solar equator they are much more rare\\nthan in latitudes 10\u00c2\u00b0 north or south. Connected with the spots, but\\nlying on or above the solar surface, are faculce, mottlings of light\\nbrighter than the general surface of the Sun. Many of the faculce,\\nare clouds of incandescent calcium.\\nSolar Axis and Equator. The spots revolve with the surface of the\\nSun about his axis, and the directions of their motions must be ap-\\nproximately parallel to his equator. Fig. 167 shows the appearances as\\nactually observed, the dotted lines representing the apparent paths\\nof the spots across the Sun s disk at different times of the year.\\nIn June and December these paths, to an observer on the Earth,\\nseem to be right lines, and hence at these times the observer must be\\nin the plane of the solar equator. At other times the paths are\\nellipses, and in Marchand September the planes of these ellipses are\\nmost oblique, showing the spectator to be then furthest from the\\nplane of the solar equator. The inclination of the solar equator to\\nthe plane of the ecliptic is about 7\u00c2\u00b0 9 and the axis of rotation is, of\\ncourse, perpendicular to it.\\nForm of the Solar Spots. The Sun-spots are probably depressions\\nin the photosphere. When a spot is first seen at the edge of the\\ndisk it appears as a notch, and is elliptical in shape. As the Sun s\\nrotation carries it further on to the disk it becomes more and more", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0310.jp2"}, "309": {"fulltext": "THE SUN.\\n287\\ncircular. At the centre it is often circular, and as it passes off the\\ndisk the shape again becomes elliptical. The appearances are shown\\nin fig. 168, and are due to perspective.\\nr\\ng N\\nc\\nMAF\\n\\\\CH J U\\n4 h\\n1\\nS S\\nSEPT DEC\\nFig. 167. Apparent Paths op the Solar Spots to an Observer\\non the Earth at Different Seasons of the Year.\\nThe Number of Solar Spots varies Periodically. The\\nnumber of solar spots that are visible varies from year to\\nyear. Although at first sight this might seem to be what\\nwe call a purely accidental circumstance, like the occur-\\nrence of cloudy and clear years on the Earth, observations\\nof sun-spots establish the fact that this number varies\\nperiodically.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0311.jp2"}, "310": {"fulltext": "288\\nASTRONOMY.\\nThat the solar spots vary periodically will appear from the follow-\\ning summary\\nFrom 1828 to 1831 the Sun was without spots on only 1 day.\\nIn 1833 139 days.\\nFrom 1836 to 1840 3\\nIn 1843 147\\nFrom 1847 to 1851 2\\nIn 1856 193\\nFrom 1858 to 1861 no day.\\nJtoitfbV M l 4 195 days.\\nFig. 168. Appearance of the Same Solar Spot near the\\nCentre of the Sun and near the Edge.,\\nEvery eleven years there is a minimum number of spots, and about\\nfive years after each minimum there is a maximum. There was a\\nmaximum of spots in 1893 the minimum occurred in 1899. If, in-\\nstead of merely counting the number of spots, measurements are\\nmade on solar photographs of the extent of spotted area, the period\\ncomes out with greater distinctness.\\nThe cause of this periodicity is as yet unknown. It probably lies\\nwithin the Sun itself, and is similar to the cause of the periodic ac-\\ntion of a geyser.\\nThe sudden outbreak of a spot on the Sun is often accompanied by\\nviolent disturbances in the magnetic needle and there is a complete", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0312.jp2"}, "311": {"fulltext": "THE SUN.\\n289\\nconcordance between certain changes in the magnetic declination\\nand the changes in the Sun s spotted area.\\nThe agreement is so close that it is now possible to say what the\\nchanges in the magnetic needle have been so soon as we know what\\nthe variations in the Sun s spotted area are.\\nThere is a direct action between the Sun and the Earth that we\\ncall their mutual gravitation and the foregoing facts show that\\nthey influence each other in yet another way. These actions take\\nplace across the space of 93,000,000 miles which separates the Sun\\nand Earth. No doubt a similar effect is felt on every planet of the\\nsolar system.\\nThe Sun s Chromosphere and Corona. Phenomena of Total Eclipses.\\nWhen a total solar eclipse is ob-\\nserved with the naked eye its\\nbeginning is marked simply by\\nthe small black notch made in\\nthe luminous disk of the Sun by\\nthe advancing edge or limb of\\nthe Moon. This always occurs\\non the western half of the Sun,\\nbecause the Moon moves from\\nwest to east in its orbit. An\\nhour or more elapses before the\\nMoon has advanced sufficiently\\nfar in its orbit to cover the Sun s\\ndisk. During this time the disk\\nof the Sun is gradually hidden\\nuntil it becomes a thin crescent.\\nThe actual amount of the Sun s\\nlight may be diminished to two\\nthirds or three fourths of its\\nordinary amount without its\\nbeing strikingly perceptible to\\nthe eye. What is first noticed is\\nthe change which takes place in the color of the surrounding land-\\nscape, which begins to wear a ruddy aspect. This grows more and\\nmore pronounced, and gives to the adjacent country that weird effect\\nwhich lends so much to the impressiveness of a total eclipse.\\nThe reason for the change of color is simple. The Sun s atmos-\\nphere absorbs a large proportion of the bluer rays, and as this\\nabsorption is dependent on the thickness of the solar atmosphere\\nthrough which the rays must pass, it is plain that just before the\\nSun is totally covered,, the rays by which we see it will be redder\\nFig. 169.\u00e2\u0080\u0094 The Solar Corona\\nat the Total Solar Eclipse\\nof January, 1889, from Pho-\\ntographs.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0313.jp2"}, "312": {"fulltext": "290 ASTRONOMY.\\nthan ordinary sunlight, as they are those which come from points\\nnear the Sun s limb, where they have to pass through the greatest\\nthickness of the Sun s atmosphere.\\nThe color of the light becomes more and more lurid up to the mo-\\nment of total eclipse. If the spectator is upon the top of a high\\nmountain, he can then begin to see the Moon s shadow rushing to-\\nward him at the rate of a kilometre in about a second. Just as the\\nshadow reaches him there is a sudden increase of darkness the\\nbrighter stars begin to shine in tbe dark lurid sky, tbe thin crescent\\nof tbe Sun breaks up into small points or dots of light, which sud-\\ndenly disappear, and the Moon itself, an intensely black ball, appears\\nto hang isolated in the heavens.\\nAn instant afterward the corona is seen surrounding the black\\ndisk of the Moon with a soft effulgence quite different from any\\nother light known to us. Near the Moon s edge it is intensely bright,\\nand to the naked eye uniform in structure 5 or 10 from the limb\\nthis inner corona has a boundary more or less defined, and from this\\nextend streamers and wings of fainter and more nebulous light.\\nThey are of various shapes, sizes, and brilliancy. No two solar\\neclipses yet observed have been alike in this respect.\\nSuperposed upon these wings may be seen (sometimes with the\\nnaked eye) the red flames or protuberances which were first discov-\\nered during a solar eclipse. They need not be more closely de-\\nscribed here, as they can now be studied at any time by aid of the\\nspectroscope.\\nThe total phase lasts for a few minutes, and during this time, as\\nthe eye becomes more and more accustomed to the faint light, the\\nouter corona becomes visible further and further away from the\\nSun s limb. At the eclipse of 1878, July 29th, it was seen to extend\\nmore than 6\u00c2\u00b0 (about 9,000,000 miles) from the Sun s limb. Photo-\\ngraphs of the corona show even a greater extension. Just before\\nthe end of the total phase there is a sudden increase of the brightness\\nof the sky, due to the increased illumination of the Earth s atmos-\\nphere near the observer, and in a moment more the Sun s rays are\\nagain visible, seemingly as bright as ever. From the end of totality\\ntill the last contact the phenomena of the first half of the eclipse are\\nrepeated in inverse order.*\\nTelescopic Aspect of the Corona. Such are the appearances to the\\nThe Total Solar Eclipse of May 28, 1900, will be visible in the United States.\\nIts track will pass from New Orleans to Norfolk in Virginia. The duration of\\nthe total phase will be about lm. 19s. in Louisiana and lm. 49s. in North Car-\\nolina. The totality occurs about 7.30 a.m. (local time) at New Orleans, and\\nabout 9 a.m. at Norfolk. The width of the shadow track is about 55 miles.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0314.jp2"}, "313": {"fulltext": "THE SUN. 291\\nnaked eye. The corona, as seen through a telescope, is, however,\\nof a very complicated structure. It is best studied on photographs,\\nseveral of which can be taken during the total phase, to be subse-\\nquently examined at leisure.\\nThe corona and red prominences are solar appendages. It was\\nformerly doubtful whether the corona was an atmosphere belonging\\nto the Sun or to the Moon. At the eclipse of 1860 it was proved by\\nmeasurements that the red prominences belonged to the Sun and not\\nto the Moon, since the Moon gradually covered them by its motion,\\nthey remaining attached to the Sun. The corona is also a solar ap-\\npendage.\\nGaseous Nature of the Prominences. The eclipse of 1868 was total\\nin India, and was observed by many skilled astronomers. A discov-\\nery of M. Janssen s will make this eclipse forever memorable. He\\nwas provided with a spectroscope, and by it observed the promi-\\nnences. One prominence in particular was of vast size, and when\\nthe spectroscope was turned upon it, its spectrum was discontinuous,\\nshowing the bright lines of hydrogen gas.\\nThe brightness of the spectrum was so marked that Janssen de-\\ntermined to keep his spectroscope fixed upon it even after the reap-\\npearance of sunlight, to see how long it could be followed. It was\\nfound that its spectrum could be seen perfectly well after the return\\nof complete sunlight and that the prominences could be observed at\\nany time by taking suitable precautions.\\nOne great difficulty was conquered in an instant. The red flames\\nwhich formerly were only to be seen for a few moments during total\\neclipses, and whose observation demanded long and expensive\\njourneys to distant parts of the world, could now be regularly\\nobserved with all the facilities offered by a fixed observatory.\\nThis great step in advance was independently made by Sir Nor-\\nman Lockyer, and his discovery was derived from pure theory, un-\\naided by observations of the eclipse itself. The prominences are\\nnow carefully mapped day by day all around the Sun, and it has\\nbeen proved that around this body there is a vast atmosphere of\\nhydrogen gas the chromosphere From this the prominences are\\nprojected sometimes to heights of 100,000 miles or more.\\nSpectrum of the Corona. The spectrum of the corona was first ob-\\nserved by two American astronomers Professors Young and Hark-\\nness at the total solar eclipse of 1869. Since that time it has been\\nregularly observed at every total eclipse and often photographed.\\nExpeditions are sent to observe all total eclipses, no matter in what\\nparts of the Earth they occur, as up to the present time there is no\\nother way of investigating the corona and its spectrum.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0315.jp2"}, "314": {"fulltext": "292\\nASTRONOMY.\\nThe spectrum of the corona consists of several bright lines super-\\nposed on a faint continuous band. The continuous spectrum is\\nprobably due to sunlight reflected from the particles (like fog or dust\\nparticles; present in the corona. The bright lines prove that the\\ncorona is chiefly made up of self-luminous gases and vapors.\\nFig. 170.\\n-Forms of the Solar Prominences as seen with\\nthe Spectroscope.\\nThe corona is a mass of inconceivably rarefied matter\\nenveloping the San and extending far oat into space. It\\nis excessively rarefied, as is proved by the fact that comets\\nmoving round the San close to it (and thus passing through\\nthe corona) are not appreciably retarded in their motions.\\nThe gas of which it is chiefly made up has, so far, not been\\ndiscovered on the Earth.\\nThe Sun s Light and Heat. The light of the Sun\\nreceived at the Earth can be compared with onr gas-jets or\\nelectric lights. Our ordinary gas-barners or electric lights\\nhave from ten to twenty candle-power. The quantity\\nof sunlight is 1,575,000,000,000,000,000,000,000,000 times\\nas great as the light of a standard candle. The Sun sends", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0316.jp2"}, "315": {"fulltext": "THE SUN. 293\\nus 618,000 times as much light as the full Moon, and\\nabout 7,000,000,000 times as much light as the brightest\\nstar Sirius.\\nAmount of Heat Emitted by the Sun. Owing to the\\nabsorption of the solar atmosphere, we receive only a por-\\ntion perhaps a very small portion of the rays emitted by\\nthe Suq s photosphere.\\nIf the Sun had no absorptive atmosphere, it would seem\\nto us hotter, brighter, and more blue in color, since the blue\\nend of the spectrum is absorbed proportionally more than\\nthe red end.\\nThe amount of this absorption is a practical question to\\nus on the Earth. So long as the central body of the Sun\\ncontinues to emit the same quantity of rays, it is plain that\\nthe thickness of the solar atmosphere determines the num-\\nber of such rays reaching the Earth. If in former times\\nthis atmosphere was much thicker, as it may have been,\\nless heat would have reached the Earth. Glacial epochs\\nmay, perhaps, be explained in this way. If the Sun has\\nhad different emissive powers at different times, as it may\\nhave had, this again would have produced variations in the\\ntemperature of the Earth in past times.\\nAmount of Heat Radiated. There is at present no way of determin-\\ning accurately either the absolute amount of heat emitted from the\\ncentral body or the amount of this heat stopped by the solar atmos-\\nphere itself. All that can be done is to measure the amount of heat\\nactually received by the Earth.\\nExperiments upon this question lead to the conclusion that if our\\nown atmosphere were removed, the solar rays would have energy\\nenough to melt a layer of ice 170 feet thick over the whole Earth\\neach year.\\nThis action is constantly at work over the whole of the Sun s sur-\\nface. To produce a similar effect by the combustion of coal at the\\nSun would require that a layer of coal nearly 20 feet thick spread\\nall over the Sun s surface should be consumed every hour. If the\\nSun were of solid coal and produced its own heat by combustion alone\\nit would burn out in 5000 years.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0317.jp2"}, "316": {"fulltext": "294 ASTRONOMY.\\nOf the total amount of heat radiated by the Sun the Earth receives\\nbut an insignificant share. The Sun is capable of heating the entire\\nsurface of a sphere whose radius is the Earth s mean distance, to the\\nsame degree that the Earth is now heated. The surface of such a\\nsphere is 2,170,000,000 times greater than the angular dimensions of\\nthe Earth as seen from the Sun, and hence the Earth receives less\\nthan one two-billionth part of the solar radiation.\\nWe have expressed the energy of the Sun s heat in terms of the ice\\nit would melt daily on the Earth. If we compute how much coal it\\nwould require to melt the same amount, and then further calculate\\nhow much work this coal would do if it were used to drive a steam-\\nengine for instance, we shall find that the Sun sends to the Earth an\\namount of heat which is equivalent to one horse-power continuously\\nacting day and night for every 25 square feet of the Earth s surface.\\nMost of this heat is expended in maintaining the Earth s tempera-\\nture but a small portion, about j^V o\u00c2\u00bb s stored away by animals and\\nvegetables.\\nSolar Temperature. From the amount of heat actually radiated by\\nthe Sun, attempts have been made to determine the actual tempera-\\nture of the solar surface. The estimates reached by various authori-\\nties differ widely, as the laws that govern the absorption within\\nthe solar envelope are almost unknown. Some law of absorption has\\nto be assumed in any such investigation, and the estimates have dif-\\nfered widely according to the adopted law.\\nProfessor Young states this temperature at about 18,000\u00c2\u00b0 Fahr.\\nAccording to all sound philosophy, the temperature of the Sun must\\nfar exceed any terrestrial temperature. There can be no doubt that if\\nthe temperature of the Earth s surface were suddenly raised to that\\nof the Sun, no single chemical element would remain in its present\\ncondition. The most refractory materials would be at once volatilized.\\nWe may concentrate the heat received upon several square feet\\n(the surface of a huge burning-lens or mirror, for instance), examine\\nits effects at the focus, and, making allowance for the condensation\\nby the lens, see what is the minimum possible temperature of the\\nSun. The temperature at the focus of the lens cannot be higher than\\nthat of the source of heat in the Sun we can only concentrate the\\nheat received on the surface of the lens to one point and examine its\\neffects. No heat is created by the lens.\\nIf a lens three feet in diameter be used, the most refractory mate-\\nrials, as fire-clay, platinum, the diamond, are at once melted or volatil-\\nized. The effect of the lens is plainly the same as if the Earth were\\nbrought closer to the Sun, in the ratio of the diameter of the focal\\nimage to that of the lens. In the case of the lens of three feet, al-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0318.jp2"}, "317": {"fulltext": "THE SUN. 295\\nlowing for the absorption, etc., this distance is yet greater than that\\nof the Moon from the Earth, so that it appears that any comet or\\nplanet so close as 240,000 miles to the Sun must be vaporized if com-\\nposed of materials similar to those in the Earth.\\nHow is the Sun s Heat Maintained It is certain that\\nthe Sun s heat is not kept np by combustion. If the Sun\\nwere entirely composed of pure coal its combustion would\\nnot serve to maintain the Sun s supply of heat for more\\nthan 5000 years. We know that the Earth has been in-\\nhabited by people of high civilization (in Egypt for example)\\nfor a much longer time than this. Moreover the Sun\\ncannot be a huge mass once very hot and now cooling\\nbecause there has certainly been no great diminution of\\nterrestrial temperatures in the past 3000 years, as is shown\\nby what is known of the history of the vine, the fig, etc.\\nA body freely cooling in space would lose its heat\\nrapidly.\\nThere are two explanations that deserve mention. The first is\\nthat the Sun s heat is maintained by the constant falling of meteors\\non its surface. It is well known that great amounts of heat and\\nlight are produced by the collision of two rapidly moving heavy\\nbodies, or even by the passage of a heavy body like a meteorite\\nthrough the atmosphere of the Earth. In fact, if we had a certain\\nmass available with which to produce heat by burning, it can be\\nshown that, by burning it at the surface of the Sun, we should pro-\\nduce less heat than if we simply allowed it to fall into the Sun. If\\nit fell from the Earth s distance, it would give 6000 times more heat\\nby its fall than by its burning.\\nThe least velocity with which a body from space can fall upon\\nthe Sun s surface is about 280 miles in a second of time, and the\\nvelocity may be as great as 350 miles.\\nNo doubt immense numbers of meteorites do fall into the Sun\\ndaily and hourly, and to each one of them a certain considerable por-\\ntion of heat is due. It is found that to account for the present\\namount of radiation meteorites equal in mass to the whole Earth\\nwould have to fall into the Sun every century. It is in the highest\\ndegree improbable that a mass so large as this is added to the Sun in\\nthis way per century, because the Earth itself and every other planet", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0319.jp2"}, "318": {"fulltext": "296 ASTRONOMY.\\nwould receive far more than its present share of meteorites, and\\nwould become quite hot from this cause alone.\\nThe meteoric theory deserves a mention, but it is probably not a\\nsufficient explanation.\\nThere is still another way of accounting for the Sun s constant\\nsupply of energy, and this has the advantage of appealing to no\\ncause outside of the Sun itself in the explanation. It is by suppos-\\ning the heat, light, etc., to be generated by a constant and gradual\\ncontraction of the dimensions of the solar sphere. As the globe cools\\nby radiation into space, it must shrink. As it shrinks, heat is pro-\\nduced and given out.\\nWhen a particle of the Sun moves towards the Sun s centre the\\nsame amount of heat is produced if its motion is caused by a slow\\nshrinking as would be developed by its sudden fall through the same\\ndistance.\\nThis theory is in complete agreement with the known laws of\\nforce. It also admits of precise comparison with facts, since the\\nlaws of heat enable us, from the known amount of heat radiated, to\\ninfer the exact amount of contraction in inches which the linear di-\\nmensions of the Sun must undergo in order that this supply of heat\\nmay be kept unchanged, as it is practically found to be.\\nWith the present size of the Sun, it is found that it is only neces-\\nsary to suppose that its diameter is diminishing at the rate of about\\n250 feet per year, or 4 miles per century, in order that the supply of\\nheat radiated shall be constant. Such a change as this may be taking\\nplace, since we possess no instruments sufficiently delicate to have\\ndetected a change of even ten times this amount since the invention\\nof the telescope.\\nIt may seem a paradoxical conclusion that the cooling of a body\\nmay cause it to give out heat. This indeed is not true when we\\nsuppose the body to be solid or liquid. It is, however, proved that\\nthis law holds for gaseous masses\u00e2\u0080\u0094 but only so long as they are gas-\\neous.\\nWe cannot say whether the Sun has yet begun to liquefy in his\\ninterior parts, and hence it is impossible to predict at present the\\nduration of his constant radiation. It can be shown that after about\\n5,000,000 years, if the Sun radiates heat as at present, and still re-\\nmains gaseous, his present volume will be reduced to one half. If\\nthe volume is reduced to one half the density will be then two times\\ngreater (since the mass will remain the same). (Z M V, see\\npage 237.) It seems probable that somewhere about this time the\\nsolidification will have begun, and it is roughly estimated, from this", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0320.jp2"}, "319": {"fulltext": "THE SUN. 297\\nline of argument, that the present conditions of heat radiation\\ncannot last greatly over 10,000,000 years.\\nThe future of the Sun (and hence of the Earth) cannot, as we see,\\nbe traced with great exactitude. The past can be more closely fol-\\nlowed if we assume (which is tolerably safe) that the Sun up to the\\npresent has been a gaseous and not a solid or liquid mass. Four\\nhundred years ago, then, the Sun was about 16 miles greater in\\ndiameter than now and if we suppose the process of contraction to\\nhave regularly gone on at the same rate (a very uncertain supposi-\\ntion), we can fix a date when the Sun filled any given space, out\\neven to the orbit of Neptune that is, to the time when the solar\\nsystem consisted of but one body, and that a gaseous or nebulous\\none.\\nIt is not to be taken for granted, however, that the amount of heat\\nto be derived from the contraction of the Sun s dimensions is infinite,\\nno matter how large the primitive dimensions may have been. A\\nbody falling from any distance to the Sun can only have a certain\\nfinite velocity depending on this distance and upon the mass of the\\nSun itself, which, even if the fall be from an infinite distance,\\ncannot exceed, for the Sun, 350 miles per second. In the same way\\nthe amount of heat generated by the contraction of the Sun s\\nvolume from any size to any other is finite and not infinite.\\nIt has been shown that if the Sun has always been\\nradiating heat at its present rate, and if it had originally\\nfilled all space, it has required some 18,000,000 years to\\ncontract to its present volume. In other words, assuming\\nthe present rate of radiation, and taking the most favor-\\nable case, the age of the Sun does not exceed 18,000,000\\nyears. The Earth is, of course, less aged.\\nThe supposition lying at the base of this estimate is that\\nthe radiation of the Sun has been constant throughout the\\nwhole period. This is quite unlikely, and any changes in\\nthis datum will affect the final number of years to be\\nassigned. While this number may be greatly in error, yet\\nthe method of obtaining it seems to be satisfactory, and\\nthe main conclusion remains that the past of the Sun is\\nfinite, and that in all probability its future is a limited one.\\nThe exact number of centuries that it is to last are of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0321.jp2"}, "320": {"fulltext": "298 ASTRONOMY.\\nno especial moment even were the data at hand to obtain\\nthem the essential point is that, so far as we can see, the\\nSun, and incidentally the solar system, has a finite past\\nand a limited futnre, and that, like other natural objects,\\nit passes through its regular stages of birth, vigor, decay,\\nand death, in one order of progress.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0322.jp2"}, "321": {"fulltext": "CHAPTER XVII.\\nTHE PLANETS MERCURY, VENUS, MARS.\\n32. Mercury Venus Mars. Mercury is the nearest\\nplanet to the Sun. Its mean distance is 36,000,000 miles,\\nabout 3%V of the Earth s distance. Its orbit is quite\\neccentric, so that its maximum distance from the Sun is\\n43,500,000 miles, and its minimum only 28,500,000. At\\nits mean distance (0.39) it would receive about 6 t 6 q- times\\nas much light and heat from the Sun as the Earth, because\\n(1.00) 2 (0.39) 2 6.6 1.0.\\nIts sidereal year is 88 days. Its time of rotation on its axis\\nis not certainly known, but the observations of Schia-\\npakelli and others make it likely that it revolves once on\\nits axis in the same time that it makes one revolution about\\nthe Sun, just as our own Moon revolves once on its axis\\nduring one of its revolutions about the Earth. The\\napparent angular diameter of Mercury can be measured\\nwith the micrometer (see page 144). Knowing the angle\\nthat the diameter of the planet subtends and knowing the\\nplanet s distance (in miles) the diameter of the planet in\\nmiles can be calculated. The diameter of Mercury is about\\n3000 miles. Its surface is of the Earth s surface and its\\nvolume about -fa. The mass of the planet is determined by\\ncalculating how much matter it must contain to affect the\\nmotions of comets as it is observed to do. In this way it\\nresults that its mass is about of the Earth s mass. Its\\ndensity is about T fi of the Earth s density.\\n299", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0323.jp2"}, "322": {"fulltext": "300 ASTRONOMY.\\nVenus 1 mean distance is 67,200,000 miles. Its sidereal\\nyear is 225 days. It is not jet certain that its period of\\nrotation may not be about 24 hours one day, but the\\nobservations of Schiaparelli and others make it likely\\nthat its rotation on its axis is performed in 225 days also.\\nIf this be so Mercury and Venus will always turn the same\\nface to the Sun, just as our Moon always turns the same\\nface to the Earth. The diameter of Venus is 7700 miles,\\nonly a little less than the diameter of the Earth (7918) and\\nit has therefore about the same volume. The mass of\\nVenus is determined by calculating how much matter the\\nplanet must contain in order to affect the motion of the\\nEarth as it is observed to do. Its mass is about T 8 F of the\\nEarth s mass and its density about T 9 that of the Earth.\\nVery little is certainly known about the geography of\\nMercury and of Venus. Mercury is never seen far distant\\nfrom the Sun and observations of the planet in the daytime\\nare unsatisfactory because the heated atmosphere of the\\nEarth is usually in constant motion and produces an effect\\non telescopic images like the twinkling of stars to the naked\\neye. Venus shows only faint markings on her surface.\\nIt is likely that Mercury has little or no atmosphere and\\nit is certain that Venus has an atmosphere of some kind\\nwhich is, in all probability, extensive. If the surface of\\nVenus which we see with the telescope is nothing but the\\nouter rim of its envelope of clouds we know nothing of the\\nreal surface of the planet. Nothing whatever is known as\\nto whether either of these planets is inhabited and very\\nlittle as to whether either of them is habitable.\\nApparent Diameters of Mercury and Venus. In Fig. 171 8 is the\\nSun, E the Earth in its orbit and LIMG the orbit of an inferior\\nplanet. If the Earth is at E and the planet at 2 the planet is at\\ninferior conjunction (nearest the Earth) if at C, at superior conjunc-\\ntion if at L or M, at elongation. The Sun will be seen from E along\\nthe line EG. It is plain that the planet can never appear at a greater\\nangle from the Sun than SEM or SEL. It is clear from the figure", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0324.jp2"}, "323": {"fulltext": "THE PLANETS MERCURY AND VENUS. 301\\nthat the apparent angular diameter of the inferior planet will vary\\ngreatly. It will be greatest when the planet is nearest the Earth\\n(inferior conjunction) and least when the planet is most distant.\\nFig. 171 \u00e2\u0080\u0094The Motion of an Inferior Planet with Refer-\\nence to the Eakth.\\nIn representing the apparent angular magnitude of these planets,\\nin Figs. 172 and 173 we suppose their whole disks to be visible, as\\nthey would be if they shone by their own light. But since they can\\nbe seen only by the reflected light of the Sun, those portions of the\\ndisk are alone seen which are at the\\nsame time visible from the Sun and from\\nthe Earth. A very little consideration\\nwill show that the proportion of the\\ndisk which can be seen by us constantly\\ndiminishes as the planet approaches\\nthe Earth, and that the planet s di-\\nameter subtends a larger angle. 1G 172. Apparent Di-\\nameter of Mercury A\\nPhases of Mercury and Venus. f T R \u00c2\u00a3Si\\nWhen the planet is at its greatest 0, at Least Distance.\\ndistance, or in superior conjunction (6 Fig. 171), its\\nwhole illuminated hemisphere can be seen from the Earth.\\nAs it moves arocmd and approaches the Earth, the illumi-\\nnated hemisphere is gradually turned from us. At the\\npoint of greatest elongation, M or X, one half the hemi-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0325.jp2"}, "324": {"fulltext": "302\\nASTRONOMY.\\nsphere is visible, and the planet has the form of the Moon\\nat first or second quarter. As it approaches inferior con-\\njunction, the apparent visible disk assumes the form of\\na crescent, which becomes thinner and thinner as the\\nplanet approaches the Sun. (See Fig. 174.)\\nFig.\\n173. Apparent Diameter of Venus; A, at Greatest\\nB, at Mean C, at Least Distance.\\nThe phases of an inferior planet were first observed by\\nGalileo in 1610. They are not visible to the naked eye\\nnnd hence their discovery dates from the invention of the\\nB c\\nc c\\nK\\nFig. 174. Phases Presented by an Inferior Planet at Dif-\\nferent Points of its Orbit K. Near Inferior A,\\nNear Superior Conjunction.\\ntelescope. If the student will turn to the plan of the\\nPtolemaic system (Fig. 124) he will see that Ptolemy\\nsupposed both Mercury and Venus to revolve about the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0326.jp2"}, "325": {"fulltext": "TEE PLANETS MERCURY, VENUS, MARS. 303\\nEarth and to be nearer to the Earth than the Snn. There\\nwas no time, according to Ptolemy s system, when the\\nwhole disk of Mercury or Venus could be seen illuminated.\\nBut Galileo s telescope showed the disk as a full circle at\\nevery superior conjunction. The inference that the\\nPtolemaic system was not true was irresistible. The failure\\nof Ptolemy s theory cleared the way for the adoption of\\nthe heliocentric theory of Copernicus.\\nTransits of Mercury and Venus. When Mercury or Venus passes\\nbetween the Earth and Sun, so as to appear projected on the Sun s\\ndisk as a dark circle the phenomenon is called a transit. If these\\nplanets moved around the Sun exactly in the plane of the ecliptic, it\\nis evident that there would be a transit at every inferior conjunction,\\nbut their orbits are inclined to the ecliptic by angles of 7\u00c2\u00b0 and 3\u00c2\u00b0 re-\\nspectively.\\nThe longitude of the descending node of Mercury at the present\\ntime is 227\u00c2\u00b0, and therefore that of the ascending node 47\u00c2\u00b0. The\\nEarth has these longitudes on May 7th and November 9th. Since a\\ntransit can occur only within a few degrees of a node, Mercury can\\ntransit only within a few days of these epochs.\\nThe longitude of the descending node of Venus is now about 256\u00c2\u00b0\\nand therefore that of the ascending node is 76\u00c2\u00b0. The Earth has these\\nlongitudes on June 6th and December 7th of each year. Transits of\\nVenus can therefore occur only within two or three days of these\\ntimes. (See page 264.)\\nTransits of Mercury will occur in 1907, 1914 etc., and of Venus in\\n2004 and 2012.\\nMars is the fourth planet in order going outwards from\\nthe Sun. Its mean distance is 141,500,000 miles, about\\ntimes the Earth s distance. Its orbit is quite eccentric so\\nthat its distance from the Sun at different times may be as\\nlarge as 153,000,000 or as small as 128,000,000 miles. Its\\ndistances from the Earth at opposition will vary in the same\\nway. When its distance from the Sun is the largest the\\ndistance from the Earth will be about 60,000,000 miles\\n153,000,000 93,000,000). When its distance from", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0327.jp2"}, "326": {"fulltext": "304 ASTRONOMY.\\nthe Sua is the smallest the distance from the Earth will be\\nabout 35,000,000 miles 128,000,000 93,000,000).\\nWhen Mars is in conjunction with the Sun its average\\ndistance is about 234,000,000 miles 141,000,000\\n93,000,000). Its greatest distance at conjunction is about\\n246,000,000 miles.\\nThe apparent angular diameter of the planet varies directly as the\\ndistance and is sometimes as small as 3 6, sometimes seven times\\nlarger (246 -4- 35 7). The amount of light received by Mars from\\nthe Sun varies as (where R Mars radius vector), so that the\\namount of light received by the Earth from Mars varies as\\nKrr*\\n(where r is the distance of Mars from the Earth). The amount of\\nlight leceived by us from the planet varies enormously at different\\ntimes, therefore.\\nThe periodic time of Mars is 687 days. Its diameter is\\n4200 miles a little more than half that of the Earth. Its\\nsurface is about and its volume is of the Earth s. Its\\nmass is determined (by calculating the effect of the planet\\non the orbits of its satellites) to be about J of the Earth s\\nmass. Its density is accurately y 1 of the Earth s density,\\nand the force of gravity at its surface is about f^ of the\\nEarth s. A body weighing 100 pounds on the Earth would\\nweigh a little less than 40 pounds on Mars.\\nMars necessarily exhibits phases, but they are not so well\\nmarked as in the case of Venus, because the hemisphere\\nwhich it presents to the observer on the Earth is always\\nmore than half illuminated. The greatest phase occurs\\nwhen its direction is 90\u00c2\u00b0 from that of the Sun, and even\\nthen six sevenths of its disk is illuminated, like that of the\\nMoon, three days before or after full moon. The phases\\nof Mars were observed by Galileo in 1610.\\nMars has little or no Atmosphere. The Moon reflects j\\\\ 7 of the\\nlight falling upon it about as much as sandstone rocks. Mercury\\nreflects ^fa. These bodies have little or no atmosphere. Venus re-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0328.jp2"}, "327": {"fulltext": "THE PLANET MAES. 305\\nfleets (from the outer surface of its envelope of clouds) T of the in-\\ncident light. Jupiter T 2 Saturn r 2 o), Uranus T Neptune\\nT 4 6 o), are all bodies surrounded by extensive atmospheres and all of\\nthem have high reflecting powers. The corresponding number for\\nMars (yo%) is so small as to indicate that this planet has little atmos-\\nphere, if any.\\nThe planet s surface has been under careful scrutiny for\\nmany years and observers are all but unanimous in their\\nreport that no clouds are visible over the surface.\\nThe centres of the disks of bodies with extensive atmos-\\npheres (the San, Jupiter, Saturn, etc.) are always brighter\\nthan the edges (see page 283). The centre of the Moon,\\nwhich has no atmosphere, is not so bright as the edge.\\nMars is like the Moon in this respect and not like Jupiter.\\nFinally the only satisfactory spectroscopic observations of\\nthe planet (made independently at the Lick Observatory\\nby Campbell and at the Allegheny Observatory by\\nKeeler) show no evidence whatever of an atmosphere to\\nMars and no sign of water-vapor about the planet. If\\nthere is any atmosphere at all it can hardly be more dense\\nthan the Earth s atmosphere at the high summits of the\\nHimalaya mountains not enough to support human life\\ntherefore. As there is no evidence of the presence of\\nwater- vapor and of clouds, etc., it follows that there is\\nlittle or no water on the planet s surface. The spectrum\\nof Mars and the spectrum of the Moon are identical in\\nevery respect. This could not be true if Mars had any\\nconsiderable atmosphere.\\nIt is proper to say that a number of astronomers hold different\\nviews and that popular writers on astronomy, with few exceptions,\\nproclaim the existence of water, air, vegetation and intelligent human\\nbeings on the planet. It is an announcement that finds thousands of\\ninterested listeners who are only too glad to welcome so momentous\\na conclusion. The popular writings referred to have little weight in\\nthemselves, but they have undoubtedly spread a general belief among\\nintelligent people that Mars is a planet much like the Earth (which\\nit certainly is not), fit for human habitation, and very likely inhabited", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0329.jp2"}, "328": {"fulltext": "306\\nASTRONOMY.\\nby beings like ourselves. The questions involved are inexpressibly-\\nimportant in themselves and they relate to matters in which every\\nhuman being is interested. The duty of Science is to investigate them\\nby every possible means (and this has been and will be done), but\\nScience can only be discredited by premature and incorrect announce-\\nments made without a proper sense of responsibility.\\nffPSi\\nI\\n\u00e2\u0096\u00a0I\\nFig. 175.\\n-Telescopic Yiew of the Surface of Mars Show-\\ning a Small Polar Cap.\\nThe important and long-continued observations of Schiaparelli\\non Mars led him to announce that the planet was provided with an\\nelaborate system of water-courses oceans, seas, lakes, canals, etc.\\nand the authority of this distinguished observer is the chief support\\nof those who maintain that this planet is fit for human habitation,\\netc. Complete explanations of all the phenomena presented by the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0330.jp2"}, "329": {"fulltext": "THE PLANET MARS.\\n307\\nplanet cannot be given in the light of our present knowledge.\\nThis is not to be wondered at in spite of the industry and ability\\nof the observers who have spent years in studying the planet. The\\ncase is much the same for the planets M ercury, Venus, Jupiter, Saturn,\\nTJranus, Neptune. We know very little of the real conditions that\\nprevail on their surfaces. We know comparatively little of the in-\\nterior of the Earth on which we live and next to nothing about the\\ninterior of other planets. There is every reason to believe that\\nFig. 176.\\n\u00e2\u0080\u00a2Drawing of Mars Made at the Lick Observatory\\nMay 21, 1890.\\ncomplete explanations will be forthcoming in time. It is, at any rate,\\ncertain that the conclusions of Schiaparelli, named above, cannot\\nbe accepted without serious modification, as will be shown in this\\nChapter.\\nAppearance of the Disk of Mars in the Telescope. The\\nappearance of Mars in large telescopes is shown in Figs.\\n175 and 176. The main body of the planet is reddish\\n(shown white in the cuts). The portions shown dark in", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0331.jp2"}, "330": {"fulltext": "308 ASTRONOMY.\\nthe pictures are bluish, greenish, or grayish in the tele-\\nscope. The cap in Fig. 175 is a brilliant white. Most\\nof the markings on Mars are permanent. They are seen\\nin the same places year after year. Observations on these\\npermanent markings prove that the planet revolves on its\\naxis once in 24 h 37 m 22 s 7. Its equator is inclined to the\\necliptic about 26\u00c2\u00b0.\\nWhen Sir William Herschel was examining Mars in\\nthe XVIII century he called the red areas of Mars land\\nand the greenish and bluish areas water. It was a\\ngeneral opinion in his day that all the planets were created\\nto be useful to man. Astronomers of the XVIII century\\nset out with this belief very much as the philosophers of\\nPtolemy s time set out with the fundamental theorem that\\nthe Earth was the centre of the motions of the planets.\\nFor example, Herschel maintained that the Suu was cool\\nand habitable underneath its envelope of fire. He says\\n(1795) The Sun appears to be nothing else than a very\\neminent, large and lucid planet most probably also\\ninhabited by beings whose organs are adapted to the\\npeculiar circumstances of that vast globe. It is certain\\nthat the Sun is not inhabited by any beings with organs.\\nThis conclusion is now as obvious as that no beings\\ninhabit the carbons of an electric street-lamp. Herschel s\\nguess that the red areas on Mars were land and the\\nblue areas water had no more foundation than his guess\\nthat the Sun might be inhabited.\\nThe next careful studies of Mars were made by Maedler\\nabout 1840. He also called the red areas of the disk\\nland and the dark areas water. In this he followed\\nHerschel. There was no reason why he should not have\\ncalled the red areas water and the dark areas land.\\nHe had no evidence on the point. The same is true of\\nlater observers down to the first observations of Schia-\\nparelli about 1877,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0332.jp2"}, "331": {"fulltext": "TEE PLANET MARS. 309\\nSchiaparelli gave reasons for these names, though his\\nreasons are not convincing. He pointed out that the\\nnarrow dark streaks canals generally ended in large\\ndark areas oceans or in smaller dark areas lakes\\nThe narrow dark streaks (very seldom less than 60 miles\\nwide) are quite straight. They cannot be rivers then.\\nIf they are water at all the name canal is not inappro-\\npriate though 60 or 100 miles is a very wide canal. If they\\nare water, then the large dark areas must be seas. The\\nnarrow dark streaks are not water, however, because it was\\ndiscovered by Dr. Schaeberle at the Lick Observatory\\nthat the so-called seas sometimes had so-called\\ncanals crossing them. A sea traversed by a\\ncanal is an absurdity. If it could be imagined it\\nwould prove the inhabitants and the engineers of\\nMars to be the exact reverse of intelligent. It is main-\\ntained by some recent observers of Mars that some of the\\ndark areas are water and some are not so. The bluish-\\ngreen color of the dark spots is said to suggest vege-\\ntation. But who can know what colors the vegetation\\non Mars may have\\nThe foregoing very brief abstract proves that the dark\\nareas on Mars are not water. The red areas are not\\nknown to be land. The spectroscopic and other evi-\\ndence proves that Mars has little or no atmosphere little\\nor no water-vapor no clouds. It is not yet known what\\nthe real nature of the red areas and of the dark areas is.\\nIt is one of the many unsolved problems of Astronomy to\\ndiscover the answer to this fundamental question. There is\\nno doubt the red areas and the large dark areas have a real\\nexistence, since some of the markings on Mars have been\\nseen for more than two centuries.\\nIt is not certain that all the canals that have been mapped really\\nexist. Some of them are probably mere optical illusions. If they\\nwere real streaks on the planet s surface (like wide fissures, broad", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0333.jp2"}, "332": {"fulltext": "310 ASTRONOMY.\\nwatercourses, etc.) they would always appear broadest when they\\nwere at the centre of the disk and would always be narrower when\\nthey were at the edges. The laws of perspective demand this. It is\\nfound by observation that the reverse is frequently true.\\nSchiaparelli was the first to observe that many of the canals\\noftentimes appear to be doubled. That is, a canal running in a certain\\ndirection which generally appeared single, thus,\\nat certain times was no longer single but attended by a companion,\\nthus:\\nMarvels of ingenious speculation have been printed to explain why\\nintelligent inhabitants having one canal not sufficient for\\ncommerce, did not widen it, but preferred to dig another parallel\\nto it, and why this second canal sometimes vanished altogether in\\na few hours. Recent experiments have proved that these com-\\npanion canals are optical illusions produced by fatigue of the eye and\\nby bad focusing. Some, at least, of the single narrow dark streaks\\ncanals have a real existence. It is probable that many of those\\nlaid down and named on the maps of Schiaparelli, Lowell and\\nothers are mere illusions. It is likely that all the double canals\\nwere so.\\nTemperature of Mars. The distance of Mars from the\\nSun is 1\u00c2\u00a3 times the Earth s distance. The heat received\\nby the Earth from the Sun is to the heat received by Mars\\nas (1.5)* 2.25 to 1. Mars receives less than one half as\\nmuch Sim heat as the Earth. If the Earth had no more\\natmosphere than the Moon the Earth s temperature would\\nbe like that of the Moon. If the Earth had no denser\\natmosphere than that on the summits of the Himalayas the\\ntemperature of the Earth would always be below zero.\\nHuman life could not exist here. The case is the same\\nwith Mars. The temperature of the whole surface of the\\nplanet must be extremely low even in its equatorial regions.\\nThe temperature at the poles of Mars must be several\\nhundred degrees (Fahrenheit) below zero when the pole is", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0334.jp2"}, "333": {"fulltext": "THE PLANET MARS. 31 1\\nturned away from the Sun and below zero even when the\\npole is turned towards the Sun.\\nBefore going further it is worth while to consider the\\ncircumstances under which Mars is seen by an observer on\\nthe Earth. The mean distance of the Moon from the\\nEarth is 240,000 miles. If it is viewed through a field-\\nglass magnifying 4 times, it is virtually brought within\\n60,000 miles of the observer (240,000 4 60,000).\\nThe nearest approach of Mars to the Earth is 35,000,000\\nmiles. The planet can very seldom be viewed to advantage\\nwith a magnifying power so high as 500. If such a power\\nis employed when Mars is nearest, the planet is virtually\\nbrought within 70,000 miles (35,000,000 -f- 500 70,000).\\nIt follows therefore that we never see Mars so advan-\\ntageously even with the largest telescopes as we mag see the\\nMoon in a common field-glass. If the student will ex-\\namine the Moon with a field-glass magnifying 4 times he\\nwill have a realizing sense of the best conditions under\\nwhich it is possible to see Mars, and he will be surprised\\nthat so much is known of the planet. The industry and\\nfidelity of observers can only be appreciated after such\\nan experiment.\\nThe Polar Caps of Mars. We have now to present\\nanother result of observation which must be interpreted in\\nthe light of the foregoing facts namely, that Mars has\\nlittle or no water- vapor and that its temperature is appal-\\nlingly low. The main facts of observation are as follows.\\nCassini, the royal astronomer of France, discovered in\\n1666 that Mars sometimes had dazzling white circular\\npatches near his poles (see Fig. 175). In 1783 Sir\\nWilliam Herschel observed these patches to wax and\\nwane and he called them snow caps, thus begging the\\nquestion as to their real nature. Herschel s observa-\\ntions and those of all later observers show that these caps\\nwax and wane with the Martian seasons. In the Martian", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0335.jp2"}, "334": {"fulltext": "312 ASTRONOMY.\\npolar summer they are smallest, or they even vanish. In\\nthe Martian polar winter they are largest. As Herschel\\nstarted out with the conviction that all planets were\\nanalogous to the Earth and were meant to be inhabited,\\nhis conclusion was that the polar winter condensed water-\\nvapor into snow and that the polar summer melted this\\nsnow and so on. A more scientific conclusion would\\nhave been that some vapor was condensed and subsequently\\ndissipated by the solar heat. It is practically certain that\\nthe phenomena of the waxing and waning of the caps\\ndepend on solar heat.\\nIf the caps are snow condensed from water- vapor the\\nlayer of snow must be exceedingly thin, because when these\\ncaps are melted no clouds appear. When snow melts\\non the Earth clouds are formed and our atmosphere is\\ncharged with the vapor of water. No clouds are seen on\\nMars and no water-vapor is to be found above its surface\\nby any spectroscopic test.\\nThe polar-caps may be formed by the vapor of some\\nother substance than water. It is worth while to inquire\\nwhether they may not be carbon-dioxyd in a solid state.\\nThis substance is a heavy gas (carbonic-acid gas) at ordi-\\nnary temperatures. It would lie at the bottom of valleys\\nand fill canons or ravines. At a temperature of about one\\nhundred Fahrenheit below zero it is a colorless liquid. At\\ntemperatures such as must obtain at the pole of Mars\\nturned away from the Sun it becomes a snow-like solid.\\nCaps of carbon-dioxyd would wax and wane at the poles of\\nMars under variations of solar heat such as obtain at these\\npoles, very much as caps of \u00c2\u00bbnow and ice wax and wane in\\nour Arctic regions which, under all circumstances, are at a\\nfar higher temperature than the poles of Mars.\\nThere is so far no observational proof that the polar-\\ncaps of Mars are formed of carbon-dioxyd. There is", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0336.jp2"}, "335": {"fulltext": "THE PLANET MARS. 313\\nconvincing proof that they are not formed of water. The\\nquestion as to the nature of the polar-caps is still an open\\none. There is little doubt that it will, one day, be settled.\\nThe scientific attitude of mind is to wait for proofs of\\nmatters still unsolved; to accept such proofs as exist; and\\nto eschew unfounded speculations. All that is now known\\ngoes to show that Mars has little or no atmosphere, little\\nor no water- vapor, no oceans, no lakes, no canals,\\nno clouds. Its general surface is rather flat, although\\na few mountain chains exist. It is not a planet like\\nthe Earth. It is much more like the Moon. It cannot\\npossibly be inhabited by beings like ourselves.\\nSatellites of Mars.\u00e2\u0080\u0094 Until the year 1877 Mars was supposed to have\\nno satellites. But in August of that year Professor Hall, of the\\nNaval Observatory, instituted a systematic search with the great\\nequatorial, which resulted in the discovery of two such objects.\\nThese satellites are by far the smallest celestial bodies known. It\\nis of course impossible to measure their diameters, as they appear in\\nthe telescope only as points of light. The outer satellite is probably\\nabout six miles and the inner one about seven miles in diameter. The\\nouter one was seen with the telescope at a distance from the Earth of\\n7,000,000 times this diameter. The proportion wo- Jd be that of a\\nball two inches in diameter viewed at a distance equal to that between\\nthe cities of Boston and New York. Such a feat of telescopic seeing\\nis well fitted to give an idea of the power of modern optical instru-\\nments in detecting faint points of light like stars or satellites.\\nThe outer satellite, called Deimos, revolves around the planet in\\n30 h 18 m and the inner one, called Phobos, in 7 1 39 m The latter is\\nonly 5800 miles from the centre of Mars, and less than 4000 miles\\nfrom its surface. It would therefore be almost possible to see an\\nobject the size of a large animal on the satellite if one of our tele-\\nscopes could be used at the surface of Mars.\\nThe short distance and rapid revolution make the inner satellite of\\nMars one of the most interesting bodies with which we are acquainted.\\nIt performs a revolution in its orbit from west to east in less than\\nhalf the time that Mars revolves on its axis. In consequence, to the\\ninhabitants of Mars it would seem to rise in the west and set in the east.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0337.jp2"}, "336": {"fulltext": "314\\nASTR0N0M7.\\nLet the student prove this statement for himself by drawing a\\nfigure somewhat like Fig. 31. Suppose iVto be Mars, a the spec-\\ntator, ZH the celestial equator, Z to be Phobos on the meridian. In\\nFig. 31\\nl h the spectator will have moved to and Phobos to in 2 h\\netc. etc.\\nThe light of Phobos is about -fa of the light of our Moon of\\nDeimos about T ^W", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0338.jp2"}, "337": {"fulltext": "CHAPTER XVIII.\\nTHE MOON\u00e2\u0080\u0094 THE MINOR PLANETS.\\n33. The Moon.\u00e2\u0080\u0094 The Moon the satellite of the Earth\\nrevolves abont its primary in a periodic-time of 27 d 32116\\nat a mean distance of 238,840 miles. Its daily motion\\namong the stars is Q011fi about 13\u00c2\u00b0 11 The apparent\\nangular diameter of the Moon is about half a degree, so\\nthat the Moon moves daily among the stars about 26 of its\\nown diameters. The interval from new moon to new moon\\nis about 29 days and the Moon comes to the meridian of\\nan observer about 51 minutes later each day (on the\\naverage). The orbit of the Moon is inclined to the plane\\nof the ecliptic by a little more than 5\u00c2\u00b0.\\nThe velocity of the Moon in her orbit is about 3350 feet\\nper second. Her diameter is 2163 miles, her surface T ^-g-\\nof the Earth s, her volume J^, and her mass -fo of the\\nEarth s. The density of the Moon is about 3.4 times the\\ndensity of water. The heaviest lavas of the Earth s crust\\nare about 3.3 in density, so that the conclusion that the\\nEarth and Moon once formed one body is not contradicted\\nby these facts. Gravity on the Moon s surface is i as great\\nas at the Earth s. Hence an explosion of subterranean\\nsteam would form a much more extensive crater on the\\nMoon than on the Earth, and mountains would stand at a\\nmuch steeper average angle on the Moon. As there is no\\nair and no water on the Moon s surface there is no frost\\nconstantly working to overthrow cliffs and sharp peaks as\\n315", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0339.jp2"}, "338": {"fulltext": "316\\nASTRONOMY.\\nFig. 177.\u00e2\u0080\u0094 Lunar Landscape {Mare Grisium) from Photographs\\nTaken at the Lick Observatory.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0340.jp2"}, "339": {"fulltext": "THE MOON. 317\\nin the case of the Earth. The albedo of the Moon is T Vo\\nabout that of weathered sandstone rocks.* The angle of\\nslope of the lunar volcanoes is about the same as the angle\\nof terrestrial lavas. These and many other facts support\\nthe conclnsion that the Earth and Moon are made of like\\nmaterials.\\nThe Moon has extremely little if any atmosphere\\nbecause the occultation of a star by the lnnar disk takes\\nplace instantaneously. If the Moon had an atmosphere,\\nthe star s rays would be refracted by it and there would be\\na change of the star s color and a gradual disappearance.\\nThe spectrum of the Moon is nothing but a fainter solar\\nspectrum. This proves that moonlight is reflected sunlight\\nand that the Moon has no absorbing atmosphere of its own.\\nNo doubt the Moon, in remote past times had an atmos-\\nphere. Its constituents have probably been absorbed by\\nthe rocks of the lunar crust as they cooled. The water on\\nthe Moon has probably been absorbed in the same way.\\nThe quantity of light received by the Earth from the\\nfull Moon is eTsV or the light received from the Sun.\\nThe temperature of the Moon s surface is probably always\\nbelow freezing-point, even in the full sunshine of a long\\nlunar day. If the Earth s atmosphere were to be\\nremoved the temperature of our summers would be ex-\\ntremely low much lower than it now is at the summits of\\nour highest mountains. The lunar night is 14 terres-\\ntrial days long. The temperature of a part of the Moon\\nafter being deprived of the Sun s light (and heat) for 14\\ndays must be extremely low several hundred degrees\\nFahr. below zero.f\\nThe Moon only Shows one Face to the Earth. The Moon rotates on\\nher axis from west to east, and the time required for one rotation is the\\nThe albedo of any substance is its power of reflecting rays of\\nlight that fall upon it. If it reflects all such rays its albedo is 100.\\nf These are the conditions that prevail on airless bodies like the\\nMoon and Mars,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0341.jp2"}, "340": {"fulltext": "318 ASTRONOMY.\\nsame as that required for one revolution in her orbit, viz., 27 days.\\nIf a line be drawn from the Earth to the Moon at any time whatever\\nthis line will always touch the same hemisphere of the Moon and the\\nMoon does not rotate at all with reference to this line. If a line be\\ndrawn through the Sun parallel to the Moon s axis, the Moon some-\\ntimes turns one face and sometimes another to this line. An observer\\non the Earth sees but one hemisphere of the moon. An observer\\non the Sun would successively see all regions of the Moon (see Fig.\\n133).\\nWhen it became clearly understood after the invention of\\nthe telescope that the ancient notion of an impassable gulf\\nbetween the character of bodies celestial and bodies terres-\\ntrial was unfounded, the question whether the Moon was\\nlike the Earth became one of great importance. The point\\nof most especial interest was whether the Moon could, like\\nthe Earth, be peopled by intelligent inhabitants. Accord-\\ningly, when the telescope was invented by Galileo, one of\\nthe first objects examined was the Moon. With every im-\\nprovement of the instrument the examination became more\\nthorough, so that at present the topography of the Moon is\\nvery well known. Photographic maps of the Moon show\\nthe details of its surface in an admirable way.\\nWith every improvement in the means of research, it has\\nbecome more and more evident that circumstances at the\\nsurface of the Moon are totally unlike those on the Earth.\\nThere are no oceans, seas, rivers, air, clouds, or vapors.\\nWe can hardly suppose that animal or vegetable life exists\\nunder such circumstances. We might almost as well\\nsuppose a piece of granite or lava to be the abode of life as\\nthe surface of the Moon.\\nThe length of one mile on the Moon would, as seen from the Earth,\\nsubtend an angle of about 1 of arc. In order that an object may be\\nplainly visible to the naked eye, it must subtend an angle of nearly\\n60. Consequently a magnifying power of 60 is required to render a\\nround object one mile in diameter on the surface of the Moon plainly\\nvisible.\\nThe following table shows the diameters of the smallest objects", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0342.jp2"}, "341": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0343.jp2"}, "342": {"fulltext": "LIST OF LUNAR CRATERS,\\nN. B.\u00e2\u0080\u0094 The Quadrants are marked I, II, III, IV on the borders of the Ma\\nI. FIRST QUADRANT.\\n1. Pallas\\n2. Gambart\\n3. Stadius\\n4. Copernicus\\n5. Reinhold\\n6. Kepler\\n7. Hevelius\\n8. Eratosthenes\\n9. Marius\\n10. Archimedes\\n11. Timocharis\\n12. Euler\\n13. Aristarchus\\n14. Herodotus\\n15. Laplace\\n16. Heraclides\\n17. Bianchini\\n18. Sharp\\n19. Mairan\\n20. Plato\\n21. Condamine\\n22. Harpalus\\nN. B. From new moon (0 da s\\nto full moon (15 d the icest limb of\\nthe moon is fully lighted. The\\nposition of the terminator for\\neach intermediate day is marked\\nby the upper set of numbers\\nalong the moon s equator 2, 3,\\n4 .15. From full moon to the\\nfollowing new moon the east\\nII. SECOND QUADRANT.\\n51. Moretus\\n52. Cysatus\\n53. Blancanus\\n54. Schemer\\n55. Clavius\\n56. Maginus\\n57. Longomontanus\\n53. Schiller\\n59. Phocylides\\n60. Wargentin\\n61. Saussure\\n62. Pictet\\n63. Tycho\\n6i. Heinsius\\n65. Hainzel\\n66. Schickai d\\n67. Hell\\n63. Gauricus\\n69. Wurzelbauer\\n70. Pitatus\\n71. Hesiodus\\n72. Clchus\\n73. Capuanus\\n74. Ramsden\\n75. Vitello\\n76. Regiomontanus\\n77. Purbach\\n78. Thebit\\n79. Mercator\\n80. Campanus\\n81. Bullialdus\\n82. Doppelmayer\\n83. Fourier\\n84. Vieta\\n85. Mersenius\\n86. Arzachel\\n87. Alphonsus\\n88. Alpetragius\\n89. Davy\\n90. Guericke\\n91. Lubiniezky\\n92. Gassendi\\n93. Billy\\n94. Hansteen\\n95. Sirsalis\\n96. Ptolemseus\\n97. Herschel\\n98. Moesting\\n99. Lalande\\n100. Damoiseau\\nThe names are those\\nof scientific men, usually\\nof astronomers.\\nV,\\nM a r\\nr p uua.ti s\\nI\\nFig. 178.\\nNC\\n-The Moon a;\\nf", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0344.jp2"}, "343": {"fulltext": "SHOWN IN FIG. 178.*\\no see the numbers plainly, a common handglass should be used.\\nlimb is fully lighted, and The\\n^_. position of the terminator for\\neach intermediate day is marked\\n_ by the lower set of numbers\\n17, 18 28, 30. These numbers\\nA V give the moon s age, in days,\\n52 when the terminator passes\\n^\u00e2\u0096\u00a0v. through their positions on the\\nX v v.C\\\\ map.\\nv/\\ne/f\\nIII.\\n101.\\n102.\\n103.\\n104.\\n105.\\n106.\\n107.\\n108.\\n109.\\n110.\\n111.\\n112.\\n113.\\n114.\\n115.\\n116.\\n117.\\n118.\\n119.\\n120.\\n121.\\n122.\\n123.\\n124.\\n125.\\n126.\\n127.\\n12s.\\n129.\\n130.\\n131.\\n132.\\n133.\\n134.\\n135.\\n136.\\n137.\\n138.\\n139.\\n140.\\n141.\\n142.\\n143.\\nIV.\\n151.\\n152.\\n153.\\n154.\\n155.\\n156.\\n157.\\n158.\\n159.\\n160.\\n161.\\n162.\\n163.\\n164.\\n165.\\n166.\\n167.\\n168.\\n169.\\n170.\\n171.\\n172.\\n173.\\n174.\\n175.\\n176.\\n177.\\n178.\\n179.\\n180.\\n181.\\n182.\\n183.\\n184.\\n185.\\n186.\\n187.\\n188.\\n189.\\nTHIRD QUADRANT.\\nManzinus\\nMutus\\nBoussingault\\nBoguslawsky\\nCurtius\\nZach\\nJacobi\\nLilius\\nBaco\\nPitiscus\\nHomme]\\nPabricius\\nMetius\\nRheita\\nNicolai\\nBarocius\\nMaurolycus\\nClah-aut\\nCuvier\\nStoeffler\\nFunerius\\nRiccius\\nZagut\\nLindenau\\nAliacenus\\nWerner\\nApian us\\nSacrobosco\\nSantbach\\nFracastor\\nPetavius\\nVendelinus\\nLangrenus\\nGoclenius\\nGuttenberg\\nTheophilus\\nCyrillus\\nCatherina\\nAlbategnius\\nParrot\\nHipparchus\\nReaumur\\nDelambre\\nFOURTH QUADRANT.\\nTaruntius\\nSabine\\nRitter\\nArago\\nAriadeeus\\nGodin\\nAgrippa\\nHyginus\\nTriesnecker\\nCondorcet\\nAzout\\nPicard\\nVitruvius\\nPlinius\\nAcherusia\\nMenelaus\\nMan il ius\\nEinmart\\nCleomedes\\nMacrobius\\nRoemer\\nLe Monnier\\nLinnaeus\\nBessel\\nGauss\\nMessala\\nGeminus\\nPosidonius\\nCalippus\\nAristillus\\nAutolycus\\nCassini\\nAtlas\\nHercules\\nFranklin\\nBiirg\\nEudoxus\\nAristotle\\nEndymion\\n:AWN BY LOHRMANN.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0345.jp2"}, "344": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0346.jp2"}, "345": {"fulltext": "THE MOON. 319\\nthat can be seen with different magnifying powers, at the Moon s\\ndistance.\\nPower 60 diameter of object 1 mile.\\nPower 150 diameter 2000 feet.\\nPower 500 diameter 600 feet.\\nPower 1000 diameter 300 feet.\\nIf telescopic power could be increased indefinitely, there would be\\nno limit to the minuteness of an object that could be seen on the\\nMoon s surface. But the imperfections of all telescopes are such that\\nonly in exceptional cases can anything be gained by increasing the mag-\\nnifying power beyond 1000. The influence of warm and cold currents\\nin our atmosphere will forever prevent the advantageous use of very\\nhigh magnifying powers.\\nCharacter of the Moon s Surface. The most striking point of dif-\\nference between the Earth and Moon is seen in the total absence\\nfrom the latter of anything that looks like the water-worn surfaces\\nof terrestrial plains, prairies, and hills. Valleys and mountain-\\nchains exist on the Moon, but they are abrupt and rugged, not in the\\nleast like our formations of the same name. The lowest surface of\\nthe Moon which can be seen with the telescope appears to be nearly\\nsmooth and fiat, or, to speak more exactly, spherical (because the\\nMoon is a sphere). This surface has different shades of color in\\ndifferent regions. Some portions are of a bright silvery tint, while\\nothers have a dark gray appearance. These differences of tint seem\\nto arise chiefly from differences of material.\\nUpon this surface as a foundation are built numerous formations\\nof various sizes, usually of a very simple character. Their general\\nform can be made out by the aid of Fig. 179, and their dimensions by\\nremembering that one inch on the figure is about 30 miles. The\\nlargest and most prominent features are known as craters. They\\nhave a typical form consisting of a round or oval rugged wall rising\\nfrom the plain in the manner of a circular fortification. These\\nwalls are frequently 10,000 feet or more in height, very rough and\\nbroken. In their interior we see the plane surface of the Moon\\nalready described. It is, however, generally strewn with fragments\\nor broken up by chasms.\\nIn the centre of the craters we frequently find a conical formation\\nrising up to a considerable height. The craters resemble the vol-\\ncanic formations upon the Earth, the principal difference being that\\nsome of them are very much larger than anything known here.\\nThe diameter of the larger ones ranges from 50 to 100 miles, while\\nthe smallest are a half-mile or less, in diameter mere crater-pits.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0347.jp2"}, "346": {"fulltext": "320 ASTRONOMY.\\nHeights of the Lunar Mountains.\u00e2\u0080\u0094 When the Moon is only a few\\ndays old, the Sun s rays strike very obliquely upon the lunar moun-\\ntains, and tliey cast long shadows. From the known position of\\nthe Sun, Moon, and Earth, and from the measured length of the\\nshadows, the heights of the mountains can be calculated. It is thus\\nfound that some of the mountains near the south pole rise to a\\nheight of 8000 or 9000 metres (from 25,000 or 30,000 feet) above the\\ngeneral surface of the Moon. Heights of from 3000 to 7000 metres\\nare very common over almost the whole lunar surface.\\nIs there any Change on the Surface of the Moon When the sur-\\nface of the Moon was first found to be covered by craters like the\\nvolcanoes of the Earth, it was very naturally thought that the lunar\\nvolcanoes might be still in activity, and exhibit themselves to our\\ntelescopes by their flames. Not the slightest evidence of any erup-\\ntion at the Moon s surface has been found.\\nSeveral instances of supposed changes of shape of features on the\\nMoon s surface have been described in recent times, however.\\nPhotographs of the Moon. To make a complete map of the Moon\\nrequires a lifetime. The map of the Moon (six feet in diameter)\\nmade by Dr. Schmidt, Director of the Observatory of Athens, occu-\\npied the greater part of his time during the years 1845-1865.\\nA photograph of the full moon can now be taken in a fraction of a\\nsecond that shows most features far better than Schmidt s map;\\nand a series of such photographs exhibits substantially every lunar\\nfeature better than any map can do. The first photographs of the\\nMoon were made in America. The best lunar photographs are\\nthose of the observatories of Mt. Hamilton (Lick Observatory) and\\nof Paris.\\nKey-chart of the Moon. The accompanying chart of the Moon will\\nbe found of use to the student who has a small telescope or even an\\nopera-glass at his command. After acquiring a general acquaintance\\nwith the lunar topography by observations continued throughout a\\nlunation, he should begin to study the craters in detail, making\\ndrawings of them as accurately as he can. Such drawings may not\\nbe of value to science, but they will be invaluable to the student\\nhimself; for they will train him to see what is to be seen, and to\\nregister it accurately. The changes in the appearance of lunar\\ncraters during a lunation are very marked, and to seek the explana-\\ntion of each particular change is a valuable discipline.\\nGalileo supposed some of the plains of the Moon to be seas, and\\nnamed them Mare Tranquilitatis (the tranquil sea), etc. The prin-\\ncipal mountain-chains on the Moon are named Apennines, Alps, Cau-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0348.jp2"}, "347": {"fulltext": "THE MOON,\\n321\\nFig. 179.\u00e2\u0080\u0094 A Drawing of the Lunar Surface,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0349.jp2"}, "348": {"fulltext": "322\\nASTRONOMY.\\ncasus, etc. The craters are usually named after noted astronomers,\\nKepler, Copernicus, Tycho.\\n34. The Minor Planets. We have next to consider the\\ngroup of minor planets, also called asteroids (because they\\nresemble stars in appearance) or planetoids (because they\\nare planets). None of them was known until the begin-\\nning of the nineteenth century.\\nFirst of all, a curious relation between the distances of the planets,\\nknown as Bode s law, must be mentioned. If to the numbers\\n0, 3, 6, 12, 24, 48, 96, 192, 384,\\neach of which (the second excepted) is twice the preceding, we add\\n4, we obtain the series\\n4. 7, 10, 16, 28, 52, 100, 196, 388,\\nThese last numbers represent approximately the distances of the\\nplanets from the Sun (except for Neptune, which was not discovered\\nwhen the law was announced) by Bode in 1772.\\nThis is shown in the following table\\nPlanets.\\nBode s Law.\\nMercury\\nVenus\\nEarth\\nM ars\\n[Ceres] (one of the asteroids)\\nJupiter\\nSaturn\\nUranus\\nNeptune\\n4\\n7.0\\n10.0\\n16.0\\n28.0\\n52.0\\n100.0\\n196.0\\nAlthough the so-called law was purely arbitrary, the agreement\\nbetween the distances predicted by the law and the actual distances\\nwas sufficiently close to draw attention to the fact that a srap existed\\nin the succession of the planets between Mars and Jupiter.\\nIt was therefore supposed by the astronomers of the\\nseventeenth and eighteenth centuries that a new major\\nplanet might be found in the region between Mars and", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0350.jp2"}, "349": {"fulltext": "THE MINOR PLANETS. 323\\nJupiter. A search for this object was instituted, bat before\\nit had made much progress a minor planet in the place of\\nthe one so long expected was found by Piazzi, of Palermo.\\nThe discovery was made on the first day of the present\\ncentury, 1801, January 1. It was named Ceres.\\nIn the course of the following seven years the astronom-\\nical world was surprised by the discovery of three other\\nplanets, all in the same region, thongh not revolving in the\\nsame orbit. Seeing four small planets where one large one\\nought to be, Olbers suggested that these bodies might be\\nfragments of a large planet that had been broken to pieces\\nby the action of some unknown force.\\nA generation of astronomers now passed away without\\nthe discovery of more than these four. It was not until\\n1845 that a fifth planet of the group was found. In 1847\\nthree more were discovered, and many discoveries have\\nsince been made. The number is now nearly 500, and\\nthe discovery of additional ones is going on as fast as ever.\\nThe frequent announcements of the discovery of planets\\nwhich appear in the public prints all refer to bodies of this\\ngroup. Seventy-seven of them have been discovered by\\nAmerican astronomers.\\nThe minor planets are distinguished from the major ones\\nby many characteristics. Among these we may mention\\ntheir small size; their positions, all but one being situated\\nbetween the orbits of Mars and Jupiter; the great eccen-\\ntricities and inclinations of their orbits. The inclination\\nof the orbit of Pallas to the ecliptic is 35\u00c2\u00b0, for example.\\nNumber of Small Planets.\u00e2\u0080\u0094 It would be interesting to know bow\\nmany of tbese planets tbere are in tbe group, but it is as vet impos-\\nsible even to guess at tbe number. As already stated, about 500 are\\nnow known, and new ones are found every year.\\nA minor planet presents no sensible disk, and therefore looks\\nexactly like a small star. It can be detected only by its motion among\\ntbe surrounding stars, which is so slow that some hours must elapse\\nbefore it can be noticed. Nowadays they are found by photograph-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0351.jp2"}, "350": {"fulltext": "324: ASTRONOMY.\\ning a region of the sky with two or three hours exposure and noticing\\nwhether any of the objects on the plate show a motion in that time.\\nA fixed star will show no motion. An asteroid will make a trail on\\nthe plate.\\nMagnitudes. \u00e2\u0080\u0094It is impossible to make any precise measurement of\\nthe diameters of the minor planets. The diameters in miles that are\\nsometimes quoted are subject to very large errors. The amount\\nof light which the planet reflects is a better guide than measures\\nmade with ordinary micrometers. Supposing the proportion oi light\\nreflected to be the same as in the case of the larger planets, the diam-\\neters of the three or four largest range between 300 and 600 kilo-\\nmetres, while the smallest are from 20 to 50 kilometres in diameter.\\nThe average diameter is perhaps less than 150 kilometres (say 90\\nmiles) that is, scarcely more than one hundredth that of the Earth.\\nThe volumes of solid bodies vary as the cubes of their diameters it\\nmight therefore take a million of these planets to make one of the\\nsize of the Earth.\\nMass and Density of the Asteroids. Nothing is known of the mass\\nof any single asteroid. If their density is the same as that of the\\nEarth the mass of the larger asteroids will be about gu^oo \u00c2\u00b0f tue\\nEarth s mass. The force of gravity on the surface of such a body\\nwould be about of the force of gravity on the Earth. A bullet\\nshot from a rifle would fly quite away from the planet and would cir-\\nculate about the Sun. It is not probable that any of them has\\nan extensive atmosphere.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0352.jp2"}, "351": {"fulltext": "CHAPTER XIX.\\nTHE PLANETS JUPITER, SATURN, URANUS, AND\\nNEPTUNE.\\n35. Jupiter. Jupiter is much the largest planet in the\\nsystem. His mean distance is 483,300,000 miles. His\\nmean diameter is 86,500 miles, the polar diameter being\\n83,000, the equatorial 88,200 miles. His linear diameter\\nis about y 1 his surface is T and his volume y^Vo that of\\nthe Sun. His mass is T oVs- His density is nearly the\\nsame as the Sun s density, that is 1^- times the density\\nof water. The densities of Venus, the Earth, the Moon,\\nand of Mars are all more than three times the density of\\nwater. A cubic foot of the materials of each of these\\nbodies weighs at least 200 lbs. A cubic foot of the stuff\\nout of which Jupiter is made weighs, on the average, no\\nmore than 83 lbs. Jupiter is, in this respect, like the Sun\\nand not like the inner planets.\\nHe is attended by five satellites, four of which were dis-\\ncovered by Galileo on January 7, 1610. He named them,\\nin honor of the Medicis, the Medicean stars. They are\\nnow known as Satellites I, II, III, and IV, I being the\\nnearest. They are large bodies, from 2100 to 3500 miles\\nin diameter, comparable in size to the Moon or to Mercury.\\nThe fifth satellite was discovered by Barnard with the\\ngreat telescope of the Lick Observatory in 1892. It is a\\nvery small object, about 100 miles in diameter, revolving\\nvery close to the surface of Jupiter. Observations show\\nthat the larger satellites revolve about Jupiter, always turn-\\n325", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0353.jp2"}, "352": {"fulltext": "326 ASTRONOMY.\\ning the same face to the planet just as our own Moon turns\\nalways the same face to the Earth.\\nThe rotation-time of the planet is not the same in all latitudes\\nnor, in the same latitude, at all depths below the outer surface of its\\nclouds. The average time of rotation is about 9 h 55 m which is notice-\\nably shorter than the rotation-times of Mars and the Earth. The\\nFig. 180. Drawing op Jupiter made at the Lick Observa-\\ntory, August 28, 1890.\\nfigure of the planet is markedly spheroidal its disk is easily seen to\\nbe elliptical in shape. The phases of Jupiter are slight scarcely\\nnoticeable. The reflecting-power {albedo) of the planet is y 6 not\\nvery much less than that of newly fallen snow T W)- I n this respect\\nJupiter and all the outer planets differ very materially from Mars\\nand all the inner planets (except Venus). The periodic-time of Jupiter", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0354.jp2"}, "353": {"fulltext": "TEE PLANET JUPITER. 327\\nis 11.86 years, about the period in which the solar spots vary from\\nmaximum to maximum again. Figure 180 shows in the upper third\\nof the disk an oval spot that has remained on the planet tor the past\\n30 years {The Great Red Spot). Its surface is red and it probably lies\\nat a deeper level than many of the whitish clouds in the same lati-\\ntudes. It is remarkable that the red spot has endured for so long a\\ntime on the surface of the planet where all other features are so\\nchangeable. The red spot is not fixed in position, but is slowly drift-\\ning to the east. It is as if Australia were slowly moving eastwardly\\non the earth. The rotation time of the red spot was 9 h 55 m 34\\\\5 in\\n1869 34 s 1 in 1879 39 s .O in 1884 40 8 .4 in 1889 41 8 .0 in 1894 41v 9\\nin 1898. It is as if an island of slag were drifting on the surface of\\na lake of liquid lava.\\nThe temperature of Jupiter is, in all probability, very\\nhigh. The planet may even be incandescent. The rapid\\nchanges observed in the surface of Jupiter prove that the\\nvisible surface is gaseous an atmospheric envelope. These\\nchanges are due to heat. As the solar heat at Jupiter is\\nonly 2* T of the solar heat at the Earth, it is likely that the\\nchanges are due to the internal heat of the planet itself.\\nThe solar heat at Saturn is only -J^- of the solar heat at the\\nEarth, and as it is also surrounded by a gaseous envelope,\\nthere is good reason for supposing Saturn, also, to be a hot\\nbody.\\nThe surface of Jupiter has been carefully studied with\\nthe telescope, particularly within the past thirty years.\\nAlthough further from us than Mars, many of the details\\non his disk are much more plainly marked. The most\\ncharacteristic features are shown in the drawings appended.\\nThese features are, first, the dark bands of his equatorial\\nregions, and, secondly, the cloud-like forms spread over\\nnearly the whole surface. Near the edges of the disk all\\nthese details become indistinct, and finally vanish, thus in-\\ndicating a highly absorptive atmosphere like that of the Sun.\\nThe light from the centre of the disk is twice as bright as\\nthat from the poles. The bands can be seen with instru-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0355.jp2"}, "354": {"fulltext": "S28\\nASTRONOMY.\\nments no more powerful than those nsed by Galileo, yet\\nhe makes no mention of them.\\nThe general color of the bands is reddish. Their posi-\\ntion varies slightly in latitude, but in the main they remain\\nas permanent features of the region to which they belong.\\nFig. 181\\n-View of Jupiter and his Satellites in a Small\\nTelescope.\\nHerschel, iu the year 1793, attributed the aspects of\\nthe bands to zones of the planet s atmosphere more tranquil\\nand less filled with clouds than the remaining portions, so\\nas to permit the true surface of the planet to be seen\\nthrough these zones, while the clouds prevailing in the\\nother regions give a brighter tint to the latter. It is not\\nlikely that we see the true surface of the planet, in the\\nbelts, but rather the oater surfaces of the inner layers of\\nthe planet s atmosphere.\\nThe clouds themselves can easily be seen at times, and\\nthey have every variety of shape. In general they are\\nsimilar in form to a series of white cumulus clouds such as\\nare frequently seen piled up near the horizon, and the\\nspaces between them have the deep salmon color of the\\nspaces between cumulus clouds before a summer storm.\\nThis color is due to the absorption of the dense atmosphere\\nof the planet, probably. The bands themselves and the red", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0356.jp2"}, "355": {"fulltext": "THE PLANET JUPITER. 329\\nspot seem frequently to be veiled over with something like\\nthe thin cirrus clouds of oar atmosphere.\\nSuch clouds can be tolerably accurately observed, and\\nmay be used to determine the rotation-time of the planet.\\nThe observations show that the clouds often have a proper\\nmotion of their own.\\nFig. 182. View of Jupiter in a Large Telescope, with a\\nSatellite and its Shadow seen on the Disk.\\nMotions of the Satellites. The satellites move about Jupiter from\\nwest to east in nearly circular orbits. When one of these satellites\\npasses between the Sun and Jupiter, it casts a shadow upon Jupiter s\\ndisk (see Fig. 182) precisely as the shadow of our Moon is thrown upon\\nthe Earth in a solar eclipse. If the satellite passes through Jupiter s\\nown shadow in its revolution, an eclipse of the satellite takes place.\\nIf it passes between the Earth and Jupiter, it is projected upon Ju-\\npiter s disk, and we have a transit of the satellite (see Fig. 182); if Ju-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0357.jp2"}, "356": {"fulltext": "330\\nASTRONOMY.\\npiter is between the Earth and the satellite, an occultation of the\\nlatter occurs. All these phenomena can be seen with a common\\ntelescope, aod the times are predicted in the Nautical Almanac.\\nThese shadows are seen black upon Jupiter s surface by contrast,\\nbecause Jupiter is very much brighter than the satellites.\\nFtg 183 \u00e2\u0080\u0094The Eclipsep of Jupttep s Satellites.\\nS is the Sun, Tthe Earth, J, J J J are different positions of Jupiter.\\nTelescopic Appearance of the Satellites. Under ordinary circum-\\nstances, the satellites of Jupiter are seen to have disks under very\\nfavorable conditions, marking s have been seen on these disks.\\nThe satellites completely disappear from telescopic view when they\\nenter the shadow of the planet. This shows that neither planet nor\\nsatellite is self-luminous to any marked degree. If the satellite were", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0358.jp2"}, "357": {"fulltext": "THE PLANET SATURN. 331\\nself-luminous, it would be seen by its own light and if the planet\\nwere luminous, the satellite might be seen by the reflected light of\\nthe planet.\\nThe Progressive Motion of Light.\u00e2\u0080\u0094 The discovery that light requires\\ntime to travel was first made by the observations of the satellites of\\nJupiter, as has been said. (See page 255.) Jupiter casts a shadow\\njust as our Earth does, and its inner satellite passes through this\\nshadow and is eclipsed at every revolution.\\nThe eclipses can be observed from the Earth, the satellite vanishing\\nfrom view as it enters the shadow, and reappearing when it leaves it.\\nThe astronomers of the seventeenth century made tables by which\\nthe times of the eclipses could be predicted. It was found by Romer\\nthat these times depended on the distance of Jupiter from the Earth.\\nWhen the Earth was nearest Jupiter, the eclipses were seen earlier\\nthan the predicted time. Jupiter and the Earth were near each other.\\nWhen the Earth was farthest from Jupiter the eclipses were seen\\nlater than the predicted time. Jupiter and the Earth were far apart.\\nThe light from the satellite required time to cross the intervening\\nspaces. The velocity with which light travels is 186,330 miles per\\nsecond. At that rate it traverses the distance from the Sun to the\\nEarth in 499 seconds. The sunlight is 8 m 19 s old when it reaches us.\\nLongitudes by Observation of the Satellites of Jupiter. The differ-\\nence of longitude of two places on the Earth is the difference of their\\nsimultaneous local times. If we know beforehand (by calculation) the\\nGreenwich time of an eclipse of one of the satellites and if we observe\\nthe eclipse by a clock keeping our own local time, the difference of\\nthe two times (observed and calculated) is our longitude from Green-\\nwich. Galileo suggested that a method like this might be useful\\nin determining terrestrial longitudes and the method has often been\\ntested. The difficulty of observing the eclipses with accuracy, and\\nthe fact that the aperture of the telescope employed has an important\\neffect on the appearances seen, have so far kept this method from a\\nwide utility, which it at first seemed to promise.\\n36. Saturn and its System. Saturn is the most distant\\nof the major planets known to the ancients. It revolves\\naround the Sun in 29\u00c2\u00a3 years, at a mean distance of about\\n886,000,000 miles. The equatorial diameter of the ball of\\nthe planet is about 75,000 miles and the polar diameter\\nabout 68,000 miles. It revolves on its axis in 10 h 14 m 24 s\\nor less than half a day, which accounts, as in the case of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0359.jp2"}, "358": {"fulltext": "332\\nASTRONOMY.\\nFig. 184.\u00e2\u0080\u0094 Drawing of Saturn made at the Lick Observa-\\ntory, January 7, 1888.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0360.jp2"}, "359": {"fulltext": "THE PLANET SATURN.\\n333\\nJupiter, for the ellipticity of the disk. The mass of the\\nplanet is only 95 times the mass of the Earth, though its\\nvolume is 760 times greater. The force of gravity at its\\nsurface is only a little greater than that of the Earth. It\\nis remarkable for its small density which is less than that\\nof any other heavenly body, and even less than that of\\nwater. No doubt the planet is in great part, if not en-\\ntirely, gaseous. The edges of the planet are fainter than\\nthe centre, as in the case of Jupiter, and for the same\\nreason.\\nFig. 185.\u00e2\u0080\u0094 View of the Saturnian System in a Small Tele-\\nscope.\\nSaturn is the centre of a system of its own, in appearance\\nquite unlike anything else in the heavens. Its most note-\\nworthy feature is a pair of rings which surround it at a\\nconsiderable distance from the planet itself. Outside of\\nthese rings revolve no less than nine satellites. The", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0361.jp2"}, "360": {"fulltext": "334 ASTRONONY.\\nplanet, rings, and satellites are altogether called the\\nSaturnian system. The general appearance of this system,\\nas seen in a small telescope, is shown in Fig. 185. Fig.\\n184 was drawn with the great telescope of the Lick\\nObservatory.\\nThe Rings of Saturn. The rings are the most remark-\\nable and characteristic feature of the Saturnian system.\\nFig. 186 gives two views of the ball and rings. The upper\\none shows one of their aspects as actually presented in the\\ntelescope, and the lower one shows what the appearance\\nwould be if the planet were viewed from a direction at right\\nangles to the plane of the ring (which it never can be from\\nthe Earth). The shadow of the ball of the planet on the\\nrings should be noticed in both views. The periodic-time\\nof the planet is a little less than 29-J years.\\nThe first telescopic observers of Saturn were unable to see the\\nrings in their true form, and were greatly perplexed to account for\\nthe appearance which the planet presented. Galileo described the\\nplanet as tri-corporate, the two ends of the ring having, in his\\nimperfect telescope, the appearance of a pair of small planets attached\\nto the central one. On each side of old Saturn were servitors who\\naided him on his way. This discovery was announced to his friend\\nKepler in this logogriph\\nsmaismrmilmepoetalevmibunenugttaviras, which, being trans-\\nposed, becomes\\nAltissimura planetam tergeminum observavi (I have observed\\nthe most distant planet to be tri-form).\\nThe phenomenon constantly remained a mystery to its first ob-\\nserver. In 1610 he had seen the planet accompanied, as he supposed,\\nby two lateral stars in 1612 the latter had vanished and the central\\nbody alone remained. Galileo inquired whether Saturn had\\ndevoured his children, according to the legend.\\nIt was not until 1655 (after seven years of observation) that the\\ncelebrated Huyghens discovered the true explanation of the remark-\\nable and recurring series of phenomena present by the tri corporate\\nplanet.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0362.jp2"}, "361": {"fulltext": "TEE PLANET SATURN.\\n335\\nFig. 186.\u00e2\u0080\u0094 The Planet Saturn.\\n1\u00c2\u00b0 as it sometimes appears to an observer on the Earth 2\u00c2\u00b0 as it would\\nappear to an observer over the polar region of the planet.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0363.jp2"}, "362": {"fulltext": "336 ASTRONOMY.\\nHe announced his conclusions in the following logogriph\\naaaaaa ccccc d eeeee g h iiiiiii 1111 mm nnnnnnnnn oooo pp q rr s\\nttttt uuuuu, which, when arranged, read\\nAnnulo cingitur, tenui, piano, nusquam coherente, ad eclipticam\\ninclinato (it is girdled by a thin plane ring, nowhere touching,\\ninclined to the ecliptic).\\nThis description is complete and accurate, as to the appearance in a\\nsmall telescope.\\nIn 1675 it was found by Cassini that what Htjyghens had seen as\\na single ring was really two. A division extended all the way around\\nnear the outer edge. The division is shown in the figures. This\\ndivision is permanent. Others are sometimes seen at different places\\nand this fact of observation suggests that the rings cannot be per-\\nmanent solids, nor liquids.\\nIn 1850 the Messrs. Bond, of Harvard College Observatory, found\\nthat there was a third ring, of a dusky and nebulous aspect, attached\\nto the inner edge of the inner ring. It is known as Bond s dusky ring.\\nIt is a difficult object to see in a small telescope. It is not separated\\nfrom the bright ring, but attached to it. The latter shades off toward\\nits inner edge, and merges gradually into the dusky ring, Fig. 184.\\nAspect of the Rings. As Saturn revolves around the Sun, the\\nplane of the rings remains parallel to itself. That is, if we consider\\na straight line passing through the centre of the planet, perpendicu-\\nlar to the plane of the ring, as the axis of the latter, this axis will\\nalways point in the same direction in space among the stars. In\\nthis respect the motion is similar to that of the Earth around the Sun.\\nThe ring of Saturn is inclined about 27\u00c2\u00b0 to the plane of its orbit.\\nConsequently, as the planet revolves around the sun, there is a\\nchange in the direction in which the Sun shines upon it similar\\nto that which produces the change of seasons upon the Earth, as\\nshown in Fig. 110.\\nThe corresponding changes for Saturn are shown in Fig. 187. Dur-\\ning each revolution of Saturn (29| years) the plane of the ring\\npasses through the Sun twice. This occurred in the years 1878 and\\n1891, at two opposite points of the orbit, as shown in the figure,\\nand will occur in 1907. At two other points, midway between these,\\nthe Sun shines upon the plane of the ring at its greatest inclination,\\nabout 27\u00c2\u00b0. Since the Earth (shown in the picture) is little more than\\none-tenth as far from the Sun as Saturn is, an observer sees Saturn\\nnearly, but not quite, as if he were upon the Sun. Hence at certain\\ntimes the rings of Saturn are seen edgeways while at other times\\nthey are at an inclination of 27\u00c2\u00b0, the aspect depending upon the posi-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0364.jp2"}, "363": {"fulltext": "THE PLANET SATUEN. 337\\ntion of Saturn in its orbit. The following are the times of some of the\\nphases\\n1878 and 1907.\u00e2\u0080\u0094 The edge of the ring is turned toward the Sun. It\\nis seen only as a thin line of light.\\n1885.\u00e2\u0080\u0094 The planet having moved forward 90\u00c2\u00b0, the south side of\\nthe rings is seen at an inclination of 27\u00c2\u00b0.\\n1891.\u00e2\u0080\u0094 The planet having moved 90\u00c2\u00b0 further, the edge of the ring\\nis again turned toward the Sun.\\nFig. 187. Different Aspects of the Ring of Saturn as seen\\nfrom the Earth in Different Years.\\n1899. The north side of the ring is inclined toward the sun, and\\nis seen at its greatest inclination.\\nThe rings are extremely thin in proportion to their extent. Conse-\\nquently, when their edges are turned toward the Earth, they appear\\nas a mere line of light, which can be seen only with powerful\\ntelesc pes.\\nConstitution of the Rings of Saturn. The nature of\\nthese objects has been a subject both of wonder and of in-\\nvestigation by mathematicians and astronomers ever since", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0365.jp2"}, "364": {"fulltext": "338 ASTRONOMY.\\nthey were discovered. They were at first supposed to be\\nsolid bodies; indeed, from their appearance it was difficult\\nto conceive of them as anything else. The question then\\narose What keeps them from falling on the planet It\\nwas shown mathematically by La Place that a homo-\\ngeneous and solid ring surrounding the planet could not\\nremain in a state of equilibrium, but must be precipitated\\nupon the central ball by the smallest disturbing force.\\nIt is now established both by mathematical processes and\\nby spectroscopic observation that the rings do not form a\\ncontinuous mass, but are really a countless multitude of\\nsmall separate particles or satellites, each of which revolves\\nin its own orbit. These satellites are individually far too\\nsmall to be seen in any telescope, but so numerous that\\nwhen viewed from the distance of the Earth they appear as\\na continuous mass, like particles of dust floating in a sun-\\nbeam.\\nThe thickness of the rings is not above 100 miles. The outer diam-\\neter of the outer ring (ring A) is 173,000 miles. It is 11,500 miles\\nwide. The Cassini division separating A from B is 2400 miles wide.\\nThe outer diameter of ring B is 145,000 miles, and it is 17,500 miles\\nwide. The outer diameter of ring G (the dusky ring) is 100,000, and\\nits inner diameter is 90,000 miles. Dr. Keeler, has proved, spectro-\\nscopically, that different parts of the rings revolve about the planet\\nat different rates, so that the rings must necessarily be composed\\nof discrete particles. The rotation time of the ball of Saturn is 10 h\\n14 m the periodic-time of the innermost particle of the dusky ring is\\n5 h 50 m Inside of this particle the space is empty.\\nSatellites of Saturn. Outside of the rings of Saturn revolve its nine\\nsatellites, the order and discovery of which are shown in the table on\\npage 339.\\nThe distances are given in radii of the planet. The satellites Mimas\\nand Hyperion and satellite No. 9 are visible only in the most power-\\nful telescopes. The brightest of all is Titan, which can be seen in a\\ntelescope of ordinary size. The mass of Titan is T of Saturn s mass,\\nand it is some 3000 miles in diameter. Japetus is nearly as bright as\\nTitan when west of the planet, and is so faint as to be visible only in\\nlarge telescopes when on the other side. Like our moon, it always", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0366.jp2"}, "365": {"fulltext": "THE PLANET URANUS.\\n339\\npresents the same face to the planet, and one side of it is dark and the\\nother side light. When west of the planet, the bright side is turned\\ntoward the Earth and the satellite is visible. On the other side of the\\nplanet, the dark side is turned toward us, and it is nearly invisible.\\nSatellites 3, 4, 5, 6, and 8 can be seen with telescopes of moderate\\npower.\\nNo.\\nName.\\nDistance\\nfrom\\nPlanet.\\nDiscoverer.\\nDate of\\nDiscovery.\\nPeriodic-time.\\nMimas\\n3.3\\nEnceladus\\n4.3\\nTethys\\n5.3\\nDione\\n6.8\\nRhea\\n9.5\\nTitan\\n20 7\\nHyperion\\n26.8\\nJapetus\\n64.4\\n225.4\\nHerschel\\n1789\\nHerschel\\n1789\\nCassini\\n1684\\nCassini\\n1684\\nCassini\\n1672\\nHuyghens\\n1655\\nBond\\n1848\\nCassini\\n1671\\nPickering\\n1899\\nAbout d 23 h\\nu ld g h\\nl d 21 h\\n2 d 18*\\n4 d 13 h\\n15 d 23 h\\n21 d 7 h\\n79 d 8 h\\n510 days.\\n37. The Planet Uranus. Uranus was discovered on\\nMarch 13, 1781, by Sir William Herschel (then an\\namateur observer) with a ten-foot reflector made by him-\\nself. He was examining a portion of the sky when one of\\nthe stars in the field of view attracted his notice by its\\npeculiar appearance. On further scrutiny, it proved to be\\na planet. We can scarcely comprehend now the enthusiasm\\nwith which this discovery was received. No new body\\n(save comets) had been added to the solar system since the\\ndiscovery of the third satellite of Saturn in 1684, and all\\nthe major planets of the heavens had been known for\\nthousands of years.\\nUranus revolves about the Sun in 84 years. Its apparent\\ndiameter as seen from the Earth varies little, being about\\n3 9. Its true diameter is about 31,000 miles, and its\\nfigure is spheroidal.\\nIn physical appearance it is a small greenish disk without\\nmarkings. The centre of the disk is slightly brighter than", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0367.jp2"}, "366": {"fulltext": "340 ASTRONOMY.\\nthe edges. At its nearest approach to the Earth, it shines\\nas a star of the sixth magnitude, and is just visible to an\\nacute eye when the attention is directed to its place. In\\nsmall telescopes with low powers, its appearance is not\\nmarkedly different from that of stars of about its own\\nbrilliancy.\\nSir William Heeschel discovered two satellites to\\nUranus. Two additional ones were discovered by Lassell\\nin 1847.\\nDays.\\nI. Ariel (Lassell) Period 2.520383\\nII. Umbriel 4.144181\\nIII. Titania (Herschel) 8.705897\\nIV. Oberon 13.463269\\nAriel varies in brightness on different sides of the planet,\\nand the same phenomenon has also been suspected for\\nTitania. This indicates that these satellites always\\npresent the same face to the planet.\\nThe most remarkable feature of the satellites of Uranus\\nis that their orbits are nearly perpendicular to the ecliptic\\ninstead of having a small inclination to that plane, like\\nthose of all the orbits of both planets and satellites pre-\\nviously known.\\nThe four satellites move in the same plane. This fact\\nrenders it highly probable that the planet Uranus revolves\\non its axis in the same plane with the orbits of the satel-\\nlites, and is therefore an oblate spheroid like the Earth.\\nIf the planes of the satellites orbits were not kept together\\nby some cause, they would gradually deviate from each\\nother owing to the attractive force of the Sun upon the\\nplanet. The different satellites would deviate by different\\namounts, and it would be extremely improbable that all the\\norbits would be found in the same plane at any particular\\nepoch. Since we now see them in the same plane, we con-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0368.jp2"}, "367": {"fulltext": "THE PLANET NEPTUNE. 341\\nclade that some force keeps them there, and the oblateness\\nof the planet is the efficient cause of such a force.\\nThe Planet Neptune. After the planet Uranus had been\\nobserved for some thirty years, tables of its motion were\\nprepared by Bouvard a French astronomer. He had not\\nonly all the observations since the date of its discovery in\\n1781, but also observations extending back as far as 1695,\\nwhen the planet was observed and supposed to be a fixed\\nstar. It was expected that the ancient observations would\\nmaterially aid in obtaining exact accordance between the\\ntheory and observation. But it was found that, after\\nallowing for all perturbations produced by the known\\nplanets, the ancient and modern observations, though un-\\ndoubtedly referring to the same object, were yet not to be\\nreconciled with each other, but differed systematically.\\nBouvard was forced to found his theory upon the modern\\nobservations alone. By so doing, he obtained a good agree-\\nment between theory and the observations of the few years\\nimmediately succeeding 1820.\\nBouvard made the suggestion that a possible cause for\\nthe discrepancies noted might be the existence of an\\nunknown planet, exterior to Uranus.\\nIn the year 1830 it was found that Bouvard s tables,\\nwhich represented the motion of the planet well during the\\nyears 1820-25, were 20 in error. In 1840 the error was\\n90 and in 1845 it was over 120.\\nThese progressive changes attracted the attention of\\nastronomers to the subject of the theory of the motion of\\nUranus. The actual discrepancy (120 in 1845 was not a\\nquantity large in itself. Two stars of the magnitude of\\nUranus, and separated by only 120 would be seen as one\\nto the unaided eye. It was on account of its systematic and\\nprogressive increase that suspicion was excited.\\nSeveral astronomers attacked the problem in various\\nways. The elder Struve, at Pulkova in Russia, searched", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0369.jp2"}, "368": {"fulltext": "342 ASTRONOMY.\\nfor a new planet with the large telescope of the Imperial\\nObservatory. Bessel, at Koenigsberg, set a student of his\\nown, Fleming, to make a new comparison of observation\\nwith theory, in order to furnish data for a new determina-\\ntion. Arago, then Director of the Observatory at Paris,\\nsuggested this subject in 1845 as an interesting field of\\nmathematical research to Le Verrier. Mr. J. C. Adams,\\na student in Cambridge University, England, had become\\naware of the problems presented by the anomalies in the\\nmotion of Uranus, and had attacked this question as early\\nas 1843.\\nIn October, 1845, Adams communicated to the As-\\ntronomer Koyal of England elements of a new planet so\\nsituated as to produce the perturbations of the motion of\\nUranus which had actually been observed. Such a predic-\\ntion from an entirely unknown student, as Adams then\\nwas, did not carry entire conviction with it. A series of\\naccidents prevented the unknown planet being lo oked for\\nby one of the largest telescopes in England, and so the\\nmatter apparently dropped. It may be noted, however,\\nthat we now know Adams elements of the new planet to\\nhave been so near the truth that if it had been really looked\\nfor by the powerful telescope which afterward discovered\\nits satellite, it could scarcely have failed of detection.\\nBessel s pupil Fleming died before his work was done,\\nand Bessel s researches were temporarily brought to an\\nend. Struve s search was unsuccessful. Le Verrier,\\nhowever, continued his investigations, and in the most\\nthorough manner. He first computed anew the perturba-\\ntions of Uranus produced by the action of Jupiter and\\nSaturn. Then he examined the nature of the irregulari-\\nties observed. These showed that if they were caused by\\nan unknown planet, it could not be between Saturn and\\nUranus, because Saturn would have been more affected\\nthan was the case.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0370.jp2"}, "369": {"fulltext": "THE PLANET NEPTUNE. 343\\nIf the new planet existed at all it was outside of Uranus.\\nIn the summer of 1846, Le Verrier obtained complete\\nelements of a new planet, which would account for the\\nobserved irregularities in the motion of Uranus, and these\\nwere published in France. They were very similar to\\nthose of Adams, and this striking fact renewed the interest\\nin Adams work. It was determined to search in the\\nheavens for the planet foretold by theory.\\nProfessor Challis, the Director of the Observatory of\\nCambridge, England, began a search for such an object, and\\nas no star-maps were at hand for this region of the sky, he\\ncommenced by mapping the surrounding stars. In so\\ndoing the new planet was actually observed, both on August\\n4 and 12, 1846, but the observations remained unreduced,\\nand the planetary nature of the object was not recognized\\ntill afterwards.\\nIn September of the same year Le Verrier wrote to\\nDr. Galle, then Assistant at the Observatory of Berlin,\\nasking him to search for the new planet, and directing him\\nto the place where it should be found. By the aid of an\\nexcellent star-chart of this region, which had just been\\ncompleted, the new planet was found September 23, 1846.\\nThe strict rights of discovery lay with Le Verrier, but Adams\\ndeserves an equal share in the honor attached to this most brilliant\\nachievement. Indeed, it was only by the most unfortunate succession\\nof accidents that the discovery did not attach to Adams researches.\\nOne thing must in fairness be said, and that is that the results of\\nLe Verrier were reached after a most thorough investigation of the\\nwhole ground, and were announced with an entire confidence which,\\nperhaps, was lacking in the other case.\\nThis brilliant discovery created even more enthusiasm\\nthan the discovery of Uranus, as it was by an exercise of\\nfar higher intellectual qualities that it was achieved. It\\nwas nothing short of marvellous that a mathematician could\\nsay to an observer that if he would point his telescope to a", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0371.jp2"}, "370": {"fulltext": "344\\nASTRONOMY.\\ncertain small area, he would find within it a major planet\\nhitherto unknown. Yet so it was. By somewhat similar\\nprocesses previously unknown companious to the bright\\nstars Sir ins and Procyon have been predicted, and these\\ncompanions have subsequently been discovered with the\\ntelescope.\\n185\\nI830 3\\n1840// A\\nff A /^830\\n9 1840\\n2\\n~~~Xl8IO\\n1 \\\\l800\\nl8l0 /1\\\\\\ny t\\n1800 1\\nx .S-\\nx\\nFig. 188.\u00e2\u0080\u0094 Perturbations op Uranus by a Planet Exterior\\nto it Neptune.\\nThe general nature of the disturbing force which revealed the new\\nplanet may be seen by Fig. 188, which shows the orbits of the two\\nplanets, and their respective motions between 1781 and 1840. The\\ninner orbit is that of Uranus, the outer one that of Neptune. The\\narrows show the directions of the attractive force of Neptune.\\nOur knowledge regarding Neptune is mostly confined to\\na few numbers representing the elements of its motion.\\nIts mean distance is more than 2,775,000,000 miles; its\\nperiodic time is 164.78 years; its apparent diameter is 2.6\\nseconds, corresponding to a true diameter of about 34,000\\nmiles. Gravity at its surface is about nine tenths of the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0372.jp2"}, "371": {"fulltext": "CONSTITUTION OF THE PLANETS. 345\\ncorresponding terrestrial surface gravity. Of its rotation\\nand physical condition nothing is known. Its color is a pale\\ngreenish blue. It is attended by one satellite, which was\\ndiscovered by Mr. Lassell, of England, in 1847. The\\nsatellite requires a telescope of twelve inches aperture or\\nupward to be well seen. It is not unlikely that the planet\\nmay have a second very faint satellite.\\n38. The Physical Constitution of the Planets.\u00e2\u0080\u0094 The solar system is\\ncomposed of three groups of planets differing- widely in their char-\\nacteristics. The first group consists of Mercury, Venus, the Earth,\\nMars the second group is the asteroids the third consists of Ju-\\npiter, Saturn, Uranus, and Neptune. The diameters of the first group\\nvary from 3000 to 8000 miles, their periodic-times are less than two\\nyears, their masses are never greater than aoo^nru \u00c2\u00b0f tne Sun s mass,\\ntheir densities are from 3 to 5\u00c2\u00a3 times the density of water. The Moon,\\nthe satellite of the Earth, belongs in this group. Its density is 3.4\\ntimes the density of water. Two planets of this group Venus and\\nthe Earth are certainly surrounded by atmospheres. The others\\nprobably have little or no atmosphere. The planets of this group\\nwere named by Alexander von Humboldt terrestrial planets. They\\nare in some respects like the Earth. At any rate, all of them are\\nmuch more like the Earth than like the giant planets beyond Mars.\\nThe asteroids are quite unique among the planets. Jupiter, Saturn,\\nUranus, Neptune present many striking resemblances. They are of\\ngiant size. Their diameters vary from 30,000 to 90,000 miles.\\nTheir masses are relatively large (^3^07 to ttjfo \u00c2\u00b0f tne Sun s mass),\\ntheir densities are all small (none greater than 1\u00c2\u00a3 times the density\\nof water). At least two of them have a very short period of rotation,\\nand all of them have a high reflecting power. Their surfaces are\\ncovered with clouds and there is good reason to believe that one of\\nthem\u00e2\u0080\u0094 Jupiter is still a very hot body. Very likely all of them\\nconsist of masses of molten matter surrounded by envelopes of vapor.\\nThis view is further strengthened by their very small specific grav-\\nity, which can be accounted for by supposing that the liquid interior\\nis nothing more than a comparatively small central core, and that the\\ngreater part of the bulk of each planet is composed of vapor of small\\ndensity. Some of the satellites of this group are about as large as\\nMars or Mercury.\\nFinally the central body of the whole system\u00e2\u0080\u0094 the Sun is im-\\nmensely larger than all the planets tak. n together it is very hot it", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0373.jp2"}, "372": {"fulltext": "346 ASTBONOMY.\\nis almost or entirely gaseous its density is less than 1 T the density\\nof water and this in spite of the immense pressure on its interior\\nparts. Mercury, Mars, the Moon, are airless, cold, dense, small.\\nWe know little of Venus except that she is covered with clouds.\\nVenus may be more like the Earth than any other planet. The aster-\\noids are mere fragments, probably all airless and cold. The giant\\nplanets are (probably) all hot, with a solid or liquid nucleus and a\\ndeep atmosphere. And at the end of the series comes the Sun, hot,\\ngaseous, immensely larger than the planets.\\nThe differences between these different bodies are chiefly due to\\ntemperature. If any one of them were to be suddenly raised to the\\nSun s temperature it would probably be a miniature Sun. Each of\\nthese bodies is cooling by the radiation of its heat into space. None\\nof the heat radiated returns to the body, so far as is known. The\\nSun in cooling will probably become a body somewhat like Jupiter.\\nJupiter in cooling will probably become a body somewhat like the\\nEarth. The Earth in cooling will probably become a body somewhat\\nlike the Moon. The Moon has already reached its permanent state.\\nIts heat has gone; it has no atmosphere; and its temperature on the\\nside turned away from the Sun is the temperature of space hundreds\\nof degrees below zero Fahrenheit.\\nThe temperature of any planet in the system thus depends, in an\\nimportant degree, on its age. It depends also on a thousand other\\ncircumstances\u00e2\u0080\u0094 on the kind of matter of which it is made up, on its\\nsize, etc. When we come to consider the Nebular Hypothesis of\\nKant and Laplace, which is an attempt to explain the evolution of\\nthe solar system, these facts (and others not here explicitly set down)\\nwill be found to be highly significant.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0374.jp2"}, "373": {"fulltext": "CHAPTEE XX.\\nMETEORS.\\n39. Phenomena of Meteors and Shooting-stars. Any\\none who watches the heavens at night for a few hours will\\nsee shooting-stars or meteors. They suddenly appear as\\nbright points of light, move along an arc in the sky and\\nthen disappear. Large meteors aerolites are often as\\nbright as Venus or even very much brighter; they are\\nusually followed by brilliant trains they frequently explode\\nin the air, like rockets, and leave clouds of meteoric dust\\nbehind them. Sometimes their bursting or their passage\\nthrough the atmosphere is accompanied by an audible noise.\\nOccasionally fragments of the aerolite fall to the Earth.\\nLarge collections of such fragments are preserved in our\\nmuseums, and some of the specimens weigh hundreds of\\npounds. Usually, however, they are much smaller.\\nMost of the specimens of aerolites aie stones; some of\\nthem are nearly pure iron alloyed with nickel, etc.\\nWhen we consider that the aerolites come from regions\\nbeyond the Earth and that they never had any direct con-\\nnection with it before their fall on its surface, it is a highly\\nsignificant fact that they contain no chemical elements not\\nfound on the Earth. It indicates that all the bodies of the\\nsolar system are similar in constitution. Moreover, of the\\nseventy or more elements known to us more than twenty\\nhave been found in meteoric masses. The minerals formed\\nby the combination of the elements are often somewhat dif-\\nferent in the aerolites from the corresponding minerals\\nfound in the Earth s crust, which seems to show that they\\n347", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0375.jp2"}, "374": {"fulltext": "348\\nASTRONOMY.\\nIta. 189. -THE GREAT CAI.IFOBWA METEOR OF\\n1894.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0376.jp2"}, "375": {"fulltext": "METEORS. 349\\nwere combined under quite different conditions of heat,\\npressure, etc. An aerolite is a little planet out of the\\ncelestial spaces, evident to our sight, it may be to our\\ntouch.\\nPath of a Meteor. The positions of a meteor can be observed\\nby referring it to neighboring stars we can draw its path on a\\nstar- map, and note the time of its appearance or bursting. If such\\nobservations are made by observers at different stations on the\\nEarth, the orbit of the meteor can be calculated. It is found that\\nmost aerolites, or large meteors, were moving in elliptic orbits about\\nthe Sun before they fell into the sphere of the Earth s attraction.\\nThe Earth, of course, alters such an orbit, and draws the body down-\\nwards into the atmosphere with a high velocity. In most cases it is\\nconsumed burned up completely in our atmosphere. Occasionally\\npieces of it fall to the ground, as has been said.\\nCause of the Light and Heat of Meteors. Why do meteors burn\\nwith so great an evolution of light on reaching our atmosphere\\nTo answer this question we must have recourse to the mechanical\\ntheory of heat. Heat is a vibratory motion in the particles of solid\\nbodies and a progressive motion in those of gases. The more rapid\\nthe motion the warmer the body. By simply blowing air against\\nany combustible body with high velocity it can be set on fire, and, if\\nthe body is incombustible, it can be made red-hot and finally melted.\\nExperiments show that a velocity of about 50 metres (about 164\\nfeet) per second corresponds to a rise of temperature of one degree\\nCentigrade. From this the temperature due to any velocity can be\\ncalculated on the principle that the increase of temperature is pro-\\nportional to the energy of the particles, which again is propor-\\ntional to the square of the velocity. A velocity of 500 metres (about\\n1640 feet) per second corresponds to a rise of 100\u00c2\u00b0 C. above the actual\\ntemperature of the air, so that if the latter was at the freezing-point\\nthe body would be raised to the temperature of boiling water. A\\nvelocity of 1500 metres (4921 feet, about twice the velocity of a\\ncannon-ball) per second would produce a red heat.\\nThe Earth moves around the Sun with a velocity of about 30,000\\nmetres (18\u00c2\u00a3 miles) per second; consequently if it met a body at rest\\nthe concussion between the latter and the atmosphere would corre-\\nspond to a temperature of more than 300,000\u00c2\u00b0. This would instantly\\nchange any known substance from a solid to a gaseous form.\\nIt must be remembered that these enormous temperatures are\\npotential, not actual, temperatures. The body is not actually raised", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0377.jp2"}, "376": {"fulltext": "350 ASTRONOMY.\\nto a temperature of 300,000\u00c2\u00b0, but the air acts upon it as if it were\\nsuddenly plunged into a furnace heated to this temperature. It is\\nrapidly destroyed just as if it were in such a furnace.\\nThe potential temperature is independent of the density of the\\nmedium, being the same in the rarest as in the densest atmosphere.\\nBut the actual effect on the body is not so great in a rare as in a\\ndense atmosphere. Every one knows that he can hold his hand for\\nsome time in air at the temperature of boiling water. The rarer the\\nair the higher the temperature the hand would bear without injury.\\nIn an atmosphere as rare as ours at the height of 50 miles, it is prob-\\nable that the hand could be held for an indefinite period, though its\\ntemperature should be that of red-hot iron hence the meteor is not\\nconsumed so rapidly as if it struck a dense atmosphere with a like\\nvelocity. In the latter case it would probably disappear like a flash\\nof lightning.\\nThe amount of beat evolved is measured not by that which would\\nresult from the combustion of the body, but by the vis viva (energy\\nof motion) which the body loses in the atmosphere. The student of\\nphysics knows that motion, when lost, is changed into a definite\\namount of heat.\\nThe amount of heat which is equivalent to the energy of motion\\nof a pebble having a velocity of 20 miles a second is sufficient to\\nraise about 1300 times the pebble s weight of water from the freezing\\nto the boiling point. This is many times as much heat as could\\nresult from burning pure carbon.\\nMeteoric Phenomena. Meteoric phenomena depend upon the sub.\\nstance out of which the meteors are made and the velocity with\\nwhich they move in the atmosphere. With very rare exceptions,\\nthey are so small and fusible as to be entirely dissipated in the\\nupper regions of the air. On rare occasions the body is so hard and\\nmassive as to reach the Earth without being entirely consumed.\\nThe potential heat produced by its passage through the atmosphere\\nis expended in melting and destroying its outer layers, the inner\\nnucleus remaining unchanged. When a meteor first strikes the\\ndenser portion of the atmosphere, the resistance becomes so great\\nthat the body is generally broken to pieces. A single large aerolite\\nmay produce a shower of small meteoric stones.\\nHeights of Meteors. Many observations have been made to deter-\\nmine the height at which meteors are seen. This is effected by two\\nobservers stationing themselves several miles apart and mapping out\\nthe courses of such meteors as they can observe.\\nMeteors and shooting-stars commonly commence to be visible at a", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0378.jp2"}, "377": {"fulltext": "METEORS. 351\\nheight of about 70 statute miles. The separate results vary widely,\\nbut this is a rough average. They are generally dissipated at about\\nhalf this height, and therefore above the highest atmosphere which\\nreflects the rays of the Sun. The Earth s atmosphere must, then,\\nextend at least as high as 70 miles.\\nWhile there are few aerolites or large meteors, there are\\nmillions of the smaller sort shooting -stars. A single\\nobserver will see, on the average, from four to eight every\\nhour. If the whole sky is watched at any one place on the\\nEarth from 30 to 60 are visible every hour. They fill\\nspace like particles of dust, only these particles of the\\ndust of space are, on the average, about 200 miles apart.\\nThe Earth sweeps along in its orbit at the rate of 18\u00c2\u00a3 miles\\nper second and in its daily journey of some 1,600,000 miles\\nit meets, or is overtaken by millions of these bodies. From\\n10 to 15 millions of meteors fall into the Earth s atmos-\\nphere every day. The mass of the single meteors is ex-\\ntremely small several thousands of them being required\\nto make up a pound s weight. If each meteor has a mass\\nof one grain the Earth is growing heavier daily by about a\\nton. Theoretically the Earth is daily receiving heat by the\\nfall of meteorites, also; but calculation shows that the Sun\\nsends us ten times as much heat in a second as is received\\nfrom meteors in a year; so that there is no noteworthy\\neffect from this cause.\\nMeteoric Showers. Shooting-stars may be seen by a\\ncareful observer on almost any clear night. In general,\\nnot more than half-a-dozen will be seen in an hour, and\\nthese are usually so minute as hardly to attract notice.\\nBut they sometimes fall in great numbers as a meteoric\\nshower. On rare occasions the shower has been so striking\\nas to fill the beholders with terror. Ancient and mediaeval\\nrecords contain many accounts of such phenomena.\\nOne shower of this class occurs at an interval of about a\\nthird of a century. It was observed by Humboldt, on the", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0379.jp2"}, "378": {"fulltext": "352 ASTRONOMY.\\nAndes, on the night of November 12, 1799, for instance,\\nand often before that time. A great shower was seen in\\nthis country in 1833. On the night of November 13, 1866,\\na remarkable shower was seen in Europe, while on the\\ncorresponding night of the year following it was again seen\\nin this country, and, in fact, was repeated for two or three\\nyears, gradually dying away, as it were. This great shower\\nwill appear in 1899, once more.\\nThe occurrence of a shower of meteors evidently shows\\nthat the Earth encounters a swarm of such bodies movirjg\\ntogether in space. The recurrence at the same time of the\\nyear (when the Earth is in the same point of its orbit)\\nshows that the Earth meets the swarm at the same point in\\nspace in successive years. All the meteors of the swarm\\nmust be moving in the same direction in space or else they\\nwould soon be widely scattered.\\nRadiant Point. Suppose that, during a meteoric shower, we mark\\nthe path of each meteor on a star-map, as in figure 190. If we con-\\ntinue the observed paths backward in a straight line, we shall find\\nthat they all meet near one and the same point of the celestial sphere;\\nthat is, they move as if they all radiated from this point. The latter\\nis, therefore, called the radiant point. In the figure the lines do not\\nall pass accurately through the same point owing to the unavoidable\\nerrors made in marking out the path.\\nIt is found that the radiant point is always in the same position\\namong the stars, wherever the observer may be situated, and that,\\nas the stars apparently move toward the west, the radiant point moves\\nwith them.\\nThe existence of a radiant point proves that the meteors that strike\\nthe Earth during a shower are all moving in the same direction.\\nTheir motions will all be parallel hence when the bodies strike our\\natmosphere the paths described by them in their passage will all be\\nparallel straight lines. A straight line in space seen by an observer\\nis projected as a great circle of the celestial sphere, with the\\nobserver at its centre. If we draw a line from the observer parallel\\nto the paths of the meteors, the direction of that line intersects the\\ncelestial sphere in a point through which all the meteor-paths will\\nseem to pass.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0380.jp2"}, "379": {"fulltext": "METEORS.\\n353\\nOrbits of Showers of Meteors\u00e2\u0080\u0094 The position of the radiant point in-\\ndicates the direction in which the meteors move relatively to the\\nFig. 190.\u00e2\u0080\u0094 The RADIANT POINT of a Meteoric Shower.\\nEarth. If we also knew the velocity with which they are really mov-\\ning in space, we could make allowance for the motion of the Earth,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0381.jp2"}, "380": {"fulltext": "354: ASTRONOMY.\\nand thus determine the direction of their actual motion in space, and\\ndetermine the orbit of the swarm around the Sun.\\nThe radiant point of the shower of August 10 (Perseids) is R. A.\\n3 h 4 m Decl. -f- 57\u00c2\u00b0 of the shower of November 13 {Leonids) R. A. 10 u\\nra Decl. -f- 23\u00c2\u00b0 of the shower of November 26 (Andromedes) R.A.\\nl h 41 m Decl. 43\u00c2\u00b0. The student should observe these showers.\\nRelations of Meteors and Comets. The velocity of the\\nmeteors in space does not admit of being determined from\\nobservation of the meteors themselves. It is necessary to\\ndetermine their velocity in the orbit from the periodic-time\\nof the swarm about the Snn. The orbit of the swarm\\ngiving the 33-year shower was calculated shortly after the\\ngreat shower of 1866 with the results that follow:\\nPeriod of revolution 33.25 years\\nEccentricity of orbit 0.9044\\nLeast distance from the sun. 0.9890\\nInclination of orbit. 165\u00c2\u00b0 19\\nLongitude of the node 51\u00c2\u00b0 18\\nPosition of the perihelion (near the] node)\\nThe orbit of the meteor-swarm presents an extraordinary\\nlikeness to the orbit of a periodic comet discovered by\\nTempel. The elements of the comet s orbit are:\\nPeriod of revolution 33.18 years.\\nEccentricity of orbit 0.9054\\nLeast distance from the sun 0.9765\\nInclination of orbit 162\u00c2\u00b0 42\\nLongitude of the node 51\u00c2\u00b0 26\\nLongitude of the perihelion 42\u00c2\u00b0 24\\nIf the two orbits are compared, the result is evident.\\nThe swarm of meteors which causes the November showers\\nmoves in the same orbit with Tempel s comet.\\nThe comet passed its perihelion in January, 1866. The\\nshower was not visible until the following November.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0382.jp2"}, "381": {"fulltext": "METEORS. 355\\nTherefore, the swarm which produced the showers followed\\nafter Tempel s comet, moving in the same orbit with it.\\nThe recurrence of the phenomenon every 33 years was\\ntraced backward in historical records and it was shown that\\nfor centuries this swarm had been revolving about the Sun.\\nThe swarm is stretched out in a long mass and the Earth\\ncrosses the orbit in November of every year. The Earth\\nfinds the swarm in its path every 33 years. The radiant\\npoint of the November shower is in the constellation Leo\\nand hence these meteors are called Leonids. The August\\nmeteors radiate from Perseus and are called Perseids. The\\nrelation between comets and meteors suggested the question\\nwhether a similar connection might not be found between\\nother comets and other meteoric showers.\\nOther Showers of Meteors. Although the November showers (which\\noccur about November 14) are the only ones so brilliant as to strike\\nthe ordinary eve, it has long been known that there are other nights\\nof the year (notably August 10) in which more shooting-stars than\\nusual are seen, and in which the large majority radiate from one\\npoint of the heavens. They also arise from swarms of meteoroids\\nmoving together around the Sun.\\nThe honor of the discovery of this remarkable and unexpected\\nrelation between meteors and comets is shared between several\\nastronomers. Professors Olmsted and Twining of Yale College\\nwere the first to show that meteors w r ere extra-terrestrial bodies re-\\nvolving in swarms about the Sun. Professors Erman of Germany,\\nLe Verrier of France, Adams of England, Schiaparelli of Italy\\nand particularly Professor Newton of Yale College developed the\\nwhole subject.\\nMany meteor-swarms revolve in the same orbits with\\ncomets. In some cases the swarms follow the comet in a\\nmore or less compact mass. In others the meteors are\\nscattered all around the orbit. If a comet, originally, is\\nnothing but a close cluster of meteors it will partially break\\nup into its parts under the influence of planetary attractions\\n(perturbations) and especially at every one of its perihelion\\npassages. The longer a comet has been in the solar system", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0383.jp2"}, "382": {"fulltext": "356 ASTRONOMY.\\nthe more the meteors will be spread out along its orbit.\\nBat it is by no means certain that comets are, in the first\\nplace, only aggregations of meteors, so that it can only be\\nsaid that there is, certainly, a very close connection between\\nmeteors and comets, and that it is likely that certain\\nmeteor-swarms are no more than the debris of comets.\\nBeside the meteors known to be connected with comets\\nthere are millions upon millions of others scattered through\\nspace.\\nThe Zodiacal Light. If we observe the western sky during the\\nwinter or spring months, about the end of the evening twilight, we\\nshall see a stream of faint light, a little like the Milky Way, rising\\nobliquely from the west, and directed along the ecliptic toward a\\npoint southwest from the zenith. This is called the Zodiacal Light.\\nIt may also be seen in the east before daylight in the morning during\\nthe autumn months, and can be traced all the way across the heavens.\\nA brighter mass opposite to the Sun s place is called the Gegenschein.\\nThe Zodiacal Light is probably due to solar light reflected from an\\nextremely thin cloud either of meteors or of semi-gaseous matter like\\nthat composing the tail of a comet, spread all around the Sun inside\\nthe Earth s orbit. Its spectrum is probably that of reflected sunlight,\\na result which gives color to the theory that it arises from a cloud of\\nmeteors revolving round the Sun. The student should trace out the\\nZodiacal Light in the sky.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0384.jp2"}, "383": {"fulltext": "CHAPTER XXL\\nCOMETS.\\n40. Aspect of Comets. Comets are distinguished from\\nthe planets both by their aspects and their motions. Only\\na few comets belong permanently to the solar system (see\\nTable IV, p. 279). Most of them are mere visitors. They\\nenter the system, go round the Sun once, and then leave it\\nforever.\\nThe nucleus of a comet is, to the naked eye, a point of\\nlight resembling a star or planet. Viewed in a telescope,\\nit generally has a small disk, but shades off so gradually\\nthat it is difficult to estimate its magnitude. In large\\ncomets it is sometimes several hundred miles in diameter.\\nThe nucleus is always surrounded by a mass of foggy\\nlight, which is called the coma. To the naked eye the\\nnucleus and-coma together look like a star seen through a\\nmass of thin fog, which surrounds it with a sort of halo.\\nThe nucleus and coma together are generally called the\\nhead of the comet. The head of the great comet of 1858\\nwas 250,000 miles in diameter.\\nThe tail of the comet is a continuation of the coma,\\nextending out to a great distance, and usually directed\\naway from the Sun. It has the appearance of a stream of\\nmilky light, which grows fainter and broader as it recedes\\nfrom the head. The length of the tail varies from 2\u00c2\u00b0 or 3\u00c2\u00b0\\nto 90\u00c2\u00b0 or more. The tail of the great comet of 1858 was\\n45,000,000 miles in length and 10,000,000 miles in breadth.\\nAll that area was filled with matter sufficiently condensed\\nto send light to the Earth and to appear as a continuous\\n357", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0385.jp2"}, "384": {"fulltext": "358\\nASTRONOMY.\\nFig. 191.\u00e2\u0080\u0094 The Great Comet of 1858.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0386.jp2"}, "385": {"fulltext": "COMETS. 359\\nsheet. The mass of comets is extremely small, so small\\nthat no comet has yet been observed to produce perturba-\\ntions in the motion of any planet. It is to be remembered\\nthat we do not see the tail of a comet in its true shape, but\\nonly its projection on the celestial sphere, and it is further-\\nmore to be noted that the tail is not the debris of the comet\\nleft behind the comet in its motion. The tail of a comet\\nis behind the nucleus as the comet approaches the Sun, but\\nit precedes the nucleus as the comet moves away from the\\nSun. The vapors that arise from the nucleus, owing chiefly\\nto the Sun s heat, are repelled by the Sun driven away\\nfrom him probably by electric repulsion. The nucleus it-\\nself is always attracted and performs its revolution about\\nthe Sun in obedience to the attraction of gravitation.\\nFig. 192. Telescopic Comet Fig. 193. Telescopic Comet\\nwithout a Nucleus and with a Nucleus, but with-\\nwithout a Tail. out a Tail.\\nWhen large comets are studied with a telescope, it is\\nfound that they are subject to extraordinary changes. To\\nunderstand these changes, we must begin by saying that\\ncomets do not, like the planets, revolve around the Sun in\\nnearly circular orbits, but in orbits always so elongated that\\nthe comet is visible in only a very small part of its course\\n(see Figs. 195, 196, 197) namely, in that part of its orbit\\nnear the Sun (and Earth).", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0387.jp2"}, "386": {"fulltext": "\u00c2\u00a360 ASTRONOMY.\\nThe Vaporous Envelopes. If a comet is very small, it may undergo\\nno changes of aspect during its entire course. If it is an unusually\\nbright one, a bow surrounding the nucleus on the side toward the Sun\\nwill develop as the comet approaches the Sun. (a, Fig. 194.) This\\nbow will gradually rise and spread out on all sides, finally assuming\\nthe form of a semicircle having the nucleus in its centre, or, to\\nspeak with more precision, the form of a parabola having the nucleus\\nnear its focus. The two ends of this parabola will extend out further\\nand further so as to form a part of the tail, and finally be joined to it.\\nOther bows will successively form around the nucleus, all slowly\\nrising from it like clouds of vapor (Fig. 194).\\nFig. 194. Formation op Envelopes.\\nThese distinct vaporous masses are called the envelopes they\\nshade off gradually into the coma so as to be with difficulty distin-\\nguished from it. The appearances are apparently caused by masses\\nof vapor streaming up from that side of the nucleus nearest the Sun\\n(and therefore hottest) and gradually spreading around the comet on\\neach side as if repelled by the Sun. The form of the bow is, of\\ncourse, not the real form of the envelopes, but only the apparent one\\nin which we see them projected against the background of the sky.\\nPerhaps their forms can be best imagined by supposing the Sun\\nto be directly above the comet (see Fig. 194) and a fountain, throwing\\na vapor horizontally on all sides, to be built upon that part of the\\ncomet which is uppermost. Such a fountain would throw its vapor\\nin the form of a sheet, falling on all sides of the cometic nucleus,\\nbut not touching it. Two or three vapor surfaces of this kind are\\nsometimes seen around the comet, the outer one enclosing each of\\nthe inner ones, but no two touching each other.\\nThe tail also develops rapidly as the comet draws near to the Sun,\\nand sometimes several tails are developed. The principal tail is\\ndirected away from the Sun, as if under electric repulsion.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0388.jp2"}, "387": {"fulltext": "COMETS. 361\\nThe Constitution of Comets. To tell exactly what a comet is, we\\nshould be able to show how all the phenomena it presents would\\nfollow from the properties of matter, as we learn them at the surface\\nof the Earth. This, however, no one has been able to do, many of\\nthe phenomena being such as we should not expect from the known\\nconstitution of matter. All we can do, therefore, is to present the\\nprincipal characteristics of comets, as shown by observation, and to\\nexplain what is wanting to reconcile these characteristics with the\\nknown properties of matter.\\nIn the first place, all comets which have been examined with the\\nspectroscope show a spectrum which indicates that the comets are\\nprincipally made up of gases mostly compounds of carbon and\\nhydrogen. Sodium and several other substances are often found.\\nPart of the comet s light is undoubtedly reflected sunlight.\\nIt is, at first sight, difficult to comprehend how a mass of gas of\\nextreme tenuity can move in a fixed orbit just as if it were a solid\\nplanetary mass. The difficulty vanishes when we remember that the\\nspaces in which comets move are practically empty as empty as\\nthe vacuum of an air-pump. In such a vacuum a feather falls as\\nfreely and as rapidly as a block of metal.\\nThe Orbits of Comets.\u00e2\u0080\u0094 Previous to the time of Newton only bright\\ncomets had been observed and nothing was known of their actual mo-\\ntions, except that no one of them moved around the Sun in an ellipse\\nas the planets moved. Newton found that a body moving under the\\nattraction of the Sun might move in anyone of the three conic\\nsections, the ellipse, parabola, or hyperbola. Bodies moving in an\\nellipse, as the planets, complete their orbits at regular intervals of\\ntime over and over again. A body moving in a parabola or an hyper-\\nbola never returns to the Sun after once passing it, but moves away\\nfrom it forever. Most comets move in parabolic orbits, and therefore\\na proach the Sun but once during their whole existence (Fig. 195).\\nA few comets revolve around the Sun in elliptic orbits, which differ\\nfrom those of the planets only in being much more eccentric. (See\\np. 279.) But nearly all comets move about the Sun in orbits which\\nwe are unable to distinguish from parabolas, though it is possible\\nthat some of them may be extremely elongated ellipses. It is note-\\nworthy that the orbits of comets are inclined at all angles to the\\necliptic and that their directions of motion are often retrograde. In\\nthese respects they differ widely from the planets.\\nIn the last chapter it was shown that swarms of minute particles,\\nsmall meteors, accompany certain comets in their orbits. This is\\nprobably true of all comets. We can only regard such meteors as", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0389.jp2"}, "388": {"fulltext": "362 ASTRONOMY.\\nfragments or debris of the comet. On this theory a telescopic comet\\nwhich has no nucleus is simply a cloud of these minute bodies. Per-\\nhaps each one of the minute particles has a little envelope of gases\\nabout it. The nucleus of the brighter comets may either be a more\\ncondensed mass of such bodies or it may be a solid or liquid body\\nitself.\\nIf the student has difficulty in reconciling this theory of detached\\nparticles with the view already presented, that the envelopes from\\nwhich the tail of the comet is formed consists of layers of vapor, he\\nmust remember that vaporous masses, such as clouds, fog, and\\nFig. 195 Elliptic and Parabolic Orbits.\\nsmoke, are in fact composed of minute and separate particles of water,\\ncarbon and so forth.\\nThe gases shut up in the cavities of meteoric stones have been\\nspectroscopieally examined, and they show the characteristic comet\\nspectrum. This gives a new proof of the connection between comets\\nand meteors.\\nFormation of the Comet s Tail. The tail of the comet is not a per-\\nmanent appendage, but is composed of masses of vapor which ascend\\nfrom the nucleus, and afterwards move away from the Sun. The", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0390.jp2"}, "389": {"fulltext": "COMETS. 363\\ntail which we see on one evening is not absolutely the same we saw\\nthe evening before. A portion of the latter has been dissipated,\\nwhile new matter has taken its place, as with the stream of smoke\\nfrom a steamship. It is an observed fact that the vapor which rises\\nfrom the nucleus of a comet is repelled by the Sun instead of being\\nattracted toward it, as larger masses of matter are as indeed the\\nnucleus itself is.\\nNo adequate expl nation of this repulsive force has yet been given.\\nIt is probably electrical.\\nFig. 196. Orbtt of Halley s Comet.\\nPeriodic Comets. The first discovery of the periodicity of a comet\\nwas made by Halley in connection with the great comet of 1682.\\nThis comet moves in an immense elliptic orbit with a periodic time\\nof 76 years. Halley predicted that it would return in 1758. Clai-\\nratjt, a French astronomer, worked out its orbit by Newton s\\nmethods, and the comet returned, obedient to law, on Christmas\\nday, 1758. (See Fig. 196.)\\nGravitation was thus, for the first time, shown to rule the erratic\\nmotions of comets as well as the orderly revolutions of the planets.\\nThe figure shows the very eccentric orbit of Halley s comet and\\nthe nearly circular orbits of the four outer planets. It attained its\\ngreatest distance from the Sun, far beyond the orbit of Neptune,\\nabout the year 1873, and then commenced its return journey. The\\nfigure also shows the position of the comet in 1874. It will return\\nto perihelion again in the year 1910.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0391.jp2"}, "390": {"fulltext": "364\\nASTBONOMY.\\nOrbit of a Parabolic Comet. \u00e2\u0080\u0094Figure 197 shows the orbit of a comet\\ndiscovered by Perrine at the Lick Observatory on November 17,\\n1895. The places of the comet in its parabolic orbit are marked for\\nNovember 20 and subsequent dates. The places of the Earth in its\\norbit are marked for the same dates. Lines joining the correspond-\\n0\u00c2\u00abVv\\\\ o\\\\ Coma c 5 5. C S\u00c2\u00abxW)\\nFig. 197\u00e2\u0080\u0094 The Orbtt of Comet C. 1895, and the Orbit of\\nthe Earth, drawn to Scale. The Sun is at the Centre\\nof the Diagram.\\ning dates in the two orbits will show the direction in which the\\ncomet was seen from the Earth. A line shows the direction of the\\nVernal Equinox. The plane of the paper is the plane of the Eclip-\\ntic. All that part of the comet s orbit which is drawn full is north\\nof the Ecliptic; the dotted portion is south of it. The line of nodes", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0392.jp2"}, "391": {"fulltext": "COMETS. 365\\nof the comet s orbit is marked on the diagram. The comet was\\nnearest to ihe Sun (at perihelion) on December 18, when its dis-\\ntance was 0.19 (the Earth s distance 1.00). The positions of the\\ncomet were\\nv. 20\\nR. A. 208\u00c2\u00b0\\nDecl.\\n0\u00c2\u00b0\\n24\\n211\u00c2\u00b0\\n3\u00c2\u00b0\\n28\\n214\u00c2\u00b0\\n5\u00c2\u00b0\\n2\\n219\u00c2\u00b0\\n10\u00c2\u00b0\\n10\\n236\u00c2\u00b0\\n22\u00c2\u00b0\\n18\\n274\u00c2\u00b0\\n31\u00c2\u00b0\\n26\\n287\u00c2\u00b0\\n23\u00c2\u00b0\\nRemarkable Comets. In former years bright comets\\nwere objects of great dread. They were supposed to\\nFig. 198. Medal of the Gkeat Comet op 1680-81.\\npresage the fall of empires, the death of monarchs, the\\napproach of earthquakes, wars, pestilence, and every other\\ncalamity that could afflict mankind. In showing the entire\\ngroundlessness of such fears, science has rendered one of\\nits greatest benefits to mankind.\\nThe number of comets visible to the naked eye, so far as\\nrecorded, has generally ranged from twenty to forty in a\\ncentury. Only a few of these, however, have been so\\nbright as to excite universal notice.\\nIn 1456 the comet, afterwards known as Halley s,\\nappeared when the Turks were making war on Christen-\\ndom, and caused such terror that Pope Calixtus III", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0393.jp2"}, "392": {"fulltext": "366 ASTRONOMY.\\nordered prayers to be offered in the churches for protection\\nagainst; it. This is the origin of the popular fable that the\\nPope once excommunicated a comet.\\nComet of 1680.\u00e2\u0080\u0094 One of the most remarkable of the brilliant comets\\nis that of 1680. It inspired such terror that a medal was struck to\\nquiet popular apprehension. A free translation of the inscription is\\nThe star threatens evil things; trust only God will turn them\\nto good. This comet is especially remarkable in the history of\\nAstronomy because Newton calculated its orbit, and showed that it\\nmoved around the Sun obedient to the law of gravitation.\\nGreat Comet of 1811. It has a period of over 3000 years, and its\\naphelion distance is about 40,000,000,000 miles.\\nGreat Comet of 1843. \u00e2\u0080\u0094It was visible in full daylight close to the Sun.\\nAt perihelion it passed nearer the Sun than any other body has\\never been known to pass, the least distance being only about\\none fifth of the Sun s semidiameter. With a very slight change of\\nits original motion, it would have actually fallen into the Sun, and\\nbecome a part of it.\\nGreat Comet of 1858. It is frequently called Donati s comet from\\nthe name of its discoverer. It was visible for about nine months and\\nwas thoroughly studied by many astronomers, particularly by Bond at\\nHarvard College. At its greatest brilliancy its tail was 40\u00c2\u00b0 in length\\nand 10\u00c2\u00b0 in bread that its outer end, about 45,000,000 and 10,000,000\\nmiles in real (no perspective) dimensions. Its period is 1950 years.\\n(See Fig. 191.)\\nGreat Comet of 1882.\u00e2\u0080\u0094 It was visible in full daylight at its bright-\\nest, and it was seen with the telescope until it actually appeared to\\ntouch the Sun s disk. It passed across the face of the Sun (half a\\ndegree) in less than fifteen minutes, with the enormous velocity of\\nmore than 300 miles per second. Its least distance from the surface\\nof the Sun was less than 300,000 miles, so that it passed through the\\ndenser portions of the Sun s Corona.\\nThe orbit of this comet has been calculated from observations\\ntaken before its perihelion passage, and also from observations taken\\nafter it. If the Corona had had any effect on the comet s motion\\nthese two orbits would have differed but they do not differ they\\n*Tho student should notice the care which the author of the inscription has\\ntaken to make it consolatory, to make it rhyme, and to give implicitly the\\nyear of the comet by writing certain Roman numerals larger than the other\\nletters.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0394.jp2"}, "393": {"fulltext": "COMETS. 367\\nagree exactly. This shows of how rare substances the Corona is\\nmade up.\\nThe periodic time of this comet is about 840 years and its orbit\\nis the same curve in space as the orbits of the comets of 1668, 1843\\nand 1880 and 1887. But the comets themselves are different bodies.\\nThe comet of 1882 and that of 1880 cannot possibly be the same,\\nbody. They travel in the same path, however, and belong to the\\nsame family of comets.\\nObservations of comets made at the Lick Observatory and elsewhere\\nhave shown that comets sometimes break up into fragments which\\nthereafter travel in similar paths one behind the other. Pho-\\ntographs of comets sometimes actually show the formation of com-\\npanion comets left behind or rejected by the main comet. From\\nthese photographs it appears that the head of a comet sends out\\nenormous quantities of matter to form the tail, so that the material\\nthat forms it on one day may not be and probably is not the same\\nmaterial that formed the tail of a few days previous. The observa-\\ntions and photographs referred to have opened a new field for investi-\\ngation, and it is likely that very many important questions as to the\\nconstitution of comets will be settled when the next bright comet\\nappears.\\nEncke s Comet and the Resisting Medium.\u00e2\u0080\u0094 The period of this\\ncomet is between three and four years. Viewed with a telescope, it\\nappears simply as a mass of foggy light. Under the most favorable\\ncircumstances, it is just visible to the naked eye. The circumstance\\nthat has lent most interest to this comet is that observations ex-\\ntended over many years indicate that it is gradually approaching the\\nSun.\\nEncke attributed this change in its orbit to the existence in space\\nof a resisting medium, so rare as to have no appreciable effect upon\\nthe motion of the planets, and felt only by bodies of extreme tenuity,\\nlike the telescopic comets. The approach of the comet to the Sun is\\nshown by a gradual diminution of the period of revolution.\\nIf the change in the period of this comet were actually due to the\\ncauses which Encke supposed, then other faint comets of the same\\nkind ought to be subject to a similar influence. But the investiga-\\ntions which have been made in recent times on these bodies show no\\ndeviations of the kind. It might, therefore, be concluded that the\\nchange in the period of Encke s comet must be due to some other\\ncause. There is, however, one circumstance which leaves us in\\ndoubt.\\nEncke s comet passes nearer the Sun than any other comet of", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0395.jp2"}, "394": {"fulltext": "368 ASTRONOMY.\\nshort period which has been observed with sufficient care to decide\\nthe question. It may, therefore, be supposed that the resisting\\nmedium, whatever it may be, is densest near the Sun, and does not\\nextend out far enough for the other comets to meet it. The question\\nis one very difficult to settle. The fact is that all comets exhibit\\nslight anomalies in their motions which prevent us from deducing\\nconclusions from them with the same certainty that we should from\\nthose of solid bodies like the planets. One of the chief difficulties in\\ninvestigating the orbits of comets with all rigor is due to the difficulty\\nof obtaining accurate positions of the centre of so ill-defined an object\\nas the nucleus.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0396.jp2"}, "395": {"fulltext": "PART III\\nTHE UNIVERSE AT LARGE.\\nCHAPTER XXII.\\nINTRODUCTION.\\n41. Although the solar system comprises the bodies\\nwhich are most important to us who live on the Earth, yet\\nthey form only an insignificant part of creation. Besides\\nthe Earth, only seven of the bodies of the solar system are\\nplainly visible to the naked eye, whereas some 2000 or\\nmore stars can be seen on any clear night. Our Sun is\\nsimply one of these stars, and does not, so far as we know,\\ndiffer from its fellows in any essential characteristic. It is\\nrather less bright than the average of the nearer stars, and\\noverpowers them by its brilliancy only because it is so much\\nnearer to us.\\nThe distance of the stars from each other, and therefore\\nfrom the Sun, is immensely greater than any of the dis-\\ntances in the solar system. In fact, the nearest known star\\nis about seven thousand times as far from us as the planet\\nNeptune. If we suppose the orbit of this planet to be\\nrepresented by a child s hoop, the nearest star would be\\nthree or four miles away. We have no reason to suppose\\nthat contiguous stars are, on the average, any nearer\\ntogether than this, except in special cases where they are\\ncollected together in clusters.\\n369", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0397.jp2"}, "396": {"fulltext": "370 ASTRONOMY.\\nThe total number of the stars is estimated by millions,\\nand they are separated one from another by these wide\\nintervals. It follows that, in going from the Sun to the\\nnearest star, we are simply taking a single step in the\\nuniverse. The most distant stars are probably a thousand\\ntimes more distant than the nearest one, and we do not\\nknow what may lie beyond the distant stars.\\nThe planets, though millions of miles away, are compara-\\ntively near us, and form a little family by themselves.\\nThe planets are, so far as we can see, worlds not exceed-\\ningly different from the Earth on which we live, while\\nthe stars are suns, generally larger and brighter than our\\nown Sun. Each star may, for aught we know, have\\nplanets revolving around it, but their distance is so im-\\nmense that even the largest planets will forever remain in-\\nvisible with the most powerf al telescopes man can construct.\\nWe shall see in what follows that only a few stars are so\\nnear to us that their light can reach the Earth in 10, 20,\\nor even 50 years. The vast majority are so distant that\\nthe light which we now see left them a century ago, or\\nmore. If one of these were suddenly destroyed it would\\ncontinue to shine for years afterwards. The aspect of\\nthe sky at any moment does not then represent the present\\nstate of the stellar universe, but rather its past history.\\nThe Sun s light is already eight minutes old when it\\nreaches us; that of Neptune left the planet about four\\nhours before; the nearest fixed stars appear as they were\\nno less than four years ago while the Milky Way shines\\nwith a light which may have been centuries on its journey.\\nThe difference between the Earth and the Sun is almost\\nentirely due to a difference in their temperature. Nearly\\nevery element in the Earth is present in the Sun. If the\\nEarth were to be suddenly raised to the Sun s temperature\\nit would become a miniature Sun; that is, a miniature star.\\nSome of the elements present in the Sun are found to be", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0398.jp2"}, "397": {"fulltext": "INTRODUCTION. 371\\nplentiful in other stars, in nebulae, and even in comets and\\nmeteors. All the bodies of the solar system appear to be,\\nin the main, of like constitution; and their wonderfully\\ndifferent physical conditions to be due, in the main, to\\ndifferences of temperature. The stars, likewise, are made\\nup of elements often the same as the elements we know on\\nthe Earth. The extraordinary diversity exhibited by the\\nbodies of the visible universe thus appears to be largely due\\nto differences in their temperature. The past and the\\nfuture of the Sun, the Earth, and the Moon can, therefore,\\nbe investigated by inquiring what temperatures these bodies\\nhave had in past times and what temperatures they are\\nlikely to have in the future.\\nGeneral Aspect of the Heavens. Constellations. When\\nwe view the heavens with the unassisted eye, the stars\\nappear to be scattered nearly at random ove*r the surface of\\nthe celestial vault. The only deviation from an entirely\\nrandom distribution which can be noticed is a certain\\napparent grouping of the brighter ones into constellations.\\nA few stars are comparatively much brighter than the rest,\\nand there is every gradation of brilliancy, from that of\\nthe brightest to those which are barely visible. We also\\nnotice at a glance that the fainter stars far outnumber the\\nbright ones; so that if we divide the stars into classes\\naccording to their brilliancy, the fainter classes will contain\\nthe most stars.\\nThere are in the whole celestial sphere about 6000 stars\\nvisible to the naked eye. Of these, however, we can never\\nsee more than a part at any one time, because one half of\\nthe sphere is always below the horizon. If we could see a\\nstar in the horizon as easily as in the zenith, one half of the\\nwhole number, or 3000, would be visible on any clear\\nnight. But stars near the horizon are seen through so\\ngreat a thickness of atmosphere as greatly to obscure their\\nlight; consequently only the brightest ones can there be", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0399.jp2"}, "398": {"fulltext": "372 ASTRONOMY.\\nseen. It is not likely that more than 2000 stars can ever\\nbe taken in at a single view by any ordinary eye. About\\n2000 other stars are so near the south pole that they never\\nrise in our latitudes. Hence out of the 6000 visible, only\\n4000 ever come within the range of our vision, unless we\\nmake a journey toward the equator.\\nThe Galaxy. The Galaxy, or Millcy Way, is a magnifi-\\ncent stream or belt of white milky light 10\u00c2\u00b0 or 15\u00c2\u00b0 iu\\nbreadth, extending obliquely around the celestial sphere.\\nDuring the spring months it nearly coincides with our\\nhorizon in the early evening, but it can be seen at all other\\ntimes of the year spanning the heavens like an arch. For\\na portion of its length it is split longitudinally into two\\nparts, which remain separate through many degrees, and\\nare finally united again. The student will obtain a better\\nidea of it by actual examination than from any description.\\nHe will see that its irregularities of form and lustre are\\nsuch that in some places it looks like a mass of brilliant\\nclouds (see Fig. 199).\\nWhen Galileo first directed his telescope to the heavens,\\nabout the year 1610, he perceived that the Milky Way was\\ncomposed of stars too faint to be individually seen by the\\nunaided eye. Huyghexs in 1656 resolved a large portion\\nof the Galaxy into stars, and concluded that it was com-\\nposed entirely of them. Kepler considered it to be a vast\\nring of stars surrounding the solar system, and remarked\\nthat the Sun must be situated near the centre of the ring.\\nThis view agrees very well with the one now received,\\nexcept that the stars which form the Milky Way, instead\\nof lying near to the solar system, as Kepler supposed, are\\nat distances so vast as to elude all our powers of imagina-\\ntion.\\nThe most recent researches have shown that the Milky\\nWay is a vast cluster of stars intermixed with nebulae, and\\nthat these stars and nebulae are, in all probability, physi-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0400.jp2"}, "399": {"fulltext": "INTRODUCTION.\\n373", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0401.jp2"}, "400": {"fulltext": "374 ASTRONOMY.\\ncally connected and not merely perspectively projected in\\nthe same part of the sky. A majority of its stars are of the\\nsame spectral type (like Sirius). Nearly all the gaseous\\nnebalse are in this region; and most of the stars with\\nbright-line spectra are here. We must then consider the\\nMilky Way as mainly a physical system, and only partly as\\na geometrical appearance.\\nLucid and Telescopic Stars. When we view the heavens with a\\ntelescope, we find that there are innumerable stars too small to be\\nseen by the naked eye. We may therefore divide the stars, with re-\\nspect to brightness, into two great classes.\\nLucid Stars are those which are visible without a telescope.\\nTelescopic Stars are those which are not so visible.\\nMagnitudes of the Stars. The stars were classified by Ptolemy\\ninto six orders of magnitude. The fourteen brightest visible in our\\nlatitudes were designated as of the first magnitude, while those barely\\nvisible to the naked eye were said to be of the sixth magnitude. This\\nclassification is entirely arbitrary, since there are no two stars of ab-\\nsolutely the same brightness. If all the stars were arranged in the\\norder of their actual brilliancy, we should find a regular gradation\\nfrom the brightest to the faintest, no two being precisely the same.\\nBetween the north pole and 35\u00c2\u00b0 south declination there are\\n14 stars of the first magnitude.\\n48\\nsecond\\n152\\nthird\\n313\\nfourth\\n854\\nfifth\\n974\\nsixth\\n5355 of the first six magnitudes.\\nOf these, however, nearly 2000 of the sixth magnitude are so faint\\nthat they can be seen only by an eye of extraordinary keenness.\\nMeasures of the light of the stars show that a star of the second\\nmagnitude is four tenths as bright as one of the first one of the third\\nis four tenths as bright as one of the second, and so on. The ratio\\n\u00e2\u0096\u00a03*3 is called the light-ratio.\\nThe Constellations and Names of the Stars. The\\nancients divided the stars into constellations, and gave", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0402.jp2"}, "401": {"fulltext": "INTRODUCTION. 375\\nspecial names to these groups and to many of the more\\nconspicuous stars also.\\nConsiderably more than 3000 years before the commencement of the\\nChristian chronology the star Sirius, the brightest in the heavens, was\\nknown to the Egyptians under the name of Sothis. The seven stars\\nof the Great Bear, so conspicuous in our northern sky, were known\\nunder that name to Homer (800 B.C.), as well as the group of the\\nPleiades, or Seven Stars, and the constellation of Orion. All the\\nearlier civilized nations, Egyptians, Chinese, Greeks, and Hindoos,\\nhad some arbitrary division of the surface of the heavens into irregu-\\nlar and often fantastic shapes, which were distinguished by names.\\nThe area within which the Sun and planets move the Zodiac was\\nprobably divided and named before the year 2000 B.C., and the 48\\nconstellations given by Ptolemy were probably formed at least as\\nearly as this time.\\nIn early times the names of heroes and animals were given to the\\nconstellations. Each figure was supposed to be painted on the sur-\\nface of the heavens, and the stars were designated by their position\\nupon some portion of the figure. The ancient and mediaeval astrono-\\nmers spoke of the bright star in the left foot of Orion the eye\\nof the Bull, the heart of the Lion, the head of Perseus, etc.\\nThese figures are still retained upon some star-charts, and are useful\\nwhere it is desired to compare the older descriptions of the constella-\\ntions with our modern maps. Otherwise they have ceased to serve\\nany really useful purpose, and are often omitted from maps designed\\nfor purely astronomical uses.\\nThe Arabians gave special names to a large number of the brighter\\nstars. Some of these names are in common use at the present time,\\nas Aldebaran, FomaUiaut, etc.\\nIn 1654 Bayer, of Germany, mapped the constellations and desig-\\nnated the brighter stars of each constellation by the letters of the\\nGreek alphabet. When this alphabet was exhausted he introduced\\nthe letters of the Roman alphabet. In general, the brightest star\\nwas designated by the first letter of the alphabet, a, the next by the\\nfollowing letter, ft, etc.\\nOn this system, a star is designated by a certain Greek letter, fol-\\nlowed by the genitive of the Latin name of the constellation to which\\nit belongs. For example a Ganis Majoris, or, in English, a of the\\nGreat Dog, is the designation of Sirius, the brightest star in the\\nheavens. The brightest stars of the Great Bear are called a Ursce\\nMajoris, ft Ursa Majoris, etc. Arcturus is a Bootis. The student", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0403.jp2"}, "402": {"fulltext": "376 ASTRONOMY.\\nwill here see a resemblance to our way of designating individuals by\\na Christian name followed by the family name. The Greek letters\\nfurnish the Christian names of the separate stars, while the name of\\nthe constellation is that of the family. As there are only fifty letters\\nin the two alphabets used by Bayer, only the fifty brightest stars in\\neach constellation could possibly be designated by this method.\\nAfter the telescope had fixed the position of many additional stars,\\nsome other method of denoting them became necessary. Flamsteed,\\nabout the year 1700, prepared an extensive catalogue of stars, in\\nwhich those of each constellation were designated by numbers in the\\norder of right-ascension. These numbers were entirely independent\\nof the designations of Bayer that is, he did not omit the Bayer\\nstars from his system of numbers, but numbered them as if they\\nhad no Greek letter. Hence those stars to which Bayer applied\\nletters have two designations, the number and the letter. The fainter\\nstars are designated nowadays either by their R.A. and Decl., or by\\ntheir numbers in some well-known catalogue of stars.\\nNumbering and Cataloguing the Stars. As telescopic power is in-\\ncreased, we still find fainter and fainter stars. But the number\\ncannot go on increasing forever in the same ratio as the brighter\\nmagnitudes, because, if it did, the whole night sky would be a blaze\\nof starlight, instead of a dark sphere dotted with brilliant points.\\nIf telescopes with powers far exceeding our present ones are made,\\nthey will, no doubt, show very many new stars. But it is highly\\nprobable that the number of such successive orders of stars would\\nnot increase in the same ratio as is observed in the 8th, 9th, and 10th\\nmagnitudes, for example.\\nIn special regions of the sky, which have been searchingly ex-\\namined by various telescopes of successively increasing apertures,\\nthe number of new stars found is by no means in proportion to the\\nincreased instrumental power. If this is found to be true elsewhere,\\nthe conclusion may be that, after all, the stellar system can be ex-\\nperimentally shown to be of finite extent, or to contain only a finite\\nnumber of stars, rather.\\nWe have already stated that in the whole sky an eye of average\\npower will see about 6000 stars. With a telescope this number is\\ngreatly increased, and the most powerful telescopes of modern times\\nwill probably show more than 100,000,000 stars.\\nIn Argelander s Durchmusterung of the stars of the northern\\nheavens there are recorded as belonging to the northern hemisphere\\n314,926 stars from the first to the 9.5 magnitude, so that there are\\nabout 600,000 in the whole heavens.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0404.jp2"}, "403": {"fulltext": "INTRODUCTION.\\n377\\nWe can readily compute the amount of light received by the Earth\\non a clear but moonless night from these stars. The brightness of\\nan average star of the first magnitude is 0.5 of that of a Lyrce. A\\nstar of the 2d magnitude will shine with a light expressed by\\n0.5 X 0.4 0.20, and so on. (See p. 374.)\\nThe total brightness of 10 1st magnitude stars is 5.0\\n\u00e2\u0080\u00a24\\n9. I\\n37\\n2d\\n128\\n3d\\n310\\n4th\\n1,016\\n5th\\n4,328\\n6th\\n13,593\\n7th\\n57,960\\n8th\\nSum 142.8\\nIt thus appears that from the stars to the 8th magnitude, inclusive,\\nwe receive 143 times as much light as from a Lyrce. a Lyrce has\\nbeen determined by Zollner to be about 44,000,000,000 times fainter\\nthan the Sun, so that the proportion of starlight to sunlight can be\\ncomputed. It also appears that the stars too faint to be individually\\nvisible to the naked eye are yet so numerous as to affect the general\\nbrightness of the sky more than the so-called lucid stars (1st to 6th\\nmagnitude). The sum of the last two numbers of the table is greater\\nthan the sum of all the others.\\nThe Star Maps printed in this book furnish a means\\nby which the constellations and principal stars can be\\nidentified by the student.\\nMaps of the stars down to the 14th or 15th magnitude\\nare now made by photography, using special telescopes and\\nlong exposures (two or three hours). Such complete maps\\nas this will throw a flood of light on the distribution and\\narrangement of the constituent stars of the Stellar Uni-\\nverse.\\nThe Stars are Suns. Spectroscopic observations prove\\nthat nearly all of the stars are suns, very like our own\\nSun. They are self-luminous and intensely hot. They\\nhave extensive atmospheres of incandescent gases and\\nmetallic vapors. The light from a whole class of stars is,", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0405.jp2"}, "404": {"fulltext": "378 ASTRONOMY.\\nso far as can be determined, precisely like sunlight in\\nquality. We may say in general that stars are suns.\\nThe light received from even the brightest star is a very small\\nquantity because even the nearest star is very distant. From Sirius,\\nthe brightest star in the sky, we receive of the light\\nreceived from the Sun. Let I be the light received from a star\\nat a distance D from us and L the light we should receive from this\\nstar if it were at the Sun s distance from us 1). Then\\nIn the case of Sirius, I as above, and D about\\n542,000 times the Sun s distance. Hence L l^ ^^n 42.\\n7,000,000,000\\nThat is, Sirius emits forty-two times as much light (and presumably\\nabout forty-two times as much heat) as the Sun. The Sun is a\\nsmall star, compared to Sirius. The pole-star, Polaris, emits about\\ntwo hundred times as much light as the Sun, while the light received\\nfrom it is insignificant compared to sunlight.\\nIf we compare stars with the Sun in this way we shall\\nsee that some of them emit several thousand times more\\nlight, while some emit perhaps yoVo P ar as m uch light.\\nThese are great differences, but they are not enormous.\\nThe masses of a few stars are known. It is found that\\nsome of these stars have masses perhaps a hundred times\\ngreater, while others have masses very much smaller, than\\nthe Sun s mass. Here again there are great differences,\\nbut the differences are not enormous. Our Sun is an\\naverage star, we may say.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0406.jp2"}, "405": {"fulltext": "CHAPTEE XXIII.\\nMOTIONS AND DISTANCES OF THE STARS.\\n42. Proper Motions. To the unaided vision, the fixed\\nstars appear to preserve the same relative position in the\\nheavens through many centuries, so that if the ancient\\nastronomers conld again see them, they coald detect only\\nthe slight changes in their arrangement. But the accurate\\nmeasurements of modern times show that there are slow\\nchanges in the positions of the brighter stars. Many of\\nthem have small motions on the celestial sphere. Their\\nright-ascensions and declinations change (slightly) from\\nyear to year, apparently with uniform velocity. The\\nchanges are called proper motions, since they are real\\nmotions peculiar to the star itself.\\nIn general, the proper motions even of the brightest stars\\nare only a fraction of a second of arc in a year, so that\\nthousands of years would be required for them to change\\ntheir place in any striking degree, and hundreds of thou-\\nsands to make a complete revolution around the celestial\\nsphere. The circumference of a sphere contains 1,296,000\\nProper Motion of the Sun. It is a priori evident that\\nstars, in general, must have proper motions, when once we\\nadmit the universality of gravitation. That any fixed star\\nshould be entirely at rest would require that the attractions\\non all sides of it should be exactly balanced. Any the\\nslightest change in the position of this star would break\\nup this balance, and thus, in general, it follows that stars\\nmust be in motion, since each of them cannot occupy such\\na critical position as has to be assumed.\\n379", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0407.jp2"}, "406": {"fulltext": "380 ASTRONOMY.\\nIf but one fixed star is in motion, all the rest are affected,\\nand we cannot donbt that every single star, our San in-\\ncluded, is in motion by amonuts which vary from small to\\ngreat. If the Sun alone has a motion, and all the other\\nstars are at rest, the consequence would be that all the fixed\\nstars would appear to be retreating en masse from that\\npoint in the sky toward which we were moving. Those\\nnearest us would move more rapidly, those more distant\\nless so. And in the same way, the stars from which the\\nsolar system was receding would seem to be approaching\\neach other.\\nIf the stars, instead of being quite at rest, as just sup-\\nposed, have motions proper to themselves, as they do, then\\nwe shall have a double complexity. They would still\\nappear to an observer in the solar system to have motions.\\nOne part of these motions would be truly proper to the\\nstars, and one part would be due to the advance of the\\nSun itself in space.\\nObservations of the positions of stars of their right-\\nascensions and declinations can show only the resultant\\nof these two motions. It is for reasoning to separate this\\nresultant into its two components. The first question is to\\ndetermine whether the results of observation indicate any\\nsolar motion at all. If there is none, the proper motions\\nof stars will be directed along all possible lines. If the Sun\\ndoes truly move in space along some line, then there will\\nbe a general agreement in the resultant motions of the stars\\nnear the ends of the line along which it moves, while those\\nat the sides, so to speak, will show comparatively less sys-\\ntematic effect. It is as if one were riding in the rear of a\\nrailway train and watching the rails over which it has just\\npassed. As we recede from any point, the rails at that\\npoint seem to come nearer and nearer together.\\nIf we were passing through a forest, we should see the\\ntrunks of the trees from which we were going apparently", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0408.jp2"}, "407": {"fulltext": "MOTIONS AND DISTANCES OF TEE STARS. 381\\ncome nearer and nearer together, while those on the sides\\nof us would remain at their constant distance, and those in\\nfront would grow further and further apart.\\nThese phenomena, that occur in a case where we are\\nsensible of our own motion, serve to show how we may de-\\nduce a motion, otherwise unknown, from the appearances\\nwhich are presented by the stars in space.\\nIn this way, acting upon suggestions which had been\\nthrown out previously to his own time, Sir William\\nHerschel demonstrated that the Sun, together with all its\\nsystem, was moving through space in an unknown and\\nmajestic orbit of its own. The centre round which this\\nmotion is directed cannot yet be assigned. We can only\\ndetermine the point in the heavens toward which our\\ncourse is directed the apex of solar motion.\\nA number of astronomers have since investigated this\\nmotion with a view of determining the exact point in the\\nheavens toward which the Sun is moving. Their results\\ndiffer slightly, but the points toward which the Sun is\\nmoving all fall in or near the constellation Hercules not far\\nfrom the bright star Alpha Lyrce (Vega). The amount of\\nthe motion is such that if the Sun were viewed at right\\nangles to the direction of motion from an average star of\\nthe first magnitude, it would appear to move about one\\nthird of a second per year.\\nSpectroscopic observations will give the direction and the\\namount of the solar motion in another and an independent\\nway (see Chapter XVII).\\nDistances of the Fixed Stars. The ancient astronomers\\nsupposed all the fixed stars to be situated at a short distance\\noutside of the orbit of the planet Saturn, then the outer-\\nmost known planet. The idea was prevalent that Nature\\nwould not waste space by leaving a great region beyond\\nSaturn entirely empty.\\nWhen Copernicus announced the theory that the Sun", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0409.jp2"}, "408": {"fulltext": "382\\nASTRONOMY.\\nwas at rest and the Earth in motion around it, the problem\\nof the distance of the stars acquired a new interest. It\\nwas evident that if the Earth described an annual orbit,\\nthen the stars would appear in the course of a year to oscil-\\nlate back and forth in corresponding orbits, unless they\\nwere so immensely distant that these oscillations were too\\nsmall to be seen.\\nThe apparent oscillation of Mars produced in this way\\nwas described p. 188 et seq. These oscillations were, in\\nfact, those which the ancients represented by the motion\\nof the planet around a small epicycle (see Fig. 124). But\\nFig. 200.\u00e2\u0080\u0094 The Theory of Parallax.\\nno such oscillation was detected in a fixed star until the\\nyear 1837; and this fact seemed to the astronomers of\\nGalileo s time to present an almost insuperable difficulty\\nin the reception of the Copernican system. As the instru-\\nments of observation were from time to time improved, this\\napparent annual oscillation of the stars was ardently sought\\nfor.\\nThe parallax of a planet (P in the figure) is the angle at\\nthe planet subtended by the Earth s radius (CS 4000\\nmiles). The annual parallax of a star (P) is the angle at", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0410.jp2"}, "409": {"fulltext": "MOTIONS AND DISTANCES OF THE STARS. 383\\nthe star subtended by the radius of the Earth s orbit\\n(CS 93,000,000 miles). See page 109. The annual\\nparallax of Saturn is about 6\u00c2\u00b0 and of Neptune it is about\\n2\u00c2\u00b0, and these are angles easily detected with the astronomi-\\ncal instruments of the ancients. It was very evident, with-\\nout telescopic observation, that the stars could not have a\\nparallax of one half a degree. A change of place of one\\nhalf a degree could be readily detected by the naked eye.\\nThey must therefore be at least twelve times as far as\\nSaturn if the Copernican system were true.\\nWhen the telescope was applied to measurement, a con-\\ntinually increasing accuracy was gained by the improve-\\nment of the instruments. Yet the parallax of the fixed\\nstars eluded measurement. Early in the present century\\nit became certain that even the brighter stars had not, in\\ngeneral, an annual parallax so great as 1 and thus it\\nbecame certain that they must lie at a greater distance than\\n200,000 times that which separates the Earth from the Sun\\n(see page 23). R 206,264\\nSuccess in actually measuring the parallax of the stars\\nwas at length obtained almost simultaneously by two\\nastronomers, Bessel of Konigsberg and Struve of Dorpat.\\nBessel selected 61 Cygni for observation, in August, 1837.\\nThe result of two or three years of observation was that\\nthis star had a parallax of about one third of a second.\\nThis would make its distance from the Sun nearly 600,000\\nastronomical units. The reality of this parallax has been\\nwell established by subsequent investigators, only it has\\nbeen shown to be a little larger, and therefore the star a\\nlittle nearer than Bessel supposed. The most probable\\nparallax is now found to be 0 .45, corresponding to a dis-\\ntance of about 400,000 radii of the Earth s orbit.\\nThe distances of the stars are frequently expressed by the\\ntime required for light to pass from them to our system.\\nThe velocity of light is, it will be remembered, about", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0411.jp2"}, "410": {"fulltext": "384\\nASTRONOMY.\\n300,000 kilometres per second, or such as to pass from the\\nSun to the Earth in 8 minutes 18 seconds.\\nThe cut shows the arrangement of some of the nearer\\nstars in space. They are shown on a plane, and not in\\nsolid space. The dot in the centre of the figure is the\\nsolar system. The circles of the figure stand for spheres,\\nFig 201.\\nwhose radii are 5, 10, 15, 20, 25, 30 light-years; that is,\\nfor spheres whose radii are of such lengths that light,\\nwhich moves 186,000 miles in a second requires 5, 10, etc.,\\nyears to traverse these radii.\\nThe time required for light to reach the Earth from a", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0412.jp2"}, "411": {"fulltext": "MOTIONS AND DISTANCES OF THE STARS. 385\\nfew of the stars, whose parallax has been measured, is as\\nfollows\\nStar.\\nYears.\\nStar.\\nYears.\\n4*\\n7\\n8\\n12\\nVega (tfLyrae)\\nAldebaran (a Tauri)\\nPolaris (a Ursae minoris).\\nArcturus xBootis)...\\n27\\n61 Cygni\\nSirius {a Canis majoris).\\nProcyon (a Canis minoris)\\n32\\n47\\n160\\nIf the star Polaris were to be suddenly destroyed now\\nthis instant its light would continue to shine for nearly\\nhalf a century more.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0413.jp2"}, "412": {"fulltext": "CHAPTER XXIV.\\nVARIABLE AND TEMPORARY STARS.\\n43. Stars Regularly Variable. Since the end of the\\nsixteenth century, it has been known that all stars do not\\nshine with a constant light. The period of a variable star\\nis the interval of time daring which it goes through all its\\nchanges, and returns to its original brilliancy.\\nThe most noted variable stars are Mira Ceti (o Ceti)\\n(star-map VI, in the southeast) and Algol (fi Per set) (star-\\nmap I, near the zenith). Mira is usually a ninth-magni-\\ntude star and is therefore invisible to the naked eye.\\nE\\\\rery eleven months it increases to its greatest brightness\\n(sometimes as high as the 2d magnitude, sometimes not\\nabove the 4th), remaining at this maximum for some time,\\nthen gradually decreases until it again becomes invisible to\\nthe naked eye, and so remains for about five or six months.\\nThe average period, from minimum to minimum, is about\\n333 days, but the period varies greatly. It has been known\\nas a variable since 1596.\\nAlgol has been known as a variable star since 1667.\\nThis star is commonly of the 2d magnitude; after remain-\\ning so about 2-J- days, it falls to 4th magnitude in the short\\ntime of 4^ hours, and remains of 4th magnitude for 20\\nminutes. It then increases in brilliancy, and in another\\n3| hours it is again of the 2d magnitude, at which point it\\nremains for the rest of its period, about 2 d 12 h\\nThese examples of two classes of variable stars give an\\nidea of the extraordinary nature of the phenomena they\\npresent.\\n386", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0414.jp2"}, "413": {"fulltext": "VARIABLE AND TEMPORARY STARS. 387\\nSeveral handred stars are known to be variable. A short\\nlist of variables is given in Table VII.\\nThe color of more than three fourths of the variable stars\\nis red or orange. It is a very remarkable fact that certain\\nstar-clusters contain large numbers of variable stars.\\nTemporary or New Stars. There are a few cases\\nknown of stars that have suddenly appeared, attained more\\nor less brightness, and slowly decreased in magnitude,\\neither disappearing totally, or finally remaining as compara-\\ntively faint objects. A new star that appeared in 134 B.C.\\nled Hipparchus to form his catalogue of stars.\\nThe most famous new star appeared in 1572, and attained\\na brightness greater than that of Jupiter. It was even\\nvisible to the eye in daylight. Tycho Brahe first observed\\nthis star in November, 1572, and watched its gradual\\nincrease in light until its maximum in December. It then\\nbegan to diminish in brightness, and in January, 1573, it\\nwas fainter than Jupiter. In February it was of the 1st\\nmagnitude, in April of the 2d, in Jnly of the 3d, and in\\nOctober of the 4th. It continued to diminish until March,\\n1574, when it became invisible to the naked eye.\\nThe history of temporary stars is, in general, similar to\\nthat of the star of 1572, except that none have attained so\\ngreat a degree of brilliancy. As more than a score of such\\nobjects are known to have appeared, many of them before\\nthe making of accurate observations, it is probable that\\nmany others have appeared without recognition. Among\\ntelescopic stars there is but a small chance of detecting a\\nnew or temporary star.\\nTheories to Account for Variable Stars. Two main\\nclasses of variable stars exist and two theories must be\\nmentioned here.\\nI. Stars in general, like the Sun, are subject to erup-\\ntions of glowing gas from their interior, and to the forma-\\ntion of dark spots on their surfaces. These eruptions and", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0415.jp2"}, "414": {"fulltext": "388 ASTRONOMY.\\nformations have in most cases a greater or less tendency to\\na regular period, like the period of a gigantic geyser.\\nIn the case of our San, the period is 11 years, but in the\\ncase of many of the stars it is much shorter. Ordinarily,\\nas in the case of the Sun and of a large majority of the\\nstars, the variations are too slight to affect the total quan-\\ntity of light to any noteworthy extent.\\nIn the case of the variable stars this spot-producing\\npower and the liability to eruptions are very much greater,\\nand we have changes of light sufficiently marked to be per-\\nceived by the eye.\\nThis theory explains why so large a proportion of the\\nvariable stars are red. It is well known that glowing\\nbodies emit a larger proportion of red rays, and a smaller\\nproportion of blue ones, the cooler they become. It is\\ntherefore probable that the red stars have the least heat.\\nThis being the case, spots are more easily produced on\\ntheir surfaces just as cooling iron is covered with a crust.\\nIf their outside surface is so cool as to become solid in\\ncertain regions, the glowing gases from the interior will\\nburst through with more violence than if the surrounding\\nshell were liquid or gaseous. The cause of the periodic\\nnature of these eruptions is probably similar to the cause\\nof the periodic outbursts of geysers.\\nII. There is, however, another class of variable stars\\nwhose variations are due to an entirely different cause;\\nAlgol is the best representative of the class. The extreme\\nregularity with which the light of this object fades away\\nand disappears suggests the possibility that a dark body\\nmay be revolving around it, partially eclipsing it at every\\nrevolution. The law of variation of its light is so different\\nfrom that of the light of most other variable stars as to sug-\\ngest a different cause. Most others are near their maximum\\nfor only a small part of their period, while Algol is at its\\nmaximum for nine tenths of it. Others are subject to", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0416.jp2"}, "415": {"fulltext": "VARIABLE AND TEMPORARY STARS. 389\\nnearly continuous changes, while the light of Algol remains\\nconstant during nine tenths of its period. Spectroscopic\\nobservations show that Algol (a bright body) is accompanied\\nby a dark satellite that revolves about it in an orbit which\\nis presented to us nearly edgewise. The satellite is about\\nas large in diameter as Algol and is about 3,000,000 miles\\ndistant from it. When the dark satellite is in front of\\nAlgol some of its light is cut off. When it is to one side,\\nAlgol shines with its full brightness, and we do not see the\\nsatellite because it is not self-luminous. Probably both\\nAlgol and its dark companion revolve about a third dark\\nstar. The diameter of Algol is about 1,000,000 miles. The\\ndiameter of the dark satellite is about 800,000 miles.\\nEach of these stars is about the size of our Sun. The\\nmass of both combined is about of the Sun s mass.\\nTheir density is therefore much less than that of water.\\nThey are like heavy spherical clouds.\\nDark Stars. The existence of dark stars is proved in several\\nways. Algol and other stars of its class are accompanied by non-\\nluminous satellites, as is shown by the phenomena of their variability.\\nSirius and Procyon are also so accompanied, as is demonstrated by\\nperiodic irregularities of their motion. There is no reason why\\nthere may not be as many dark stars as bright ones. A bright\\nstar is one that is (comparatively) young. Its heat is still so ardent\\nas to make it self-luminous. A dark star is one that has lost its heat\\nin the lapse of centuries probably thousands of centuries. In our\\nown solar system Jupiter was probably a self-luminous planet not so\\nvery many centuries ago. The Earth and other planets are dark,\\nbut still have some of their native heat. The moon is dark (i. e., not\\nself-luminous) and it is also cold.\\nWe must figure the stellar universe to ourselves as containing not\\nonly the stars that we see, but also as containing perhaps as many\\nmore that we shall never see, because they have lost the light and\\nheat that they (probably) once possessed. Most of the dark stars will\\nforever remain unknown to us, but occasionally we meet with cases\\nlike those of Algol or of Sirius, which make it certain that dark\\nstars exist. Their is reason to believe that their number is very\\nlarge.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0417.jp2"}, "416": {"fulltext": "CHAPTER XXV.\\nDOUBLE, MULTIPLE, AND BINARY STARS.\\n44. Double and Multiple Stars. When we examine the\\nheavens with telescopes, we find many cases in which two\\nor more stars are extremely close together, so as to form a\\npair, a triplet, or a group. It is evident that there are two\\nways to account for this appearance.\\n1. We may suppose that the stars happen to lie nearly\\nin the same straight line from the Earth, but have no con-\\nnection with each other. It is evident that in this case\\na pair of stars might appear doable, although one was\\nhundreds or thousands of times farther off than the other.\\nIt is, moreover, impossible, from mere inspection, to deter-\\nmine which is the farther off. (See Fig. 3, t, t, t).\\n2. We may suppose that the stars are really near\\ntogether, as they appear, and do, in fact, form a connected\\npair or group.\\nA couple of stars in the first case is said to be optically\\ndouble.\\nStars that are really physically connected are said to be\\nphysically double. Their physical connection can only be\\nproved by observations which show that the two stars are\\nrevolving about their common centre of gravity. There\\nare tens of thousands of stars in the sky that appear to be\\ndouble and hundreds that have already been proved to be\\nphysically connected.\\nThere are several cases of stars which appear double to the naked\\neye. e Lyrm is such a star and is an interesting object in a small\\ntelescope, from the fact that each of the two stars which compose it\\n390", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0418.jp2"}, "417": {"fulltext": "DOUBLE, MULTIPLE, AND BINARY STARS. 391\\nis itself double. This minute pair of points, capable of being distin-\\nguished as double only by the most perfect eye (without the tele-\\nscope), is really composed of two pairs\\nof stars wide apart, with a group of\\nsmaller stars between and around\\nthem. The figure shows the appear-\\nance in a telescope of considerable\\npower.\\nRevolutions of Double Stars\u00e2\u0080\u0094 Binary\\nSystems. It is evident that if stars\\nphysically double are subject to the\\nforce of gravitation, they must be\\nrevolving around each other, as the\\nEarth and planets revolve around the\\nSun, else they would be drawn together FlG 202.\u00e2\u0080\u0094 The Quadruple\\ni Star e Lyrje.\\nas a single star.\\nThe method of determining the period of revolution of a pair of\\nstars, A and B, is illustrated by the figure, whLh is supposed to rep-\\nresent the field of view of an inverting telescope pointed toward the\\nsouth. The arrow shows the\\ndirection of the apparent diur-\\nnal motion. The telescope is\\npointed so that the brighter star\\nis in the centre of the field.\\nThe angle of position of the\\nsmaller star (NAB) is measured\\nby means of a divided ciicle,\\nand their distance apart (AB) is\\nmeasured with the micrometer\\n(see page 141) at the same time.\\nIf, by measures of this sort,\\nextending through a series of\\nyears, the distance or position-\\nangle of a pair of stars is found\\nto change periodically, it shows\\nthat one star is revolving around\\nthe other. Such a pair is called\\na binary star or binary system.\\nThe only distinction that we\\ncan make between binary systems and ordinary double stars is\\nfounded on the presence or absence of this observed motion. It is\\nprobable that nearly all the very close double stars are really binary\\n1\\nKb\\n31\\nH B\\nKfluB\\nH 1\\nFig. 203.\u00e2\u0080\u0094 Position-angle of a\\nDouble Star.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0419.jp2"}, "418": {"fulltext": "892 ASTRONOMY.\\nsystems, but that many hundreds of years are required to perform\\na revolution in some instances, so that their motion has not yet been\\ndetected.\\nCertain pairs of binary stars whose components are entirely too\\nclose to be separable by the telescope have been discovered by the\\nspectroscope. If two stars, A and B, are binary, and therefore re-\\nvolving in orbits, they will sometimes be in this position to an ob-\\nserver on the Earth, thus\\nAB\\nI\\nEarth.\\nIf they are too close to be separated by the telescope, still the spec-\\ntrum of the pair will show the lines of both stars. That is, certain\\nof the spectrum lines will appear double. At other times one star\\nwill be behind the other, as seen from the Earth, thus\\nEarth.\\nand the spectrum lines will be seen single. If changes like these\\noccur periodically, as they do, then the orbit of one star about the\\nother can be calculated. In this way a number of spectroscopic\\nbinary stars has been found. The star Zeta Ursa Majoris (Mizar)\\n(see Fig. 95) is a binary of this class, whose period is about 52 days.\\nThe mass of this system is about 40 times the Sun s mass.\\nThe existence of binary systems shows that the law of gravitation\\nincludes the stars as well as the solar system in its scope, and thus\\nthat it is truly universal.\\nWhen the parallax of a binary star is known, as well as the orbit,\\nit is possible to compute the mass of the binary system in terms of\\nthe Sun s mass. It is an important fact that the stars of such binary\\nsystems as have been investigated do not differ very greatly in mass\\nfrom our Sun.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0420.jp2"}, "419": {"fulltext": "CHAPTER XXVI.\\nNEBULA AND CLUSTERS.\\n45. Nebulae. In the star-catalogues of Ptolemy and\\nthe earlier writers, there was included a class of nebulous\\nor cloudy stars, which were in reality star-clusters. They\\nwere visible to the naked eye as masses of soft diffused\\nlight like parts of the Milky Way. The telescope shows\\nthat most of these objects are clusters of stars.\\nAs the telescope was improved, great numbers of such\\npatches of light were found, some of which could be\\nresolved into stars, while others could not. The latter\\nwere called nebulm and the former star-clusters.\\nAbout 1656 Huyghens described the great nebula of\\nOrion, one of the most remarkable and brilliant of these\\nobjects. It is just visible to the naked eye as a cloudiness\\nabout the middle star of the sword of Orion (a line from\\nthe r of Orion in Fig. 204 to the r of Eridanus passes\\nthrough the nebula). The student should look for this\\nnebula with the eye on a clear winter s night. An opera-\\nglass will show the nebulosity distinctly; but a telescope is\\nneeded to show it well. Sir William Herschel with his\\ngreat telescopes first gave proof of the enormous number\\nof these masses. In 1786 he published a catalogue of one\\nthousand new nebulae and clusters. This was followed in\\n1789 by a catalogue of a second thousand, and in 1802 by\\na third catalogue of five hundred new objects of this class.\\nSir John Herschel added about two thousand more\\n393", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0421.jp2"}, "420": {"fulltext": "394\\nASTRONOMY.\\nnebulas. About nine thousand nebulas, mostly very faint,\\nare now known.\\nClassification of Nebulae and Clusters.\u00e2\u0080\u0094 In studying these objects, the\\nfirst question we meet is this Are all these bodies clusters of stars\\nr\\nTau,ru.s blades.\\nAh\\nGeirvLruv\\nOrion,\\n11 i 9\\nvi\\nm PI\\nEvidanus\\nm Wxm\\nFig.\\n204 \u00e2\u0080\u0094The Constellation Orion as Seen with the\\nNaked Eye.\\nwhich look diffused only because they are so distant that our tele-\\nscopes cannot distinguish the separate stars? or are some of them\\nin reality what they seem to be namely, diffused masses of matter\\nIn his early memoirs, Sir William Herschel took the first view.\\nHe considered the Milky Way as nothing but a congeries of stars, and\\nall nebulae seemed to be but stellar clusters, so distant as to cause the\\nindividual stars to disappear in a general milkiness or nebulosity.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0422.jp2"}, "421": {"fulltext": "NEBULA AND CLUSTERS. 395\\nIn 1791, however, he discovered a nebulous star (properly so called)\\n\u00e2\u0080\u0094that is, a star which was undoubtedly similar to the surrounding\\nstars, and which was encompassed by a halo of nebulous light. His\\nreasoning on this discovery is instructive.\\nHe says Supposing the nucleus and halo to be connected, we\\nmay, first, suppose the whole to be of stars, in which case either the\\nnucleus is enormously larger than other stars of its stellar magnitude,\\nFig. 205.\u00e2\u0080\u0094 Spfral Nebttla.\\nor the envelope is composed of stars indefinitely small or, second,\\nwe must admit that the star is involved in a shining fluid of a nature\\ntotally unknown to us.\\nThe shining fluid might exist independently of stars. The light\\nof this fluid is no kind of reflection from the star in the centre. If\\nthis matter is self-luminous, it seems more fit to produce a star by its\\ncondensation than to depend on the star for its existence.\\nThis was the first exact statement of the idea that, beside stars and", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0423.jp2"}, "422": {"fulltext": "396\\nASTRONOMY.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0424.jp2"}, "423": {"fulltext": "NEBULjE and CLUSTERS. 397\\nstar-clusters, we have in the universe a totally distinct series o^ ob-\\njects, probably much more simple in their constitution. Observations\\non the spectra of these bodies have entirely confirmed the conclusions\\nof Herschel. The spectroscope shows that the true nebulae are\\ngaseous.\\nNebulae and clusters are divided into classes. A planetary nebula\\nis circular or elliptic in shape, with a definite outline like a planet.\\nSpiral nebulae are those whose convolutions have a spiral shape. This\\nclass is quite numerous.\\nThe different kinds of nebulae and clusters will be better under-\\nstood lrom the cuts and descriptions which follow than by formal\\nFig. 207. The Moon Passing near the Pleiades.\\ndefinitions. It must be remembered that there is an almost infinite\\nvariety of such shapes. The real shape of the nebula in space ap-\\npears to us much changed by perspective.\\nVast areas of the sky are covered with faint nebulosity.\\nStar-clusters.\u00e2\u0080\u0094 The most noted of all the clusters is the Pleiades,\\nwhich may be seen during the winter months to the northwest of the\\nconstellation Taurus The average naked eye can easily distinguish\\nsix stars within it, but under favorable conditions ten, eleven, twelve,\\nor more stars can be counted. With the telescope, several hundred\\nstars are seen.\\nThe clusters represented in Figs. 208 and 209 are good examples of\\ntheir classes. The first is globular and contains several thousand\\nsmall stars. The second is a cluster of about 200 stars, of magni-\\ntudes varying from the ninth to the thirteenth and fourteenth, in\\nwhich the brighter stars are scattered.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0425.jp2"}, "424": {"fulltext": "398\\nASTRONOMY.\\nClusters are probably subject to central powers or forces. This\\nwas seen by Sir William Herschel in 1789. He says\\nNot only were round nebulae and clusters formed by central\\npowers, but likewise every cluster of stars or nebula that shows a\\ngradual condensation or increasing brightness toward a centre.\\nSpherical clusters are probably not more different in size among\\nthemselves than different individuals of plants of the same species.\\nAs it has been shown that the spherical figure of a cluster of stars is\\nowing to central powers, it follows that those clusters which, cceteris\\nparibus, are the most complete in this figure must have been the\\nlongest exposed to the action of these causes.\\nFig. 208.\u00e2\u0080\u0094 Globular Cluster.\\nThe maturity of a sidereal system may thus be judged from the\\ndisposition of the component parts.\\nThough we cannot see any individual nebula pass through all its\\nstages of life, we can select particular ones in each peculiar stage,\\nand thus obtain a single view of their entire course of development.\\nSpectra of Nebulae and Clusters. In 1864, five years after the in-\\nvention of the spectroscope, the examination of the spectra of the\\nnebulas by Sir William Huggins led to the discovery that while the\\nspectra of stars were invariably continuous and crossed with dark\\nlines similar to those of the solar spectrum, those of many nebulas\\nwere discontinuous, showing these bodies to be composed of glowing\\ngas. The nebulas have proper motions just as do the stars. The\\ngreat nebula of Orion is moving away from the Sun eleven miles\\nevery second.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0426.jp2"}, "425": {"fulltext": "NEBULM AND CLUSTERS. 399\\nThe spectrum of most clusters is continuous, indicating that the\\nindividual stars are truly stellar in their nature. In a few cases,\\nCompressed Cluster.\\nhowever, clusters are composed of a mixture of nebulosity (usually\\nnear their centre) and of stars, and the spectrum in such cases is\\ncompound in its nature, so as to indicate radiation from both gaseous\\nand solid matter.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0427.jp2"}, "426": {"fulltext": "CHAPTER XXVII.\\nSPECTRA OF FIXED STARS.*\\n46. Stellar spectra are found to be, in the main, similar\\nto the solar spectrum; i.e., composed of a continuous band\\nof the prismatic colors, across which dark lines or bands\\nare laid, the latter being fixed in position. These results\\nshow the fixed stars to resemble our own Sun in general\\nconstitution, and to be composed of an incandescent\\nnucleus surrounded by a gaseous and absorptive atmosphere\\nof lower temperature containing the vapors of metals,\\netc.\u00e2\u0080\u0094 iron, magnesium, hydrogen, etc. The atmosphere\\nof many stars is quite different in constitution from that of\\nthe Sun, as is shown by the different position and intensity\\nof the various dark lines that are due to the absorptive\\naction of the atmospheres of the stars.\\nDifferent Types of Stars. In a general way the spectra of all stars\\nare similar. All of tbem are bodies of the same general kind as the\\nSun. Yet there are characteristic differences between star and star,\\nand certain large groups into which stars can be classified certain\\ntypes of stellar spectra. It is probable that these different types rep-\\nresent different phases in the life-history of a star. Of two stars of\\nthe same size and general constitution the whitest is probably the\\nhottest and the youngest the reddest is probably the coolest and\\noldest. The hottest stars have the simplest spectra the red stars\\nhave complicated spectra and are often variable. The bright stars\\nof the constellation of Orion have spectra of the simplest type their\\natmospheres are mainly made up of helium and hydrogen gases.\\nStars like Sirius have little helium in their atmospheres, but much\\nSee Appendix.\\n400", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0428.jp2"}, "427": {"fulltext": "SPECTBA OF FIXED STARS. 401\\nhydrogen and a little calcium. Stars like Procyon have hydrogen\\nand calcium and magnesium in marked quantities, besides other me-\\ntallic lines. Stars like Arcturus are characterized by many metallic\\nlines in their spectra, such as those of iron. Our Sun belongs to\\nthis class. Stars with considerably less extensive hydrogen atmos-\\npheres and with considerably more metallic vapors surrounding\\nthem form the next class (like Alpha Orionis, Alpha Herculis and\\nthe variable star Mir a Cetis). The red stars, none of which are very\\nbright, and most of which are variable, form the last type.\\nIt appears that the stars can be arranged in classes corresponding\\nto diminishing temperatures. The hottest stars have extensive hy-\\ndrogen atmospheres, simple in constitution. They are analogous to\\nnebulae in many respects and probably are condensed from nebulous\\nmasses. As a star grows older and cooler its spectrum grows more\\nunlike a nebulous spectrum, more complex, more individual, so to\\nspeak. After passing through a stage like that of our Sun it\\nreaches the stage of pronounced variability, like the red stars, and\\nfinally becomes a dark star like the companion to Algol, for\\nexample.\\nStellar Evolution An irregular and widely extended nebula sub-\\nject to gravitating forces tends to become a spherical mass spherical\\nmasses of nebulosity subject to central powers tend to become more\\ncondensed and to form nuclei at their centres. It appears to be\\nlikely that such nebula? may condense still further into stars. Stars\\nvery hot and white go through a cycle of changes, and after losing\\nall their light and heat become dark stars. This is, in general,\\nthe final stage. If, however, two stellar systems moving through\\nspace should collide, all the bodies of both systems would be quickly\\nraised to very high temperatures, and in this way a dark star\\nmight be re-created and begin a new cycle of existence. If a dark\\nstar like the Earth, for example, were to be suddenly raised to a\\nvery high temperature it would become a gaseous body a miniature\\nSun, for example. It is probable that the phenomena of some of\\nthe new stars are to be explained in this way.\\nMotion of Stars in the Line of Sight.\u00e2\u0080\u0094 Spectroscopic\\nobservations of stars not only give information in regard to\\ntheir chemical and physical constitution, but have been\\napplied so as to determine approximately the velocity in\\nmiles per second with which the stars are approaching to\\nor receding from the Earth along the line joining Earth", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0429.jp2"}, "428": {"fulltext": "402 ASTRONOMY.\\nand star (the line of sight). The theory of such a de-\\ntermination is briefly as follows:\\nIn the solar spectrum we find a group of dark lines, as a, b, c,\\nwhich always maintain their relative position. From laboratory ex-\\nperiments, we can show that the three bright lines of incandescent\\nhydrogen (for example) have always the same relative position as the\\nsolar dark lines a, b, c. From this it is inferred that the solar dark\\nlines are due to the presence of hydrogen in the absorptive atmosphere\\nof the Sun.\\nNow, suppose that in a stellar spectrum we find three dark lines,\\na V c whose relative position is exactly the same as that of the\\nsolar lines a, b, c. Not only is their relative position the same, but\\nthe characters of the lines themselves, so far as the fainter spectrum\\nof the star will allow us to determine them, are also similar that is,\\na and a, b and b, c and c are alike as to thickness, blackness, nebu-\\nlosity of edges, etc., etc. From this it is inferred that the star con-\\ntains in its atmosphere the substance whose existence has been shown\\nin the Sun hydrogen, for example.\\nIf we contrive an apparatus by which the stellar spectrum is seen\\nin the lower half, say, of the eyepiece of the spectroscope, while the\\nspectrum of hydrogen is seen just above it, we find in some cases this\\nremarkable phenomenon. The three dark stellar lines, a b\\\\ c in-\\nstead of being exactly coincident with the three hydrogen lines a, b,\\nc, are seen to be all thrown to one side or the other by a like amount\\nthat is. the whole group a\\\\ b c while preserving its relative dis-\\ntances the same as those of the comparison group a, b, c, is shifted\\ntoward either the violet or red end of the spectrum by a small yet\\nmeasurable amount. Repeated experiments by different instruments\\nand observers always show a shifting in the same direction, and of\\nlike amount. The figure shows a shifting of the F line in the\\nspectrum of Sirius, compared with one fixed line of hydrogen. The\\nbright line of hydrogen is nearer to one side of the dark line in the\\nstellar spectrum than to the other.\\nThis displacement of the spectral lines is accounted for by a motion\\nof the star toward or from the Earth. It is shown in Physics that if\\nthe source of the light which gives the spectrum a b c is moving\\naway from the Earth, this group will be shifted toward the red end\\nof the spectrum if toward the Earth, then the whole group will be\\nshifted toward the blue end. The amount of this shifting depends\\nupon the velocity of recession or approach, and this velocity in miles\\nper second can be calculated from the measured displacement. This\\nhas already been done for many stars.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0430.jp2"}, "429": {"fulltext": "SPECTRA OF FIXED STARS.\\n403\\nThe principle upon which the calculation is made can\\nbe understood by an analogy drawn from the phenomena\\nof sound. Every one who has ridden in a railway train has\\nnoticed that the bell of a passing engine does not always give\\nout the same note. As the two trains approach the sound of\\nthe bell is pitched higher, and as they separate after passing\\nthe sound of the bell is lower. It is certain that the driver\\nof the passing engine always hears his bell give out one and\\nthe same note. The explanation of this phenomenon is as\\nfollows the bell of the passing engine gives out the note\\nFig. 210.\u00e2\u0080\u0094 F Line of Hydrogen Superposed on the Spectrum\\nov Sirius VR).\\nC (the middle C of the pianoforte) let us say. That is\\nit gives out 512 vibrations, sound-waves, in every second.\\nAny sonorous body giving out 512 waves per second makes\\nthe note C. If more than 512 sound-waves reach the ear\\nin a second the note is higher C jf for example. If fewer\\nthan 512 waves reach the ear in a second the note is lower\\nCo for example. The engineer hears 512 vibrations\\nevery second. The note of his bell is Cft. All the air\\naround him is filled with sound-waves of this frequency.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0431.jp2"}, "430": {"fulltext": "404 ASTRONOMY.\\nThe traveller approaching the bell hears the 512 vibrations\\ngiven out by the bell every second, and also other vibrations\\nthat his swiftly moving train meets the note of the bell to\\nhim is (?j( let us say, because his ear collects more than 512\\nvibrations every second. The traveller receding from the\\nbell hears fewer than 512 vibrations per second. Not all\\nof the waves given out by the bell can overtake him as he\\nmoves swiftly away the note of the bell is to him G\\\\ let\\nus say.\\nThe case is the same for light- waves. The F line of\\nhydrogen gives out in the laboratory a certain number of\\nwaves per second. If a star is at rest with respect to the\\nEarth just as many waves reach the observer s eye from the\\ninline of the star as reach it from the inline of a compari-\\nson-spectrum of hydrogen. Both sources of light are at\\nrest with respect to him. If he is moving swiftly towards\\nthe star his eye receives not only the waves sent out by the\\nstar, but also all those that he overtakes. If he is moving\\nswiftly away from the star his eye receives fewer waves than\\nthe star sends out because not all of them can overtake\\nhim. (It is as if the F$ of the star became F$ in one\\ncase, F\\\\) in the other.) A shifting of the star-line towards\\nthe violet end of the spectrum indicates an approach of the\\nEarth to the star; a shifting towards the red end indicates\\na recession. The velocity of the motion of approach or\\nrecession is proportional to the amount of the shifting. It\\nis by a principle of this kind that we can calculate from the\\nobserved shifting of lines in the stellar spectrum the\\nvelocity with which the Earth is approaching a star, or\\nreceding from it.\\nMotion of the Solar System in Space. If observation\\nshows that the Earth is approaching a star at the rate of\\n40 miles per second, we know that the Sun and all the\\nplanets must be moving towards that star, since the Earth\\nmoves in her orbit only 18 miles per second. By making", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0432.jp2"}, "431": {"fulltext": "SPECTRA OF FIXED STARS.\\n405\\nallowance for the Earth s motion, the exact velocity of the\\nSun towards the star can be calculated. The Sun carries\\nall his family all the planets with him as he moves\\nthrough space. Astronomers are now engaged in solving,\\nby spectroscopic means, the problem of how fast the solar\\nsystem is moving in space, and in what direction it is\\nmoving.\\nThe method employed is somewhat as follows A large\\nnumber of stars is spectroscopically observed and the ve-\\nlocity with which the Sun is approaching each separate\\nstar is accurately determined.\\nA\\n3*\\nc\\nD\\nFig. 211.\\nSuppose the observations to show that the Sun (O) is ap-\\nproaching the group of stars A with an average velocity of\\n12 miles per second; that it is receding from the group of\\nstars B (180\u00c2\u00b0 away from A opposite to A in the celestial\\nsphere) at the same velocity; then it follows that the Sun\\nwith the whole solar system is moving through space to-\\nwards A with a velocity of 12 miles per second.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0433.jp2"}, "432": {"fulltext": "406 ASTRONOMY.\\nSome of the stars of group A may be moving towards the\\nSun; some of them may be moving away from the Sun; if\\na great many stars are contained in the group their average\\nmotion with respect to the Sun will be zero: there is no\\nreason to suppose that stars in general have any tendency\\nto move towards our Sun or away from it. Groups of stars\\nat C and D and all around the celestial sphere are observed\\nin the same way, and the final result is made to depend on\\nall the observed velocities. Eesearches like this are in\\nprogress at Potsdam, Paris, at the Lick Observatory, and\\nelsewhere. Final conclusions have not yet been reached.\\nAll that can now be said is that the solar system is moving\\ntowards a point near to the bright star Alpha Lyrm with a\\nvelocity of about 12 miles per second. It will require\\nsome years yet to reach final values. So far as we know\\nthe solar motion is uniform and in a straight line.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0434.jp2"}, "433": {"fulltext": "CHAPTER XXVIII.\\nCOSMOGONY.\\n47. A theory of the operations by which the physical\\nuniverse received its present form and arrangement is called\\nCosmogony. This subject does not treat of the origin of\\nmatter, but only of its transformations.\\nThree systems of Cosmogony have prevailed at different\\ntimes:\\n(1) That the universe had no beginning, but existed from\\neternity in the form in which we now see it. This was the\\nview of the ancients.\\n(2) That it was created in its present shape in six days.\\nThis view is based on the literal sense of the words of the\\nOld Testament. Theological commentators have assumed\\nthat it was created out of nothing, but the Scripture\\ndoes not say so.\\n(3) That it came into its present form through an ar-\\nrangement of previously existing materials which were be-\\nfore without form and void. This maybe called the\\nevolution theory. No attempt is made to explain the ori-\\ngin of the primitive matter. The theory simply deals with\\nits arrangement and changes.\\nThe scientific discoveries of modern times show conclu-\\nsively that the universe could not always have existed in\\nits present form that there was a time when the materials\\ncomposing it were masses of glowing vapor, and that there\\nwill be a time when the present state of things will cease.\\nGeology proves beyond a doubt, that the arrangement of\\n407", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0435.jp2"}, "434": {"fulltext": "408 ASTRONOMY.\\nthe primitive matter to form a habitable Earth has required\\nmillions of years, and Anthropology proves also beyond a\\ndoubt, that the Earth has been inhabited by men for many\\nthousands. It was not until the latter half of the XVIII\\ncentury that such opinions could be held without fear of\\npersecution, for the lesson that a scientific fact is as sacred\\nas a moral principle has only been fully learned within\\nthe last half century.\\nAn explanation of the processes through which the Earth\\nand all the planets came into their present forms was first\\npropounded by the philosophers Swedenborg, Kant, and\\nLaplace, and, although since greatly modified in detail,\\ntheir fundamental views are, in the main, received. The\\nnebular hypotheses proposed by these philosophers all start\\nwith the statement that the Earth and Planets, as well as\\nthe Sun, were once a fiery mass.\\nIt is certain that the Earth has not received any great supply of\\nheat from outside since the early geological ages, because such an\\naccession of heat at the Earth s surface would have destroyed all life,\\nand even melted all the rocks. Therefore, whatever heat there is in\\nthe interior of the Earth must have been there from before the com-\\nmencement of life on the globe, and remained through all geological\\nages.\\nThe interior of the Earth is very much hotter than its surface, and\\nhotter than the celestial spaces around it. It is continually losing\\nheat, and there is no way in which the losses are made up. We\\nknow by the most familiar observation that if any object is hot inside,\\nthe heat will work its way through to the surface. Therefore, since\\nthe Earth is a great deal hotter at the depth of 50 miles than it is at\\nthe surface, and much hotter at 500 miles than at 50, heat must be\\ncontinually coming to the surface. On reaching the surface, it must\\nbe radiated off into space, else the surface would have long ago be-\\ncome as hot as the interior.\\nMoreover, this loss of heat must have been going on since the be-\\nginning, or at least since a time when the surface was as hot as the\\ninterior. Thus, if we reckon backward in time, we find that there\\nmust have been more and more heat in the Earth the further back we\\ngo, so that we reach a time when the Earth was so hot as to be", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0436.jp2"}, "435": {"fulltext": "COSMOGONY. 409\\nmolten, and finally reach a time when it was so hot as to be a mass of\\nfiery vapor.\\nThe Sun is cooling off like the Earth, only at an incomparably more\\nrapid rate. The Sun is constantly radiating heat into space, and, so\\nfar as we know, receiving none back again. A very small portion of\\n%\\\\iis heat reaches the Earth, and on this portion depends the existence\\nof life and motion on the Earth s surface. If our supply of solar heat\\nwere to be taken away, all life on the Earth would cease. The\\nquantity of heat which strikes the Earth is only about ^w^^wmos^\\nthat which the Sun radiates. This fraction expresses the ratio of the\\napparent surface of the Earth, as seen from the Sun, to that of the\\nwhole celestial sphere.\\nSince the Sun is constantly losing heat, it must have had more heat\\nyesterday than it has to-day more two days ago than it had yester-\\nday; and so on. The further we go back in time, the hotter the Sun\\nmust have been. Since we know that heat expands all bodies, it fol-\\nlows that the Sun must have been larger in past ages than it is now,\\nand we can calculate the size of the Sun at any past time.\\nThus we are led to the conclusion that there must have been a time\\nwhen the Sun filled up the whole of the space now occupied by the\\nplanets. It must then have been a very rare mass of glowing vapor.\\nThe planets could not then have existed separately, but must have\\nformed a part of this mass of vapor. The glowing vapor a fiery\\nmist was the material out of which the solar system was formed.\\nThe same process may be continued into the future. Since the Sun\\nby its radiation is constantly losing heat, it must grow cooler and\\ncooler as ages advance, and must finally radiate so little heat that fife\\nand motion can no longer exist on our globe.\\nIt is a noteworthy confirmation of this hypothesis that the revolu-\\ntions of all the planets around the Sun take place in the same direc-\\ntion and in nearly the same plane. This similarity among the differ-\\nent bodies of the solar system must have had an adequate cause. The\\nSun and planets were once a great mass of vapor, larger than the\\npresent solar system, that revolved on its axis in the same plane in\\nwhich the planets now revolve.\\nThe spectroscope shows the nebulae to be masses of glowing vapor.\\nWe thus actually see matter in the celestial spaces under the very\\nform in which the nebular hypothesis supposes the matter of our solar\\nsystem to have once existed. Some of these nebulae now have the\\nvery form that the nebular hypothesis assigns to the solar nebula in\\npast ages. (See the frontispiece.) The nebulae are gradually cooling.\\nThe process of cooling must at length reach a point when they will", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0437.jp2"}, "436": {"fulltext": "410 ASTRONOMY.\\ncease to be vaporous and will condense into objects like stars and\\nplanets. All the stars must, like the Sun, be radiating heat into space.\\nThe telescopic examination of the planets Jupiter and Saturn shows\\nthat changes on their surfaces are constantly going on with a rapidity\\nand violence to which nothing on the surface of our Earth can com-\\npare. Such operations can be kept up only through the agency of\\nheat or some equivalent form of energy. At the distance of Jupiter\\nand Saturn, the rays of the Sun are entirely insufficient to produce\\nsuch changes. Jupiter and Saturn must be hot bodies, and must\\ntherefore be cooling off like the Sun, stars, and Earth.\\nThese and many other allied facts lead to the conclusion that most\\nbodies of the universe are hot, and are cooling off by radiating their\\nheat into space.\\nThere is no way known to us in which the heat radiated by the Sun\\nand stars might be collected and returned to them. It is a funda-\\nmental principle of the laws of heat that heat can never pass from\\na cooler to a warmer body that a body can never grow warmer in\\na space that is cooler than the body itself.\\nAll differences of temperature tend to equalize themselves, and the\\nonly state of things to which the universe can tend, under its present\\nlaws, is one in which all space and all the bodies contained in space\\nwill be at a uniform temperature, and then all motion and change of\\ntemperature, and hence the conditions of vitality, must cease. And\\nthen all such life as ours must cease also unless sustained by entirely\\nnew methods.\\nThe general result drawn from all these laws and facts\\nis, that there was once a time when all the bodies of the\\nuniverse formed either a single mass or a number of masses\\nof fiery vapor, having slight motions in various parts, and\\ndifferent degrees of density in different regions. A grad-\\nual condensation around the centres of greatest density then\\ntook place in consequence of the cooling and the mutual at-\\ntraction of the parts, and thus arose a number of separate\\nnebulous masses. One of these masses formed the material\\nout of which the Sun and planets are supposed to have\\nbeen formed. It was probably at first nearly globular, of\\nnearly equal density throughout, and endowed with a very\\nslow rotation in the direction in which the planets now", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0438.jp2"}, "437": {"fulltext": "COSMOGONY. 411\\nmove. As it cooled off, it grew smaller and smaller, and\\nits velocity of rotation increased in rapidity.\\nThe rotating mass we have described had an axis around which it\\nrotated, and an equator everywhere 90\u00c2\u00b0 from this axis. As the velocity\\nof rotation increased, the centrifugal force also increased. This force\\nvaries as the radius of the circle described by any particle multiplied\\nby the square of its angular velocity. Hence when the masses, being\\nreduced to half the radius, rotated four times as fast, the centrifugal\\nforce at the equator would be increased X 4 s or eight times. The\\ngravitation of the mass at the surface, being inversely as the square\\nof the distance from the centre, or of the radius, would be increased\\nonly four times. Therefore, as the masses continued to contract, the\\ncentrifugal force increased more rapidly than the central attraction.\\nA time would therefore come when they would balance each other at\\nthe equator of the mass.\\nThe mass would then cease to contract at the equator, but at the\\npoles there would be no centrifugal force, and the gravitation of the\\nmass would grow stronger and stronger in this neighborhood.\\nIn consequence the mass would at length assume the\\nform of a lens or disk very thin in proportion to its extent.\\nThe denser portions of this lens would gradually be drawn\\ntoward the centre, and there more or less solidified by\\ncooling. At length, solid particles would begin to be\\nformed throughout the whole disk. These would grad-\\nually condense around each other and form a single planet,\\nor break up into small masses and form a group of planets.\\nAs the motion of rotation would not be altered by these\\nprocesses of condensation, these planets would all rotate\\naround the central part of the mass, which condensed to\\nform our Sun.\\nThese planetary masses, being very hot, were composed of a central\\nmass of those substances which condensed at a very high tempera-\\nture, surrounded by the vapors of other substances which were more\\nvolatile. We know, for instance, that it takes a much higher tem-\\nperature to reduce lime and platinum to vapor than it does to reduce\\niron, zinc, or magnesium. Therefore, in the original planets, the\\nlimes and earths would condense first, while many other metals would\\nstill remain in a state of vapor.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0439.jp2"}, "438": {"fulltext": "412 ASTRONOMY.\\nEach of the planetary masses would rotate more rapidly as it grew\\nsmaller, and would at length form a mass of melted metals and vapors\\nin the same way as the larger mass out of which the Sun and planets\\nwere formed. These separate masses would then condense into a\\nplanet, with satellites revolving around it, just as the original mass\\ncondensed into Sun and planets.\\nAt first the planets would be in a molten condition, each shining\\nlike the Sun. They would, however, slowly cool by the radiation of\\nheat from their surfaces. So long as they remained liquid, the sur-\\nface, as fast as it grew cool, would sink into the interior on account\\nof its greater specific gravity, and its place would be taken by hotter\\nmaterial rising from the interior to the surface, there to cool off in its\\nturn.\\nThere would, in fact, be a motion something like that which occurs\\nwhen a pot of cold water is set upon the fire to boil. Whenever a\\nmass of water at the bottom of the pot is heated, it rises to the sur-\\nface, and the cool water moves down to take its place. Thus, on the\\nwhole, so long as the planet remained liquid, it would cool off equally\\nthroughout its whole mass, owing to the constant motion from the\\ncentre to the circumference and back again.\\nA time would at length arrive when many of the earths and metals\\nwould begin to solidify. At first the solid particles would be carried\\nup and down with the liquid. A time would finally arrive when tbey\\nwould become so large and numerous, and the liquid part of the gen-\\neral mass so viscid, that their motion would be obstructed. The\\nplanet would then begin to solidify. Two views have been enter-\\ntained respecting the process of solidification.\\nAccording to one view, the whole surface of the planet would\\nsolidify into a continuous crust, as ice forms over a pond in cold\\nweather, while the interior was still in a molten state. The interior\\nliquid could then no longer come to the surface to cool off, and could\\nlose no heat except what was conducted through this crust. Hence\\nthe subsequent cooling would be much slower, and the globe would\\nlong remain a mass of lava, covered over by a comparatively thin solid\\ncrust like that on which we live.\\nThe other view is that, when the cooling attained a certain stage,\\nthe central portion of the globe would be solidified by the enormous\\npressure of the superincumbent portions, while the exterior was still\\nfluid, and that thus the solidification would take place from the\\ncentre outward.\\nIt is still an unsettled question whether the Earth is now solid to\\nits centre, or whether it is a great globe of molten matter with a com-", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0440.jp2"}, "439": {"fulltext": "COSMOGONY. 413\\nparatively thin crust. Astronomers and physicists incline to the\\nformer view some geologists to the latter one. Whichever view-\\nmay be correct, it appears certain that there are lakes of lava im-\\nmediately beneath the active volcanoes.\\nIt must be understood that the nebular hypothesis is not\\na perfectly established scientific theory, but only a philo-\\nsophical conclusion founded on the widest study of nature,\\nand supported by many otherwise disconnected facts. The\\nwidest generalization associated with it is that, so far as\\ncan now be known, the universe is not self-sustaining, but\\nis a kind of organism which, like all other organisms known\\nto us, must come to an end in consequence of those very\\nlaws of action which keep it going. It must have had a\\nbeginning within a certain number of years that cannot\\nyet be calculated with certainty, but which cannot in any\\nevent much exceed 20,000,000, and it must end in a system\\nof cold, dead globes at a calculable time in the future,\\nwhen the Sun and stars shall have radiated away all their\\nheat, unless it is re-created by the action of forces at present\\nunknown to science.\\nIt must be carefully noted that these conclusions, which\\nare correct in the main, relate entirely to the transformations\\nof matter in the past and future time, and say nothing as to\\nits origin. The original nebula must have contained all the\\nmatter now in the universe, and it must have possessed, po-\\ntentially, all the energy now operative as light, heat, etc.,\\nbesides the vast stores of energy that have been expended in\\npast ages. The process by which the physical universe was\\ntransformed from one condition to a later one is the subject\\nof the nebular hypothesis. The field of physical science is\\na limited one, although within that field it deals with pro-\\nfound problems. Astronomy has nothing to say on the\\nquestion of the origin of matter nor on the vastly more im-\\nportant questions as to the origin of life, intelligence,\\nwisdom, affection.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0441.jp2"}, "440": {"fulltext": "CHAPTER XXIX.\\nPRACTICAL HINTS ON OBSERVING.\\n48. A few Practical Hints on Making Observations.\\nLists of a few Interesting Celestial Objects. Stars, Double\\nStars, Variable Stars, Nabulse, Clusters. Maps of the Stars.\\nIn the paragraphs that follow a few hints are given for the\\nbenefit of the student who wishes to begin to make simple\\nobservations for himself. Long and detailed instructions\\nmight be set down which would perhaps save many mis-\\ntakes. But it is by mistakes made and corrected that one\\nlearns. A genius is a person who never makes the same\\nmistake twice. The rest of mankind must educate them-\\nselves by slow and patient correction of the errors they\\ncommit. Therefore only enough is here set down to start\\nthe student on his way. It will depend on himself and his\\nopportunities how far he goes.\\nObservations of the Planets. The accurate places of the planets are\\nprinted in the Nautical Almanac (address Nautical Almanac office,\\nNavy Department, Washington, D. C); and many other almanacs\\ngive their approximate positions. The Publications of the Astro-\\nnomical Society of the Pacific (address 819 Market Street, San Fran-\\ncisco), and the journal Popular Astronomy (address Northfield,\\nMinnesota), contain such information, in a form useful to amateurs.\\nLists of the eclipses of each year, and of morning and evening stars,\\nare printed in most diaries, as well as the phases of the Moon, and\\nthe hours of sunrise and moonrise, etc. The daily newspapers fre-\\nquently print articles naming the planets and stars that are in a favor-\\nable position for observation.\\nMercury is often to be seen, if one knows just where to look. Its\\ngreatest elongation from tbe Sun is about, 29\u00c2\u00b0, so that it is seldom vis-\\nible in our latitudes more than two hours afte r sunset, or before sun\\n414", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0442.jp2"}, "441": {"fulltext": "HINTS ON OBSERVING. 415\\nrise. The student will do well to know its place (from some almanac)\\nbefore looking for it, so that no time may be lost in discovering this\\nplanet over agrin. The greatest elongation of Venus from the Sun\\nis about 45\u00c2\u00b0, so that this planet is usually not visible more than about\\nthree hours after sunset, or before sunrise. In a clear sky, however,\\nVenus may be seen in the daytime, if the position is known. Mars is\\neasy to distinguish from the other planets by his ruddy color. Jupiter\\nis the planet next in brightness to Venus, and both Jupiter and Venus\\nare brighter than the most brilliant fixed star Sirius. The place of\\nSirius in the sky can be found on any one of the star-maps, and hence\\nSirius can always be distinguished from the planets. Saturn looks\\nlike a rather dull (not sparkling) fixed star. These are the planets\\neasily visible to the naked eye. If the student finds a bright object\\nin the sky, he can decide from the star-maps whether it is a fixed\\nstar. If it is not a star, it will not be difficult for him to determine\\nwhich of the planets he has found. Uranus is occasionally (just)\\nvisible to the naked eye, but Neptune always is invisible, except in a\\ntelescope. At least one of the asteroids Vesta) is sometimes visible\\nto the naked eye.\\nThe motions of the planets may be studied with the unaided eye,\\nbut nothing can be known of their disks or of their phases without\\na telescope. An opera-glass (which usually magnifies about 2 or 3\\ntimes) or, better, a field-glass, will be of much use in viewing the\\nMoon, and if nothing better is available it should be used to view the\\nplanets. But even a small telescope is much more satisfactory.\\nThe student must not expect to see the planetary disks as they are\\nshown in the drawings of this book. These drawings have usually\\nbeen made with large telescopes. Even under very favorable condi-\\ntions such observations are more or less disappointing to observers\\nwho are not practised.\\nObservations of Stars, Nebulae, Comets, etc.\u00e2\u0080\u0094 The brighter stars can\\nbe identified in the sky from the star-maps in this book. Some of\\nthe variable stars and clusters are marked in Fig. 213. Tables V to\\nVIII (pages 417 to 421) give the places of some of the principal\\nfixed stars, double-stars, etc. These objects (if they are bright\\nenough) should first be identified with the naked eye and then studied\\nwith the best telescope available. An opera-glass is better than\\nnothing a good field-glass or a spy-glass is better yet (it represents\\nGalileo s equipment), but a telescope of several inches aperture\\nwith a magnifying power of 50 diameters or more, on a firm stand,\\nshould be used if it is possible to obtain it.\\nPhotography in observation.\u00e2\u0080\u0094 If the student understands photogra", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0443.jp2"}, "442": {"fulltext": "416 ASTRONOMY.\\nphy let him try his camera on the heavens. If he directs it to the\\nnorth pole and gives an exposure of a couple of hours he will obtain\\nthe trails of the brighter circumpolar stars (see Fig. 29). An expo-\\nsure of a few minutes on a bright group of stars near the zenith or\\nin the south (the Pleiades or Orion, for example) will give trails of a\\ndifferent kind (see Fig. 80). In both these observations the camera\\nmust remain fixed, undisturbed by wind or jars of any kind.\\nIf he can strap his camera to the tube of a telescope (like that\\nshown in Fig. 79) he can follow a group of stars in their motion\\nfrom rising to setting by using the telescope as a finder in the follow-\\ning way: I. Select the group to be photographed. It should be\\nvisible in the camera and some bright star of the group should be\\nvisible in the telescope at the same time. The eyepiece of the tele-\\nscope should be provided with a pair of cross wires, thus -j-, which\\nthe observer can easily insert, if necessary. II. The image of one of\\nthe group of stars must be kept on the cross-wires (by gently and\\nconstantly moving the telescope from east towards west from rising\\ntoward setting) so long as the exposure is going on. In this way\\nfairly long exposures can be made. If the image of the guiding-star\\nis put slightly out of focus the guiding is sometimes easier. This\\nmethod is also available for photographing a bright comet; only the\\nstudent must remember to use the comet itself as a guiding-star (in\\nthe telescope), because the comet has a motion among the stars.\\nPhotographs of the Moon (and Sun) can be made with small cameras,,\\nbut unless the camera has a long focus they are disappointingly\\nsmall in size. Let the student try to make them, however. For\\nthe Moon, use the quickest plates. For the Sun, use the slowest\\nplates, the smallest stop and the quickest exposure. In these, as in\\nall observations, the important matter to the student is to make them\\nand to find out what is wrong and then to make them over again,\\ncorrecting mistakes and so on until a satisfactory result is obtained.\\nIt is desirable that the school should own apparatus to be used by\\nthe students under the direction of the master. A short list follows:\\nA celestial globe; a cheap watch regulated to sidereal time; a straight-\\nedge some three feet long a plumb-line a field-glass a small\\ntelescope a star-atlas (Upton s, McClure s edition of Klein s,\\nProctor s, are good); books on practical Astronomy (begin with\\nServiss Astronomy with an Opera-Glass, Proctor s Half Hours with\\nthe Stars, J. Westwood Oliver s Astronomy for Amateurs, Webb s\\nCelestial Objects for Common Telescopes, and add to these as needs\\narise); books on descriptive Astronomy (begin with the works of Sir\\nRobert Bull, Miss Clerke s History of Astronomy in the XIX\\nCentury, Flammarion s Popular Astronomy, etc., and add to these as", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0444.jp2"}, "443": {"fulltext": "LIST OF BRIGHT STARS.\\n417\\nopportunity offers); text-books of Astronomy (begin with Young s\\nGeneral Astronomy)\\nTABLE V.\\nMean Right Ascension and Declination of a few Bright\\nStars, visible at Washington, for January 1, 1899.\\nName of Star.\\n15\\nAndromedas\\nCassiopeiee Var\\nCeti\\nUrsae Miuoris (Pole Star).\\nArietis\\nArietis\\nCeti\\nPersei\\nTauri\\nEridani\\nTauri (Aldebaran)\\nAurigae\\nAurigae (Capella)\\nOrionis (Rigel)\\nTauri\\nOrionis\\nLeporis\\nOrionis\\nColumbae\\nOrionis\\nGeminorum\\nCanis Majoris (Sirius)\\nCanis Majoris\\nGeminorum (Castor)\\nCanis Minoris (Procyon)\\nGeminorum (Pollux)\\nArgus\\nUrsae Majoris\\nHydrae\\nUrsae Majoris\\nLeonis (Regulus)\\nLeonis\\nUrsae Majoris\\nLeonis\\nUrsae Majoris\\nCorvi\\nCorvi\\nCanum Venaticorum\\nVirginis (Spica)\\nUrsse Majoris\\nBootis (Arcturus)\\nLibrae\\nUrsae Minoris\\nLibrae.\\nCoronas Borealis\\nSerpentis\\nScorpii\\nDraconis\\nScorpii (Antares)\\nHerculis\\nDraconis\\nMag.\\n2\\n2^\\n2\\n3\\n3\\n1\\n2^\\n1\\n2\\n2^\\n2)4\\n2\\n2^\\n1\\n2\\n1\\n2\\n1\\n1\\n2\\n3\\n1\\n2^\\n2^\\n2\\n2y 2\\n2^\\n3\\n1\\n3K 2\\nRight\\nAscension.\\nm. s.\\n3 9.9\\n34 46.3\\n38 31.2\\n22 8.0\\n49 3.5\\n1 28.7\\n2 56 59.9\\n3 17 6.5\\n3 41 28.7\\n3 53 18.9\\n30 7.4\\n50 24.9\\n9 13.5\\n9 41.0\\n19 54.4\\n26 50.7\\n28 16.5\\n31 5.3\\n5 35 59.5\\n5 49 42.2\\n6 31 52.6\\n6 40 41.9\\n6 54 39.3\\n7 28 9.4\\n7 34 1.0\\n7 39 8.2\\n8 3 14.5\\n8 52 17.7\\n9 22 37.4\\n9 26 6.3\\n10 2 59.6\\n10 14 24.3\\n10 57 29.8\\n11 43 54.5\\n11 48 31.2\\n12 4 55.7\\n12 29 4.8\\n12 51 18.2\\n13 19 52.2\\n13 43 33.7\\n14 11 3.2\\n14 4F 17.3\\n14 50 59.7\\n15 11 34.2\\n15 30 24.6\\n15 39 17.5\\n15 59 33.7\\n16 22 37.4\\n16 23 12.7\\n17 10 2.5\\n17 28 9.0\\nAnnual\\nVaria-\\ntion.\\ns.\\n3.08\\n3.37\\n3.00\\n+24.99\\n3.30\\n3:36\\n3.13\\n4.26\\n4- 3.56\\n-J- 2.79\\n4- 3.43\\n4- 3.90\\n4- 4.42\\n4- 2.88\\n4- 3.79\\n4- 3.06\\n4- 2.65\\n4- 3 04\\n4-2.17\\n4- 3.25\\n4- 3.46\\n4- 2.68\\n4- 2.36\\n3.85\\n4- 3.19\\n4- 3.73\\n4- 2.56\\n4- 4.17\\n4- 2.95\\n4- 4.14\\n4- 3.22\\n4- 3.29\\n4- 3.76\\n4- 3.10\\n4- 3.17\\n4- 3.08\\n4- 3.14\\n4- 2.83\\n4- 3.16\\n2.38\\n4- 2.81\\n4- 3.32\\n0.21\\n4- 3.23\\n4- 2.53\\n4- 2.94\\n4- 3.48\\n0.81\\n3.67\\n4- 2.74\\n4- 1.36\\nDeclination.\\n28\\n4-55\\n18\\n4-88\\n20\\n4-22\\n4- 3\\n4-49\\n4-23\\n13\\n16\\n-f-33\\n4-45\\n8\\n4-28\\n17\\n1\\n34\\n4- 7\\n4-16\\n-16\\n28\\n32\\n5\\n28\\n24\\n48\\n8\\n52\\n12\\n20\\n62\\n15\\n54\\n22\\n-22\\n38\\n10\\n49\\n19\\n15\\n74\\n9\\n4-27\\n6\\n19\\n61\\n-26\\n4-14\\n52\\n31 58\\n59\\n32 28\\n46 8\\n18 52\\n59 6\\n41 36\\n30 6\\n47 34\\n47 45\\n18 23\\n23\\n53 43\\n19 5\\n31 20\\n22 26\\n53 41\\n15 59\\n7 40\\n23 18\\n29 8\\n34 41\\n50 4\\n6 36\\n29 3\\n16 12\\n48\\n26 18\\n13 15\\n8 15\\n27 39\\n21 9\\n17 46\\n8 12\\n15 23\\n3 30\\n50 18\\n51 50\\n38 3\\n49 2\\n42 30\\n37 20\\n34 6\\n38\\n3 16\\n44 36\\n31 45\\n44 34\\n12 28\\n30 19\\n22 34\\nAnnual\\nVaria-\\ntion\\n20.1\\n19.8\\n19.8\\n4-18.8\\n4-17.8\\n4-17.3\\n4-14.4\\n13.1\\n11.4\\n10.5\\n7\\n6.0\\n4.4\\n4.4\\n3.5\\n2.9\\n2.8\\n2.5\\n2.1\\n0.9\\n2.8\\n3.5\\n4.7\\n7.5\\n8.0\\n8.4\\n10.3\\n13.7\\n15.5\\n15.7\\n17.5\\n18.0\\n19.3\\n20.0\\n-20.0\\n-20.0\\n19.9\\n19.6\\n18.8\\n18.0\\n16.9\\n15.1\\n14.7\\n13.4\\n12.2\\n11.6\\n10.1\\n8.3\\n8.2\\n4.3\\n2.8", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0445.jp2"}, "444": {"fulltext": "418\\nASTRONOMY.\\nTABLE V .\u00e2\u0080\u0094Continued.\\nName of Star.\\nOphiuchi\\nLyrae (Vega)\\nLyrae (var.)\\nAquilae (Altair)\\nCygni\\nCephei\\nAquarii\\nAquarii\\nPiscisAustralis(FomaZ/iau\u00c2\u00a3)\\nPegasi (Markab)\\nPiscium\\nAnnual\\nAnnual\\nMag.\\nRight\\nAscension.\\nVaria-\\ntion.\\nDeclination.\\nVaria-\\ntion.\\nh. m. s.\\ns.\\no\\na\\na\\n2\\n17 30 14.7\\n2.78\\n12\\n38\\n2.6\\n1\\n18 33 31.1\\n2.01\\n38\\n41\\n22\\n2.9\\n3*-4*\\n18 46 21.0\\n2.21\\n33\\n14\\n43\\n4.0\\n1\\n19 45 51.3\\n2.89\\n8\\n36\\n5\\n8.9\\nm\\n20 37 59.3\\n2.04\\n44\\n55\\n9\\n12.8\\n2%\\n21 16 10.1\\n1.41\\n62\\ny\\n27\\n15.1\\n3\\n21 26 14.5\\n3.16\\n6\\n56\\n15.7\\n3\\n22 35.7\\n3.08\\n48\\n38\\n17.4\\n1V\u00c2\u00ab\\n22 52 4.1\\n3.30\\n-30\\n9\\n27\\n19.2\\n*y\u00c2\u00bb\\n22 59 43.7\\n2.98\\n14\\n39\\n42\\n19.4\\n4\\n23 54 7.4\\n3.07\\n6\\n18\\n15\\n20.0\\nN.B. The Mean Right Ascension and Declination for any other year than 1899\\nmay be found from this table by multiplying the annual variation by the num-\\nber of years elapsed, and applying the result to the quantities given in this\\ntable. If the required date be earlier than 1899, the signs of the annual varia-\\ntions must be changed. In applying such corrections to the Declinations the\\ncorrections must be added algebraically. For example, the mean place of\\nAldebaran for July 1, 1901 1901.5) is R. A. 4 h 30 m 16 8 .0 Decl. 16\u00c2\u00b0 18 42\\nN.B.\u00e2\u0080\u0094 The Nautical Almanac gives the apparent R.A. s of these and other\\nstars at intervals of ten days.\\nN.B. When any one of these stars is on the observer s meridian at any date,\\nhis local sidereal time is equal to that star s apparent right ascension on that\\ndate.\\nThe foregoing table will serve to set the observer s watch to side-\\nreal time within a few minutes so soon as he knows his meridian\\n(see page 151). A watch set approximately to sidereal time is, of\\ncourse, necessary in identifying objects in the sky.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0446.jp2"}, "445": {"fulltext": "LIST OF DOUBLE STARS.\\n419\\n3 -s\\no 2\\n5\\nbe u\\n3\\nS o\\ni 1\\na a\\no S\\n3~\\n5 3\\n\u00e2\u0096\u00a02*2\\nO T3cm OJ a\\nc\\nZA\\n\u00c2\u00ab1qJ\\ne3^ oo\\n.22\u00c2\u00abc3\\ncfl S\\na\\nUC0 aS\\nW 4J\\nWC\\n.S be\\nPQ OE-\\n*s\\ng o v O\\n\u00e2\u0080\u00a2o is\\n.5\\ni 00 CO CO 00 CO t-\\n2^\\nCO t-Oi if3 c\\n4- be\\n35 c\\n04\\nOOt-\u00c2\u00abOifl^OOO o\u00c2\u00bb o -fl o o o\\n\u00c2\u00ab\u00c2\u00abnO! CO r Oi niiWWW\\ncfl\\nt \u00c2\u00abOt\u00c2\u00a9COG0GD 00 I- K! Otf3\\noj\u00c2\u00abnineO 3 t--i in co ioeo eo^-^o\\nQ0ODNQDCO\u00c2\u00abO)fflJ!\\no eo WOO IJ Oi iO\\n00 CO th IO\\nitji oi t- ir; o \u00c2\u00abo\\n+4- +-fi\\nOOOthhhKOOI JOCO t~CCO H M ION\\nI\\n-2.2SS-~\\n1 f 2Ig\\nuj S 8 (J g g\\nmm p b\\nS\\nOS S-S-S S I :o\u00c2\u00a3\\nII s\u00c2\u00a7!ss Sis s\\nit\\nSo 5", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0447.jp2"}, "446": {"fulltext": "420\\nASTRONOMY.\\nTABLE VII.\\nA List of a few Variable Stars.\\nStar.\\nMir a Ceti\\nAlgol (jB Persei)\\ne Aurigce\\nGeminorum\\nR Leonis\\nR Ursoz Majoris\\nR Hydros,\\na Herculis\\nX Sagittarii\\n/3 Lyrce\\nCephei\\nR Cassiopeia\\nR. A.\\nMagnitude.\\nMax.\\nMin.\\nh m\\no\\nDays.\\n2 14\\n3\\n26\\n331\\n1.7\\n9.5-j\\n3 2\\n40\\n34\\n2%\\n2.3\\n3.5J\\n4 55\\n43\\n41\\n3\\n4.5-j\\n4.5\\n10\\n13\\n9.7\\n3.9\\n6\\n6 58\\n9 42\\n10 38\\n13 24\\n1? 10\\n17 41\\n20\\n11\\n69\\n22\\n14\\n-27\\n43\\n54\\n18\\n46\\n30\\n48\\n10\\n313\\n305\\n497\\n90?\\n7\\n3.7\\n5.2\\n6\\n3.5\\n3.1\\n4\\n18 46\\n33\\n15\\n12.9\\n3.4\\n4.5]\\n22 25\\n57\\n54\\n5y 3\\n3.7\\n4.9-j\\n23 53\\n50\\n50\\n429\\n5\\n12\\nRemarks.\\nIt is best seen about\\nOctober.\\nObserve it in Octo-\\nber November.\\nIrregular. Of the\\nAlgol type.\\nIrregular period.\\nObserve it in June.\\nObserve it at mid-\\nsummer.\\nObserve it in Aug.\\nand September.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0448.jp2"}, "447": {"fulltext": "LIST OF NEBULA AND CLUSTERS.\\n421\\na 5dK\\no \u00c2\u00ab,_, c^-\\no\\n15 Z, bC\\n5 Oil-\\n45 1- c3 a\\ncs\\n45\\npq\\ng 45 03\\nWJ y\\na. y\\na. z\\n\u00c2\u00ab\u00c2\u00ab5 .3\\nco sr as a) g 2\\n\u00e2\u0096\u00a09sa\u00c2\u00a7\\n5 \u00e2\u0080\u00a2\u00e2\u0080\u00a2SS-fil S fcJ-S c a\\n;h=s\\nc 5\\n45 cs\\nx H\\n\u00e2\u0096\u00a0gbS-SP^g-S\\n,3\\nv be\\n2 -O\\n5 3\\ntsb-g g:\\nP\u00c2\u00abJC BCC\\n\u00e2\u0096\u00a0213 ^T^\\nG-J^ .Is 45 43\\nCcO\\nI\u00e2\u0080\u0094 tX4 45\\n2--S as\\n^35\\nS B .Sf^ gS 45 45 2.$X* g C fa ^3\\n03 S J= j: .^45 r --o v c a d a v\\nSSI b*s b\u00c2\u00b0bbw b\u00c2\u00a3sJ h\u00c2\u00b0\\ni:x ^45 t:,s:a3454 Ea ^oao3-=.\u00e2\u0080\u0094 eg\\n?ir\u00c2\u00abiOTHWiHfflooci:iflM\u00c2\u00bbifit-o\u00c2\u00ab5Ct-M(oa!COOtDa\\noj cow CO\\n.i ic rt\\nI I I I I\\n1 i-w\u00c2\u00ab5Tjio Mcso aiiO\u00c2\u00a3\u00c2\u00abioinTtiaM \u00c2\u00bbW!DfflNff)QO qo\\nStoNi-icam t- os in to ed od os co co i ib co c t- eo co\\nCO Tf OJ CO nWO^TfTrrn i-iT-n-iOJCVCOCOOJCOCOi-iT-c\\nOO i\u00c2\u00ab\u00c2\u00abMiniC0Dffl0iOOi-irt0!WNN\u00c2\u00abKMM\u00c2\u00bbinm\\na)\\ns eo co th t t- t~ r* i-i oi to m o eo N n i~\\n*00\u00c2\u00abWWL-COOC.\u00c2\u00bbCOt-00", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0449.jp2"}, "448": {"fulltext": "422\\nASTRONOMY.\\n.2 03\\ncs o) a a\\nin s\\n03\\nw-t;\\ni 3\\n.O 03 5\\nc h5\\n03 03\\nJ2 *T in i 3\\nfe a o.\\nSSP-i\\nC 7? 03 2 O O\\nSfefl\\nS 03 03 S3 C _-,\\n03O\u00c2\u00bb^yO^ t \u00c2\u00ab-S5j\u00c2\u00a3 W)!3 SE-030303\\nCCi O\\n03 m co co\\n\u00c2\u00ab2 =5\\n.I3.fi\\noowoio toioouo\\no SJ m 55\\nI l l I I++++I+\\n.ot-iooocoooNto^NOiflinoiwT-MiiT-ito\\ni O OS OO OS 05\\nCO CD CD CO CO CDCOCOCOCOCOCOt -C -i\\nTS\u00c2\u00a3 fee\\ncS cS tp\\n8*81\\n73\\nlife!\\nfiJlcIS\\no *H g 1\\n1351\\nsill\\nW 03 03 _\\n03 -u \u00c2\u00ab3J3\\no\\ny *3+j\\ni n 03 .Si.\\n\u00c2\u00ab5 c.ff\\nHii\\n03 g-^CQ\\nO o\\nU 03\\nS otf-3\\nJ? 03 t_ S\\n\u00e2\u0080\u00a25 13 03\\nSg-2 8\\n5 03^ a\\n43\\n43 03\\nO\\nCO\\nO \u00c2\u00ab8\\nCO\\nI s S3\\n03 03", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0450.jp2"}, "449": {"fulltext": "HINTS ON OBSERVING. 423\\nTo see a nebula with advantage it is sometimes advisable to set tbe\\ntelescope a very little west of it so that the nebula may enter tbe field\\nof view by its diurnal motion and pass slowly across it. This can be\\nrepeated as often as desired. Nearly all of tbese objects are so faint\\nFig. 212.\u00e2\u0080\u0094 Map of the Stars (to Fourth Magnitude Inclu-\\nsive) near the North Celestial Pole.\\nThe names of these stars can be found in figures 214 to 219 following.\\nthat no artificial lights should be near the observer s place. The word\\nbright in the descriptions is a relative term. A bright nebula is\\nfaint compared to a planet.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0451.jp2"}, "450": {"fulltext": "424 ASTRONOMY.\\nMAPS OF THE STAES.\\nThe Northern Stars. The constellations near the pole\\ncan be seen on any clear night, while most of the southern\\nones can only be seen during certain seasons, or at certain\\nhours of the night. Fig. 212 shows all the stars down to\\nthe fourth magnitude, inclusive, within 50\u00c2\u00b0 of the pole.\\nThe Roman numerals around the margin show the\\nmeridians of right ascension, one for every hour. In order\\nto have the map represent the northern constellations as\\nthey are, it must be held so that the hour of sidereal\\ntime at which the observer is looking at the heavens shall\\nbe at the top of the map. The names of the months\\naround the margin of the map show the regions near the\\nzenith during those months. Suppose the observer to\\nlook at nine o clock {mean solar time) in the evening, to\\nface the north, and to hold the map with the month up-\\nward, he will have the northern heavens as they appear,\\nexcept that the stars near the bottom of the map may be\\ncut off by his horizon.\\nThe Equatorial Stars. The folded map, Figure 213,\\nshows the equatorial stars lying between 30\u00c2\u00b0 north and 30\u00c2\u00b0\\nsouth declination. The outlines of the constellations are\\nindicated by dotted lines. The figures of men and animals\\nwith which the ancients covered the sky are omitted.\\nThe Latin name within each boundary is the name of the\\nconstellation. The Greek letters serve to name the bright-\\nest stars. The parallels of declination (for every 15\u00c2\u00b0) and\\nthe hour-circles (every hour) are laid down.\\nThe magnitudes of the stars are indicated by the sizes of\\nthe dots. To use this map it must be remembered that as\\nyou face the south greater right ascensions are on your left\\nhand, less on your right. The right ascensions of the stars\\nimmediately to the south between 6 and 7 p.m. are:", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0452.jp2"}, "451": {"fulltext": "MAPS OF THE STARS. 425\\nFor January 1, 1 hour; For July 1, 13 hours;\\nFebruary 1, 3 hours; August 1,15\\nMarch 1, 5 September 1, 17\\nApril 1, 7 October 1, 19\\nMay 1, 9 November 1, 21\\nJune 1, 11 December 1, 23\\nThis map and the map preceding it will be found use-\\nful in various ways. The six star-maps that follow are\\nmore convenient for ordinary use, however.\\nSix Star-maps showing the Brighter Stars visible in the\\nNorthern Hemisphere.* The star-maps in this series were\\noriginally adapted to a north latitude of about 52\u00c2\u00b0, so that,\\nfor the latitudes of the United States, they will be slightly\\nin error, but not so much as to cause inconvenience. Under\\neach map will be found the date and time at which the sky\\nwill be as represented in the accompanying map; e. g., Map\\nNo. 1 shows the sky as it appears on November 22d at mid-\\nnight, December 5th at 11 o clock, December 21st at 10\\no clock, January 5th at 9 o clock, and January 20th at 8\\no clock.\\nThe maps are intended for use between the hours of 8\\no clock in the evening and midnight, and the titles are\\ngiven with reference to such a use.\\nIt should be borne in mind, however, that the same map represents\\nthe aspect of the constellations on other dates than those given, but\\nat a different hour of the night. Map No. I, for example, shows the\\naspect of the sky on October 23d at 2 a. m., September 23d at 4 a.m.,\\nand also on February 20th at 6 p. m., as well as on the dates and at\\nthe hours given in the map. For any date between those given, the\\nmap will represent the sky at a time between the hours given for\\ninstance, on November 26th, Map No. I will represent the sky at\\n11:45 o clock, on November 30th at 11:30 o clock, and on December 2d\\nat 11:15 o clock.\\nIf the maps are held with the centre overhead and the\\ntop pointing to the north, the lower part of the map will be\\nFrom the publications of the Astronomical Society of the Pacific,\\n1898.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0453.jp2"}, "452": {"fulltext": "426\\nASTRONOMY.\\nto the south, the right-hand portion will be to the west, and\\nthe left-hand to the east, and the circle bounding the map\\nwill represent the horizon. Each map is intended to show\\nthe whole of the sky visible at these times.\\nThe names of the constellations are inserted in capitals,\\nwhile the names of stars and other data are in small letters.\\nConstellations on the meridian about midnight\\nJanuary Camelopardus, Lynx, Gemini, Monoceros, Orion, Canis\\nmajor.\\nFebruary Ursa major, Lynx, Cancer, Hydra.\\nMarch Ursa major, Leo, Hydro,.\\nApril: Bootes, Libra.\\nMay: Hercules, Ophiuchus, Scorpio.\\nJune Lyra, Hercules, Sagittarius.\\nJuly: Cygnus, Aquila, Sagittarius.\\nAugust: Cepheus, Cygnus, Capricornus.\\nSeptember: Cepheus, Pegasus, Aquarius.\\nOctober: Cassiopeia, Andromeda, Pisces.\\nNovember: Perseus, Aries, Getus.\\nDecember: Camelopardus, Taurus, Orion", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0454.jp2"}, "453": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0455.jp2"}, "454": {"fulltext": "180 165 150 135 120 105 90\\n30\\nIS\\nr\\nJ.wrig a\\nlv...-\\nZ e o\\n*9\\nCan cer\\n05J.\\nGemini\\na**\\n5 9\\n/5\\nCanis\\n*fi\\nv #1\\n/?*j\\nSextans\\nminor\\n1 j\\n15\\n6\\nCrater\\nH y d 7\\na\\nJ/o ?!OC\\neros\\n6\\n4*\\n7* j\\nM\\nCanis */3\\nLepn^\\n6 /S\\n30\\nHydra\\nNa vi s\\nK\\nwi a j r\\ny+\\nj.\\nA\\nColumba\\nII XI X IX VIII VII VI\\n360 315 330 315 300 285 270\\n-t-\\n30\\n4-\\n15\\nIS\\n30\\n7T\\nCy gnu s\\n*y\\nV\\n*,9\\nPegasus\\nh\\nr u lp eculd\\nDelphinus\\ny*\u00c2\u00b0-\\nSa-* gitta\\nV r a\\n\u00e2\u0096\u00a01\\nX t\\n6\\nHe r c\\nw\\nt +0\\nPisces *y\\n.fi\\nEquul\\nVU8\\ni\\ny\\nMiuila\\nB^\\nP\\n*8\\n*9\\n1\\ntp.\\\\\\n9*\\n+T t\\nV 0*\\nScutum\\ni Op\\nv\\n7T\\nCapri\\n*i\\n*i\u00c2\u00bb\\nco rnus\\n\u00e2\u0080\u00a21\\n\u00e2\u0096\u00a0Sagi\\nA\\nt t ariusX\\nx ff 8\\ny\\nPisces austrinus\\nx\\nIV XXIII XXII XXI XX XIX XVIII\\nFig. 213 .-Map of Star^ between 30 Ifortli\\n^to 22\u00c2\u00bb 3?", "height": "3582", "width": "2296", "jp2-path": "elementaryastron00hold_0456.jp2"}, "455": {"fulltext": "75 CO 45 30 15 345\\nl *Pe r sc u s\\nTriangulum\\n7f\\n30\\n15\\n15\\nSO\\nTaurus\\ne\\na* V\\n-^a +y\\nv..\\nay\\ne\\nAries\\n7\\nv\\nIPisces\\n0* j.\\ny\\ns\\nPeg asut\\n7T\\nauruso\\na\\n*y\\n+7\\n6\\nt +0\\nPisces\\ny\\nk\\no\\n1\\ny*\\nv i\\ne\\nA qu arius\\nZJrit\\nanus\\nT T\\nT\\nT\\nT\\nGetu\\nS\\nV 2+\\nIV III II 1 XXIV XX\\nII\\n255 240 225 210 195 180 105\\ne i- c i^ A\\nP\\nf*\\n30\\n15\\n15\\n30\\n5\\nl DC\\ny\\ny\\nBootes\\nIT\\nBe\\nV 4\\n1\\nComa\\nr en ice s\\n-5* 0*\\nX\\nA+T\\nto\\nT\\ne o\\n8\\ni\\nchus\\nj H\\nj*\u00c2\u00a3\\n/S\\na\\nFir gr o\\na\\ne\\ne\\nCrater*\\nw\\n9 j\\nS cor plus\\n#s\\n7T\\na*\\ny\\n7r.\\nCentaurus\\ny*\\nCorvus 6\\ni\\ny\\ndr a\\nT\\n1/S\\nTc-n-i\\n1 w\\nXVII XVI XV XIV XIII XII X\\n30 South Declination.\\n19 var. cluster\\n(Henry Sol t Co..2iswTofk)", "height": "3582", "width": "2296", "jp2-path": "elementaryastron00hold_0457.jp2"}, "456": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0458.jp2"}, "457": {"fulltext": "MAPS OF THE STABS.\\n427\\nMAP I.\\nNorth.\\nLrf\\n*-s\\n\u00e2\u0080\u00a2\u00e2\u0096\u00a0\u00e2\u0096\u00a0z\\n4\\n,j*P\\n/Z*\\n*\u00c2\u00abU\\n*4b;wi g\\n*t\\nfe s\\n*i\\nV\\ntec\\n\u00c2\u00ab?4\\n^J;\\n%j\\ns\\ni\\n^t*-*...V**t--,\\nSouth.\\nFig. 214.\\nThe sky on November 22, at 12 o clock p.m.\\nDecember 6, at 11 o clock p.m.\\nDecember 21, at 10 o clock p.m.\\nJanuary 5, at 9 o clock p.m.\\nJanuary 20, at 8 o clock p.m.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0459.jp2"}, "458": {"fulltext": "428\\nASTRONOMY.\\nMAPH.\\nNorth.\\nW\\nrS W\\n^Ayj\\nV--\\nA\\nS *\u00c2\u00abN ,fc\\nV!.ON\u00c2\u00bb\\\\AVSUft\\nA\\nS 3l\\n\u00e2\u0096\u00a0Lto\\nv..\\nCwA\u00c2\u00abj\\nI,\\n2f\\n\u00e2\u0096\u00a0H\\n*s\\nGov y,\\nCAMS\\n\u00e2\u0080\u00a2p\\nt\\nJkf\\nfOt^o^.l\\nOTUON\\njS.f oce\\nfc\\n.,3,\\n1 v\\n^Z MA JO*\\nSouth.\\nFig. 215.\\nThe sky on January 20, at 12 o clock p.m.\\nFebruary 4, at 11 o clock p.m.\\nFebruary 19, at 10 o clock p.m.\\nMarch 6, at 9 o clock p.m.\\nMarch 21, at 8 o clock p.m.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0460.jp2"}, "459": {"fulltext": "MAPS OF THE STARS.\\n429\\nMAP III.\\nNorth,\\n12--*\\n.V\\nC\\nfs\\n\u00c2\u00abn Sl\\n^W\\n\u00e2\u0080\u00941\\nv.-^\\nOYiUSE-\\ni\\ncaV.o..\\nr\u00c2\u00ab-^\\n\u00e2\u0080\u00a2S U\\n\\\\**1\\n*4A %a\\noi:\\nDC\\nS f\\nr ...V\\nf\u00c2\u00bb 5\\nW*\\n.3*\\ntf\u00c2\u00bb*\\n.X^V\\nSouth.\\nFig. 216.\\nThe sky on March 21, at 12 o clock p.m.\\nApril 5, at 11 o clock p.m.\\nApril 20, at 10 o clock p.m.\\nMay 5, at 9 o clock p.m.\\nMay 21, at 8 o clock p.m.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0461.jp2"}, "460": {"fulltext": "430\\nASTRONOMY.\\nMAP IV.\\nNorth.\\nt^\\n3*H*\\niiiw\\n^i\\n*W\\n1\\nv\\nA\\n4*\\nV^\\n-O\\nS*\\n*_A\\ntyv\\nSCO),,\\nSouth.\\nFig. 217.\\nThe sky on May 21. at 12 o clock p.m.\\nJune 5, at 11 o clock p.m.\\nJune 21, at 10 o clock p.m.\\nJuly 7, at 9 o clock p.m.\\nJuly 22, at 8 o clock p.m.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0462.jp2"}, "461": {"fulltext": "MAPS OF THE STABS.\\n431\\nMAP V.\\nNorth.\\nV a\\nvnmnv\\n\u00c2\u00b0u *\u00c2\u00bb3 J\\nfca\\nsj^Vwio\\nW\\nV,fe\\n_^\\ngl#*\\nt\\nr*f\\nVy\\nV*\\nv\\nVion,,\\n5*\\n\u00c2\u00abAS\\nfi\\nA*\\n1 *\u00c2\u00ab*J\\n\\\\W\\n.*;v\\n*A\\n8?\\nfa.\\nv*\\nV\\n\u00e2\u0080\u00a2V\\ne Miua*\\nCA ?*icon\\nNWS c,^\\nSouth.\\nFig. 218.\\nThe sky on July 22, at 12 o clock p.m.\\nAugust 7, at 11 o clock p m.\\nAugust 23, at 10 o clock p.m.\\nSeptember 8, at 9 o clock p.m.\\nSeptember 23, at 8 o clock p.m.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0463.jp2"}, "462": {"fulltext": "432\\nASTRONOMY.\\nMAP VI.\\nNorth.\\nV\\nA\\nc?\\nr\\n1 i,5 V 5 awj\u00e2\u0080\u009e\\n\u00e2\u0080\u00a2\u00e2\u0080\u00a2\u00e2\u0080\u00a2f .4\u00e2\u0080\u00944\\nSo,\\nH 4J,.\\ng ^Jv\\n\u00c2\u00bb^3\\nV\u00e2\u0080\u0094\\n*K\\n\u00e2\u0080\u00a2a*\\n?V\\nC\\nKv\\nJto*\u00c2\u00ab\\nSouth.\\nFra. 219.\\nThe sky on September 23, at 12 o clock p.m.\\nOctober 8, at 11 o clock p.m.\\nOctober 23, at 10 o clock p. m.\\nNovember 7, at 9 o clock p.m.\\nNovember 22, at 8 o clock p.m.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0464.jp2"}, "463": {"fulltext": "APPENDIX.\\nSPECTRUM ANALYSIS.\\nAlthough the subject of Spectrum Analysis belongs\\nproperly to physics, a brief account of its relations to\\nastronomy may be useful here.\\nTo understand the instruments and methods of Spectrum\\nAnalysis it will be necessary to recall the optical properties\\nof a prism, which are demonstrated in all treatises on phys-\\nics.\\nThe Prism. When parallel rays of homogeneous light, red for ex-\\nample, fall on a face of a prism they are bent out of their course, and\\nwhen they emerge from the prism they are again bent, but they still\\nremain parallel; thus the rays rr, r r are bent into the final di-\\nrection r r This is true for parallel rays of every color. They re-\\nmain parallel after deviation by the prism. This can be shown by\\nexperiment. If the incident rays r r, in Fig. 220, are red, they will\\ncome to the screen at r r If they are violet rays, they will come to\\nv v on the screen, after having been bent more from their original\\ncourse than the red rays. The violet rays, with the shortest wave-\\nlength, are the most refrangible. The red, with the longest wave-\\nlength, are the least refrangible.\\nThe experiments of Sir Isaac Newton (1704) proved\\nthat white light (as sunlight, moonlight, starlight) was not\\nsimple, but compound. That is, white light is made up of\\nlight of different wave-lengths. Difference of wave-length\\nshows itself to the eye as difference of color. Seven colors\\nwere distinguished by Newton; viz., violet, indigo, blue,\\ngreen, yellow, orange, red. (Memorize these in order. It\\nis the order of the colors in the rainbow.) If parallel rays\\nof white light, as sunlight, r r, fall on a prism, the red rays\\n433", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0465.jp2"}, "464": {"fulltext": "434 APPENDIX.\\nof this beam will still fall at r r\\\\ and the violet rays will\\nfall at v v Between v and r the other rays will fall, in\\nthe order just given; that is, in the order of their refrangi-\\nbility. The rainbow-colored streak on the screen is called\\nthe spectrum; it is a solar, a lunar, or a stellar spectrum\\naccording as the source of the rays is the Sun, Moon, or a\\nFig. 220.\u00e2\u0080\u0094 The Action op a Prism on a Beam of White Light.\\nstar. The solar spectrum is very bright; the lunar spec-\\ntrum is much fainter; and the spectrum of a star is far\\nfainter than either.\\nIf we let parallel rays, r r, of red light come through a circular\\nhole at Q (Fig. 220), they will form a circular image of the hole at\\nr r If the hole is square or triangular, a square or a triangular\\nimage will be formed. If it is a narrow slit, a narrow streak of red\\nlight will be projected at r r\\nWhen wliite light is passed through a circular hole at Q, circular\\nimages of the hole are formed all along the line r r to v v the red\\nimages at r r the orange, yellow, green, blue images in succession,\\nand the violet image at v v If the hole is of any size these images\\nwill overlap, so that the colors are not pure. If white light falls", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0466.jp2"}, "465": {"fulltext": "SPECTRUM ANALYSIS.\\n435\\nthrough a narrow slit at Q, placed parallel to the edge A of the\\nprism, the purest spectrum is obtained. The different spectra do not\\noverlap.\\nFraunhofer tried this experiment in 1804, and he\\nfound that the spectrum of the Sun was interrupted by cer-\\ntain dark lines, fixed in relative position. These are the\\nFraunhofer lines, so called. He made a map of the solar\\nspectrum, and on the map he placed the various lines in\\ntheir proper places. These lines appear in the same rela-\\ntive position no matter whether a slit or a very small cir-\\nFig. 221. The Spectroscope.\\ncular hole is used, and they belong to the incident light\\nand are not produced by the apparatus. This simply ren-\\nders them visible. They are not seen when the light comes\\nthrough wide apertures, on account of the overlapping of\\nthe various images. (See Fig. 222.)\\nThe Spectroscope. A spectroscope consists essentially of\\none or more prisms (or any other device, as a diffraction\\ngrating) by means of which a spectrum is produced; of a\\nmeans to make the spectrum pure (a slit and collimator),\\nand of a means to see it well (a small telescope).", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0467.jp2"}, "466": {"fulltext": "436\\nAPPENDIX.\\nFig. 221 shows the arrangement of a one-prism spectro-\\nscope. The light enters the slit 8, which is exactly in the\\nfocus of the objective A of the collimator. The rays there-\\nfore emerge from A in parallel lines. They are deviated\\nby the prism P, and enter the objective B, forming an\\nimage of the spectrum at 0, which is viewed by the eye\\nat E.\\nThe Solar Spectrum. Part of this image (of the solar\\nspectrum) is shown in Fig. 222, except as to color. The\\n1 1\\nI\\n111\\nIII\\n111 HI\\n111\\nin\\nIII ill\\n1\\n11\\nII\\nI\\nalili\\nill\\n1\\nIII\\n|1 1\\nHill\\nA a B C D El F\\nFig. 222.\u00e2\u0080\u0094 A Part op the Solar Spectrum.\\nvarious colors extend in succession from end to end of the\\nspectrum. In each color are certain dark lines which have\\na definite position. The most conspicuous of these lines\\nare called the Fraunhofer lines, and are lettered A, B, C,\\nD, E, F, G, H. A is below the easily visible red, B is at\\nits lower edge, C is near the middle of the red, D is a double\\nline in the orange, Eis in the green, i^is in the blue, G in the\\nindigo, and H in the violet. There are at least 500 lines\\nbesides which can be seen with spectroscopes of moderate\\npower. Each and every one of these has a definite position.\\nWhen the instrument drawn in Fig. 221 is pointed toward the Sun\\n(so that the Sun s rays fall on 8), the spectrum seen is that of the\\nwhole Sun. If we wish to examine the spectrum of a part of the\\nSun, as of a spot for example, we must attach the whole instrument\\nto a telescope, so that S is in the principal focal plane of the tel-\\nescope-objective. An image of the Sun will then be formed by", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0468.jp2"}, "467": {"fulltext": "SPECTRUM ANALYSIS. 437\\nthe telescope-objective on the slit plate S, and the light from any\\npart of that image can be examined at will. The spectroscope is also\\nused in order to examine stars. We employ a telescope in this case\\nso that its objective may collect more light and present it at the slit\\nof the spectroscope.\\nSpectra of Solids and Gases. A solid body, heated so\\nintensely as to give off light, has a continuous spectrum.\\nThat is, there are no Fraunhofer lines in it, but prismatic\\ncolors only. A gaseous body, heated so intensely as to give\\noff light, has a discontinuous spectrum.* That is, the\\ncolors red to violet are no longer seen, but on a dark back-\\nground the spectrum shows one or more bright lines.\\nThese lines have a definite relative position and are char-\\nacteristic of the particular gas. The vapor of sodium, for\\nexample, gives two bright lines, whose relative position is\\nalways the same, as laboratory experiments show.\\nIf the source of light is a solid body, intensely heated,\\nthe spectroscope will show a continuous spectrum without\\nlines, as has been said. If between the solid body and the\\nslit of the spectroscope we place a glass vessel containing\\nthe vapor of sodium, the spectrum will no longer be without\\nlines. Two dark lines will appear in the orange. If we\\nremove the vapor of sodium, the lines will go also. They\\nare produced by the absorptive action of this vapor on the\\nincident light.\\nIf we register exactly the spot in the field of view of the\\nspectroscope where each of these dark lines appears, and if\\nwe then remove the sodium vapor and replace the solid\\nbody (the source of light) by intensely heated sodium vapor,\\nwe shall find the new spectrum to be composed of two\\nbright lines, as has been said and these two bright lines\\nwill occupy exactly the same places in the field of view that\\nthe two dark lines formerly occupied.\\nUnless under great pressure, when the spectrum is continuous, as\\nin the case of our Sun, and of stars of similar constitution to the Sun.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0469.jp2"}, "468": {"fulltext": "438 APPENDIX.\\nThe two dark lines are a sign of the kind of light that is\\nabsorbed by sodium vapor the two bright lines are a sign\\nof the kind of light that is emitted by sodium vapor. These\\ntwo kinds are the same. What is true of sodium vapor is\\ntrue or every gas. Every gas absorbs light of the same hind\\n(wave length) as that which it emits.\\nIf a spectroscopist had to determine what kind of gas was contained\\nin a certain jar, he might do it in two ways. He might heat it in-\\ntensely, and measure the positions of the bright lines of its spectrum;\\nor he might place the gas between the slit of his spectroscope and a\\nhighly heated solid body, and measure the positions of the dark lines\\nof its absorption-spectrum. The positions of the lines will be the\\nsame in both cases. By comparing the measures with previous meas-\\nures for known gases, the name of the particular gas in question\\nwould become known to him. New chemical elements have been\\ndiscovered by the spectroscope. The spectrum of the mixture that\\ncontained them showed previously unknown spectrum lines. They\\nwere first detected by the presence of these unknown lines and then\\nseparated from the known gases present in the mixture.\\nComparison of the Spectra of Incandescent Gases with\\nthe Solar Spectrum. Laboratory experiments on known\\ngases show the positions of the spectral lines characteristic\\nof each gas or vapor. The positions of the lines of magne-\\nsium or of hydrogen, for example, are accurately known.\\nThe positions of the dark lines in the solar spectrum are\\nalso known with accuracy. It is found that nearly every one\\nof the thousands of dark lines of the solar spectrum has a\\nposition corresponding exactly to that of some one of the\\nlines of some known gas or of the vapor of some known\\nmetal. For example, the vapor of iron has several hundred\\nlines, whose positions are accurately known by laboratory\\nexperiments. In the solar spectrum there are several\\nhundred whose positions precisely correspond to the lines\\nof iron vapor. The same is true of many other substances,\\nhydrogen, sodium, potassium, magnesium, nickel, copper,\\netc., etc.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0470.jp2"}, "469": {"fulltext": "SPECTR KM ANAL TS18. 439\\nFrom this it is inferred that the Sun s atmosphere con-\\ntains the metal iron in an incandescent state, as well as the\\nvapors of the other substances named.\\nLet us see the process of reasoning which led Kirchhoff and\\nBunsen (1859) to this interpretation of the observation.\\nWe have seen (Part II., Chap. XVI) that the Sun is composed of a\\nluminous surface, the pholosp7iere, surrounded by a gaseous envelope.\\nThe photosphere alone would give a continuous spectrum (with no\\ndark lines). The gaseous envelope will absorb the kind of light that\\nit would itself emit. The absorption is characteristic. If a solid in-\\ncandescent body were placed in a laboratory and surrounded by the\\nvapors of iron, hydrogen, sodium, etc., we should see the same spec-\\ntrum that we do see when we examine the Sun.\\nThe kind of evidence is easily understood from the foregoing. Only\\nthe spectroscopist can fully appreciate the force of it. The resulting\\ninference that the Sun s atmosphere contains the vapors of the metals\\nnamed is certain. These vapors exist uncombined in the Sun s atmos-\\nphere. The temperature and the pressure are too high to allow their\\nchemical combination.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0471.jp2"}, "470": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0472.jp2"}, "471": {"fulltext": "INDEX.\\nAberration of light, 257.\\nAchromatic telescope described,\\n121.\\nAdams s work on perturbations\\nof Uranus, 342.\\nAlgol (variable star), 388.\\nAltitude of a star defined, 81.\\nAnaximander, (b. c. 610), 6.\\nAnaxagoras (b. c. 500), 6.\\nAngles, 22.\\nAnnular eclipses of the Sun. 230.\\nApex of solar motion, 381.\\nApparent motion of the Sun, 154.\\nApparent motion of a planet, 180.\\nApparent time, 90.\\nARCHIMEDES, (B. C. 287), 7.\\nAristotle, (b. c. 384), 7.\\nAsteroids, 322.\\nAstronomical instruments (in\\ngeneral), 112.\\nAstronomy (defined), 1.\\nAtmosphere of the Moon, 317.\\nAtmospheres of the planets. See\\nMercury, Venus, etc.\\nAzimuth defined, 81.\\nBarnard discovers satellite of\\nJupiter, 325.\\nBessei/s parallax of 61 Cygni\\n(1837), 383.\\nBinary stars, 391.\\nBond s discovery of the dusky\\nring of Saturn, 1850, 336.\\nBooks (a list of), 416.\\nBouvard on Uranus, 341.\\nBradley discovers aberration in\\n1729 256.\\nBun sen, 439.\\nCalendar, 247.\\nCassini discovers four satellites\\nof Saturn (1684-1671), 339.\\nCatalogues of stars, 376.\\nCelestial globe, 74.\\nCelestial photography, 145, 415.\\nCelestial sphere, 18.\\nCentre of gravity of the solar\\nsystem, 275.\\nChange of the Day, 101.\\nChronology, 247.\\nChronometers, 115.\\nClocks, 112.\\nClusters of stars, 393.\\nComets, 357.\\nComets orbits, 361.\\nComets tails, repulsive force,\\n363.\\nConjunction (of a planet with the\\nSun) defined, 183.\\nConstellations, 371.\\nConstruction of the heavens, 369.\\nCo-ordinates of a star, 77.\\n441", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0473.jp2"}, "472": {"fulltext": "442\\nINDEX.\\nCOPERNICUS; 8, 191.\\nCosmogony, 407.\\nCorona of the Sun, 282, 290.\\nDark stars, 389.\\nDay, how subdivided into hours,\\netc., 83.\\nDays, mean solar and solar, 90.\\nDeclination of a star defined, 30.\\nDistance of the fixed stars, 381.\\nDistribution of the stars, 371.\\nDiurnal motion, 41, 59.\\nDonati s comet (1858), 358.\\nDouble (and multiple) stars,\\n390.\\nEarth, general account of, 232.\\nEarth s density, 238.\\nEarth s dimensions, 234.\\nEarth s mass, 237.\\nEclipses of the Moon, 224.\\nEclipses of Sun and Moon, 222.\\nEclipses of the Sun, explanation,\\n228.\\nEclipses of the Sun, physical\\nphenomena, 289.\\nEclipses, their recurrence, 230.\\nEcliptic defined, 161.\\nElements of the orbits of the\\nmajor planets, 276.\\nElongation (of a planet), 183.\\nEncke s comet, 367.\\nEpicycles, 190.\\nEquation of time, 150.\\nEquator (celestial) defined, 30.\\nEquatorial telescope, 133.\\nEquinoxes, 160, 163.\\nEratosthenes, (b. c. 276), 7.\\nEyepieces of telescopes, 121.\\nFabritius observes solar spots\\n(1611), 285.\\nFigure of the Earth, 232.\\nFrauenhofer s Experiments\\nwith the Prism, 435.\\nFuture of the solar system, 413.\\nGalaxy or milky way, 372.\\nGalileo invents the telescope\\n(1609), 117.\\nGalileo observes solar spots\\n(1611), 285.\\nGalileo s discovery of satellites\\nof Jupiter (1610), 325.\\nGalle observes Neptune (1846).\\n343.\\nGases, spectra of incandescent,\\n437; in meteoric stones, 362.\\nGeodetic surveys, 235.\\nGlobe (celestial), 74.\\nGravitation extends to stars, 392.\\nGravitation resides in each par-\\nticle of matter, 209.\\nGravity, terrestrial, 204, 237.\\nGregorian calendar, 247.\\nHalley predicts the return of a\\ncomet (1682), 363.\\nHall s discovery of satellites of\\nMars, 313.\\nHerschel (W.) discovers two\\nsatellites of Saturn (1789), 339.\\nHerschel (W.) discovers two\\nsatellites of Uranus (1787), 340.\\nHerschel W. discovers Uranus\\n(1781), 339.\\nHerschel s catalogues of nebu-\\nlas, 393.\\nHerschel (W.) states .that the\\nsolar system is in motion (1783),\\n381.\\nHerschel s (W.) views on the\\nnature of nebulae, 395.\\nHints on observing, 414.\\nHipparchus (b. c. 160), 7.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0474.jp2"}, "473": {"fulltext": "INDEX.\\n443\\nHorizon (celestial sensible) of\\nan observer defined, 30, 31.\\nHour-angle of a star defined, 78.\\nHuggins first observes the spec-\\ntra of nebulae (1864), 397.\\nHuyghens discovers a satellite\\nof Saturn (1655), 339.\\nHuyghens explanation of the\\nappearances of Saturn s rings\\n(1655), 334.\\nInferior planets defined, 185.\\nJanssen first observes solar\\nprominences in daylight, 291.\\nJulian year, 247.\\nJupiter, 325.\\nKant s nebular hypothesis, 408.\\nKepler s laws enunciated, 198.\\nKirchhoff, 439.\\nLaplace s nebular hypothesis,\\n408.\\nLaplace s investigation of the\\nconstitution of Saturn s rings,\\n338.\\nLassell discovers Neptune s sat-\\nellite (1847), 345.\\nLassell discovers two satellites\\nof Uranus (1847), 340.\\nLatitude of a place on the earth\\ndefined, 26, 59.\\nLatitude of a point on the earth\\nis measured by the elevation of\\nthe pole, 59.\\nLatitudes and longitudes (celes-\\ntial) defined, 164.\\nLatitudes (terrestrial), how deter-\\nmined, 105.\\nLe Verrier computes the orbit\\nof meteoric shower, 355.\\nLe Verrier s work on perturba-\\ntions of Uranus, 342.\\nLight-gathering power of an ob-\\nject-glass, 122.\\nLight-ratio (of stars) is about\\n374.\\nList of bright stars, 417.\\nList of double stars, 419.\\nList of variable stars, 420.\\nList of nebulse and clusters, 421.\\nLocal time, 95.\\nLongitude of a place, 26, 96.\\nLongitude of a place on the\\nearth (how determined), 98.\\nLongitudes (celestial)defined, 164.\\nLucid stars defined, 374.\\nLunar phases, nodes, etc. See\\nMoon s phases, nodes, etc.\\nMagnifying power of an eye-\\npiece, 120.\\nMajor planets defined, 270.\\nMaps of the stars, 423 et seq.\\nMars, 303.\\nMars s satellites discovered by\\nHall (1877), 313.\\nMass of the Sun in relation to\\nmasses of planets, 265.\\nMasses of the stars, 378.\\nMean solar time defined, 90.\\nMercury 299.\\nMeridian (celestial) defined, 34.\\nMeridian circle, 129.\\nMeridian line (established), 152.\\nMeridian (terrestrial) defined, 34.\\nMeteoric showers, 351.\\nMeteoric stones, gases in, 362.\\nMeteors and comets, their rela-\\ntion, 354.\\nMeteors, 347.\\nMicrometer, 141.\\nMilky Way, 372.\\nMinor planets defined, 270.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0475.jp2"}, "474": {"fulltext": "444\\nINDEX.\\nMinor planets, general account,\\n322.\\nMir a Ceti (variable star), 386.\\nModel of a meridian circle, 132.\\nModel of an equatorial, 138.\\nMonths, different kinds, 246.\\nMoon, general account, 315.\\nMoon s light gisViiff \u00c2\u00b0f Sun s, 317.\\nMoon s phases, 216.\\nMoon s parallax, 262.\\nMoon photographs, 320.\\nMoon, spectrum of the, 317.\\nMoon s surface, does it change\\n320.\\nMotion of solar system in space,\\n404.\\nMotion of stars in the line of\\nsight, 401.\\nNadir of an observer denned, 30.\\nNautical almanac described, 150.\\nNebulae and clusters in general,\\n393.\\nNebular hypothesis stated, 407.\\nNeptune, discovery of, by Le\\nVerrier and Adams (1846),\\n341.\\nNeptune, 341.\\nNew stars, 387.\\nNewton (H. A.) on meteors, 355.\\nNewton (I.), The Principia\\n(1687), 8; calculates orbit of\\ncomet of 1680, 361 Spectrum\\nAnalysis experiments, 433,\\nObjectives, or object-glasses, 120.\\nObliquity of the ecliptic, 171.\\nOccultations of stars by the Moon\\n(or planets), 230.\\nOlbers s hypothesis of the origin\\nof asteroids, 323.\\nOld style (in dates), 247.\\nOpposition (of a planet to the\\nSun) denned, 183.\\nParallax (in general) defined, 107.\\nParallax of the Sun, 262.\\nParallax of the stars, general ac-\\ncount, 109.\\nPendulum, 115.\\nPeriodic comets, 363.\\nPenumbra of the Earth s or\\nMoon s shadow, 131.\\nPerturbations, 213.\\nPhotography its use in astron-\\nomy, 145.\\nPhotographic star- charts, 323.\\nPhotosphere of the Sun, 281.\\nPiazzi discovers the first asteroid\\n(1801), 323.\\nPlanets, their relative size ex-\\nhibited, 277.\\nPlanetary nebulae defined, 397.\\nPlanets, their apparent and real\\nmotions, 179.\\nPlanets, their physical constitu-\\ntion, 345.\\nPole of the celestial sphere de-\\nfined, 46.\\nPrecession of the equinoxes, 248.\\nPrism, The, 434.\\nProblem of three bodies, 213.\\nProgressive motion of light, 254,\\n331.\\nProper motion of the sun, 379.\\nProper motions of stars, 379.\\nPtolemy (b. c. 140), 7, 190.\\nPtolemy s system of the world,\\n190.\\nPythagoras (b. c. 582), 6.\\nRadiant point of meteors, 352.\\nRadius vector, 195.\\nReflecting telescopes, 123.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0476.jp2"}, "475": {"fulltext": "INDEX.\\n445\\nRefracting telescopes, 119.\\nRefraction of light in the atmos-\\nphere, 242.\\nResisting medium in space, 367.\\nReticle of a transit instrument,\\n126.\\nRetrogradations of the planets\\nexplained, 187.\\nRight ascension of a star defined,\\n30, 80.\\nRight ascensions of stars, how\\ndetermined by observation, 127.\\nRoemer discovers (1675) that\\nlight moves progressively, 254.\\nSaturn, 331.\\nSeasons, The, 174\\nSextant, 146.\\nSidereal time explained, 83.\\nSidereal year, 246.\\nSigns of the Zodiac, 169.\\nSolar corona, etc. See Sun.\\nSolar heat, its amount, 293.\\nSolar motion in space, 404.\\nSolar parallax, 262.\\nSolar prominences gaseous, 291.\\nSolar system, description, 269.\\nSolar system, its future, 413.\\nSolar temperature, 294.\\nSolar time, 90.\\nSolstices, 162, 163.\\nSpace, 15.\\nSpectroscope, The, 435.\\nSpectrum Analysis, 433.\\nSpectrum Solar corona, 291\\nLunar, 317 Nebulae and Clus-\\nters, 398; Fixed Stars, 400 as\\nindicating motions of stars, 401;\\nSolids and Gases, 437 Solar,\\n436.\\nStandard time (U. S.), 99.\\nStar-clusters, 397.\\nStars had special names 3000\\nb. c, 375; magnitudes, 374;\\nparallax and distance, 381, 382;\\nabout 2000 seen by the naked\\neye, 371; proper motions, 379;\\nspectra, 400.\\nStar-maps, 423 et seq.\\nStruve (W.) determines stellar\\nparallax (1838), 383.\\nSummer solstice, 162.\\nSun s apparent path, 159.\\nSun s atmosphere, 281, 289.\\nSun s constitution, 280.\\nSun-dial, 114.\\nSun s (the) existence cannot be\\nindefinitely long, 413.\\nSun s mass over 700 times that\\nof the planets, 275.\\nSun, physical description, 280.\\nSun s proper motion, 404.\\nSun s rotation-time about 25 days,\\n286.\\nSun, Spectroscopic observations\\nof the, 436.\\nSun-spots and faculse, 285.\\nSun-spots are confined to certain\\nparts of the disk, 286.\\nSun-spots, their periodicity,\\n287.\\nSuperior planets (defined), 185\\nSwedenborg s nebular hypothe-\\nsis, 408.\\nTelescopes, 119.\\nTelescopes (reflecting), 123.\\nTelescopes (refracting), 119.\\nTempel s comet, its relation to\\nNovember meteors, 354.\\nTemporary stars, 386.\\nThales (b. c. 640), 5.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0477.jp2"}, "476": {"fulltext": "446\\nINDEX.\\nTides, 219.\\nTime, 83, 94.\\nTotal solar eclipses, description\\nof, 289.\\nTrails (of stars), 51, 52.\\nTransit instrument, 124.\\nTransits of Mercury and Venus,\\n303.\\nTransits of Venus, 264.\\nTriangulation, 235.\\nTwilight, 243.\\nTycho Brahe observes new star\\nof 1572, 387.\\nUnits of mass and distance,\\n260.\\nUniversal gravitation discovered\\nby Newton, 214.\\nUranus, 339.\\nVariable and temporary stars,\\n386.\\nVariable stars, theories of, 387.\\nVelocity of light, 255.\\nVenus, 300.\\nVernal equinox, 160.\\nWeight of a body defined, 237.\\nWinter solstice, 163.\\nYears, different kinds, 246.\\nZenith denned, 30.\\nZodiac, 169.\\nZodiacal light, 356.", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0478.jp2"}, "477": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0479.jp2"}, "478": {"fulltext": "DEC 13 1899", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0480.jp2"}, "479": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0481.jp2"}, "480": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0482.jp2"}, "481": {"fulltext": "", "height": "3558", "width": "2206", "jp2-path": "elementaryastron00hold_0483.jp2"}, "482": {"fulltext": "Hflr\\nfM\\n1WMI\\nWD\\njBHDPBPPgfl|\\nKjHfi\\nm", "height": "3705", "width": "2386", "jp2-path": "elementaryastron00hold_0484.jp2"}}