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^V THE SAME AUTHOR.IN STARRY REALMS.

yth Thousand, Demy 8vo, gilt edges, 7s. 6d.

IN THE HIGH HEAVENS.5th Thousand, Demy 8vo, gilt edges, 7s. 6d.


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GREAT ASTRONOMERS

BY

Sir ROBERT S. BALL D.Sc. LL.D. F.R.S.LOAVNDEAN PROFESSOR OF ASTRONOMY AND GEOMETRY IN THE

UNIVERSITY OF CAMBRIDGEAUTHOR OF "IN STARRY REALMS'' "IN THE HIGH HEAVENS"

ETC.

WITH NUMEROUS ILLUSTRATIONS

LONDONISBISTER AND COMPANY Limited15 & 16 TAVISTOCK STREET COVENT GARDEN

PHILADELPHIA: J. B. LIPPINCOTT COMPANY1895


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PREFACE.It has been my object in these pages to present thelife of each astronomer in such detail as to enable thereader to realise in some degree the man's character andsurroundings ; and I have endeavoured to indicate asclearly as circumstances would permit the main featuresof the discoveries by which he has become known.

There are many types of astronomersfrom the star-gazer who merely watches the heavens, to the abstractmathematician who merely works at his desk; it has,consequently, been necessary in the case of some lives toadopt a very different treatment from that which seemedsuitable for others.

"While the work was in progress, some of the sketchesappeared in Good Words. The chapter on Brinkley hasbeen chiefly derived from an article on the " History ofDunsink Observatory," which was published on the occa-sion of the Tercentenary celebration of the University ofDublin in 1892, and the life of Sir William EowanHamilton is taken, with a few ^alterations and omissions,from an article contributed to the Quarterly Review on


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viii PREFACE.Grraves' life of the great mathematician. The remainingchapters now appear for the first time. For many of thefacts contained in the sketch of the late Professor Adams,I am indebted to the obituary notice written by my friendDr. J. W. L. Glaisher, for the Royal Astronomical Societywhile with regard to the late Sir George Airy, I have asimilar acknowledgment to make to Professor H. H.Turner. To my friend Dr. Arthur A. Rambaut I owemy hearty thanks for his kindness in aiding me in therevision of the work.

R. S. B.The Obseevatoey, Cambridge.

October, 1895.


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CONTENTS.

Inteodtjction 1Ptolemy 7copeenicus 30Tycho Beahe 44Galileo 07Kepler 96Isaac Newton 116Flamsteed 147Halley . . . . . . . . . . . .162Beadley . . . . . . . . . . .187WlLLLA-M HeESCHEL 200Laplace 219Beinkley 233John Heeschel 247The Eael of Rosse 272AiEY 289Hamilton 303Le Veeeiee 335APAMg ,,,,,,,,,,,, 354


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LIST OF ILLUSTRATIONS.PAGBThe Oeservatoey, Q-eeenwich Frontispiece

Ptolemy 9Ptolemt's Planetaey Scheme 24,, Theoey op the Movement op Maes .... 26

Thorn, feom an Old Print 32Copernicus 33rRAUENBURG, FROM AN OlD PrINT 37Explanation of Planetary Movements . . . . . .41Tycho Brahe 47Tycho's Cross Staff .50

"New Star" Sextant of 1572 51,, Trigonio Sextant 52,, Astronomio Sextant ... .... 53,, Equatorial Armlllary 54.The Great Augsburg Quadrant 55

Tycho's "New Scheme of the Terrestrial System," 1577 . 56Uraniboeg and its Grounds 57Ground-Plan of the Observatory 5gThe Observatory of Ueaniborg, Island of Hven ... 59Effigy on Tycho's Tomb at Prague. By permission of Messrs.

A. & C. Black gjTycho's Mural Quadeant, Ueaniboeg 53Galileo's Pendulum ^0Galileo ^3The Villa Aecetei 32Facsimile Sketch of Lunar Surface by Galileo . . .87Crest of Galileo's Family 94Kepler's System of Regular Solids 100Kepler ............ 103Symbolical Eepresentation qts the Planetary System . .106


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Xll LIST OF ILLUSTRATIONS,PAGEThe Commemoeation of the Rudolphine Tables . . .111

"WooLSTHOEPE Manor . . .117Trinity College, Cambeidge 121Diagram of a Sunbeam . . ~ 123Isaac Newton 126Sir Isaac Newton's little Eeflectoe 129

,, ,, Sun-Dial 132,, ,, Telescope 135,, ,, Astrolabe 138,, Sun-Dial in the Royal Society . . 146

Flamsteed's House . . 151Flamsteed 155Halley 167Greenwich Obseevatoey in Halley's Time . . . .1857, New Kjng Steeet, Bath. From a Photo, by John Poole, Bath 204William Heeschel 206Caroline Herschel 207Street View, Heeschel House, Slough \ From / 209Garden View, " f Photographs by I 212Observatory, ,, " ( "^^^^ ^ Saunders, j 216The 40-Foot Telescope, ,, j Eton \ 217Laplace 227The Observatory, Dunsink. From a Photograph by W. Law-

rence, Dublin 239ASTEONOMETER MADE BY SlE JOHN HeRSCHEL .... 249Sir John Herschel 253Nebula in Southern Hemisphere 256The Cluster in the Centaur . . . . . . .259Obseevatory at Feldhausen 262Granite Column ,, 265The Earl or Rosse 273Birr Castle. From a Photograph by W. Lawrence, Dublin . 275The Mall, Paesonstown. ,, . 277LoED Rosse's Telescope. ,, . 281Roman Catholic Chuech, Paesonstown. ,, . 285AiEY. From a Photograph by E. P. Adams, Greenwich . . 293Hamilton 313Adams 357The Obsejiyatort, Cambridge . , i i , , , 3QQ


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INTEODUCTIOKOf all the natural sciences there is not one which offerssuch sublime objects to the attention of the inquirer asdoes the science of astronomy. From the earliest ages thestudy of the stars has exercised the same fascination asit possesses at the present day. Among the most primitivepeoples, the movements of the sun, the moon, and thestars commanded attention from their supposed influenceon human affairs.The practical utilities of astronomy were also obvious

in primeval times, Maxims of extreme antiquity showhow the avocations of the husbandman are to be guidedby the movements of the heavenly bodies. The positionsof the stars indicated the time to plough, and the time tosow. To the mariner who was seeking a way across thetrackless ocean, the heavenly bodies offered the onlyreliable marks by which his path could be guided. Therewas, accordingly, a stimulus both from intellectual curiosityand from practical necessity to follow the movements ofthe stars. Thus began a search for the causes of theever-varying phenomena which the heavens display.Many of the earliest discoveries are indeed prehistoric.

The great diurnal movement of the heavens, and theB


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2 GREAT ASTRONOMERS.annual revolution of the sun, seem to have been known intimes far more ancient than those to which any humanmonuments can be referred. The acuteness of the earlyobservers enabled them to single out the more importantof the wanderers which we now call planets. They sawthat the star-like objects, Jupiter, Saturn, and Mars, withthe more conspicuous Yenus, constituted a class of bodieswholly distinct from the fixed stars among which theirmovements lay, and to which they bear such a superficialresemblance. But the penetration of the early astro-nomers went even further, for they recognized thatMercury also belongs to the same group, though thisparticular object is seen so rarely. It would seem thateclipses and other phenomena were observed at Babylonfrom a very remote period, while the most ancient recordsof celestial observations that we possess are to be foundin the Chinese annals.The study of astronomy, in the sense in which we under-

stand the word, may be said to have commenced underthe reign of the Ptolemies at Alexandria. The mostfamous name in the science of this period is that ofliipparchus, who lived and w^orked at Ehodes about theyear 160 B.C. It was his splendid investigations that firstwrought the observed facts into a coherent branch ofknowledge. He recognized the primary obligation whichlies on the student of the heavens to compile as completean inventory as possible of the objects which are there to befound. Hipparchus accordingly commenced by undertak-ing, on a small scale, a task exactly similar to that on whichmodern astronomers, with all available appliances of meri-dian circles, and photographic telescopes, are constantly


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INTRODUCTION, 3engaged at tlie present day. Pie compiled a catalogueof the principal fixed stars, whicli is of special value toastronomers, as being the earliest work of its kind which hasbeen handed down. He also studied the movements of thesun and the moon, and framed theories to account forthe incessant changes which he saw in progress. He founda much more difficult problem in his attempt to interpretsatisfactorily the complicated movements of the planets.With the view of constructing a theory which should givesome coherent account of the subject, he made manyobservations of the places of these wandering stars. Howgreat were the advances which Hipparchus accom-plished may be appreciated if we reflect that, as a pre-liminary task to his more purely astronomical labours, hehad to invent that branch of mathematical science bywhich alone the problems he proposed could be solved.It was for this purpose that he devised the indis-pensable method of calculation which we now know sowell as trigonometry. Without the aid rendered by thisbeautiful art it would have been impossible for any reallyimportant advance in astronomical calculation to have beeneffected.But the discovery which shows, beyond all others, that

Hipparchus possessed one of the master-minds of all timewas the detection of that remarkable celestial movementknown as the precession of the equinoxes. The inquirywhich conducted to this discovery involved a most pro-found investigation, especially when it is remembered thatin the days of Hipparchus the means of observation of theheavenly bodies were only of the rudest description, andthe available observations of earlier dates were extremely


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4 GREAT ASTRONOMERS.scanty. We can but look with astonishment on thegenius of the man who, in spite of such difficulties, wasable to detect such a phenomenon as the precession, andto exhibit its actual magnitude. I shall endeavour toexplain the nature of this singular celestial movement,for it may be said to offer the first instance in the historyof science in which we find that combination of accurateobservation with skilful interpretation, of which, in thesubsequent development of astronomy, we have so manysplendid examples.The word equinox implies the condition that the night

is equal to the day. To a resident on the equator thenight is no doubt equal to the day at all times in theyear, but to one who lives on any other part of the earth,in either hemisphere, the night and the day are notgenerally equal. There is, however, one occasion inspring, and another in autumn, on which the day andthe night are each twelve hours at all places on the earth.When the night and day are equal in spring, the pointwhich the sun occupies on the heavens is termed thevernal equinox. There is similarly another point inwhich the sun is situated at the time of the autumnalequinox. In any investigation of the celestial movementsthe positions of these two equinoxes on the heavens areof primary importance, and Hipparchus, with the instinctof genius, perceived their significance, and commenced tostudy them. It will be understood that we can alwaysdefine the position of a point on the sky with referenceto the surrounding stars. No doubt we do not see thestars near the sun when the sun is shining, but they arethere nevertheless. The ingenuity of Hipparchus enabled


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INTRODUCTION. Shim to determine the positions of each, of the two equi-noxes relatively to the stars which lie in its immediatevicinity. After examination of the celestial places ofthese points at different periods, he was led to theconclusion that each equinox was moving relatively tothe stars, though that movement was so slow that twenty-five thousand years would necessarily elapse beforea complete circuit of the heavens was accomplished.Hipparchus traced out this phenomenon, and establishedit on an impregnable basis, so that all astronomers haveever since recognised the precession of the equinoxes asone of the fundamental facts of astronomy. Not untilnearly two thousand years after Hipparchus had madethis splendid discovery was the explanation of its causegiven by Newton.From the daj^s of Hipparchus down to the present hour

the science of astronomy has steadily grown. One greatobserver after another has appeared from time to time,to reveal some new phenomenon with regard to thecelestial bodies or their movements, while from time totime one commanding intellect after another has arisento explain the true import of the facts of observations.The history of astronomy thus becomes inseparable fromthe history of the great men to whose labours itsdevelopment is due.

In the ensuing chapters we have endeavoured to sketchthe lives and the work of the great philosophers, bywhose labours the science of astronomy has been created.We shall commence with Ptolemj^, who, after the founda-tions of the science had been laid by Hipparchus, gaveto astronomy the form in which it was taught throughout


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6 GREAT ASTRONOMERS.tlie Middle Ages. We shall next see the mighty revo-lution in our conceptions of the universe which areassociated with the name of Copernicus. We then passto those periods illumined by the genius of Galileo and]^ewton, and afterwards we shall trace the careers ofother more recent discoverers, by whose industry andgenius the boundaries of human knowledge have beenpo greatly extended. Our history will be brought downlate enough to include some of the illustrious astronomerswho laboured in the generation which has just passedaway.


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PTOLEMY.The career of the famous man whose name stands at thehead of this chapter is one of the most remarkable in thehistory of human learning. There may have been otherdiscoverers who have done more for science than everPtolemy accomplished, but there never has been anyother discoverer whose authority on the subject of themovements of the heavenly bodies has held sway over theminds of men for so long a period as the fourteen cen-turies during which his opinions reigned supreme. Thedoctrines he laid down in his famous book, " The Alma-gest," prevailed throughout those ages. No substantialaddition was made in all that time to the undoubtedtruths which this work contained. 'No important correc-tion was made of the serious errors with which Ptolemy'stheories were contaminated. The authority of Ptolemyas to all things in the heavens, and as to a good manythings on the earth (for the same illustrious man wasalso a diligent geographer), was invariably final.Though every child may now know more of the actual

truths of the celestial motions than ever Ptolemy knew,yet the fact that his work exercised such an astonishingeffect on the human intellect for some sixty generations,


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8 GREAT ASTRONOMERS.shows that it must have been an extraordinary produc-tion. We must look into the career of this wonderfulman to discover wherein lay the secret of that marvelloussuccess which made him the unchallenged instructor ofthe human race for such a protracted period.

Unfortunately, we know very little as to the personalhistory of Ptolemy. He was a native of Egypt, andthough it has been sometimes conjectured that he be-longed to the royal families of the same name, yet thereis nothing to support such a belief. The name, Ptolemy,appears to have been a common one in Egypt in thosedays. The time at w^hich he lived is fixed by the factthat his first recorded observation was made in 127 a.d.,and his last in 151 a.d. When we add that he seems tohave lived in or near Alexandria, or to use his ownwords, "On the parallel of Alexandria," we have saideverything that can be said so far as his individuality isconcerned.Ptolemy is, without doubt, the greatest figure in ancient

astronomy. He gathered up the wisdom of the philoso-phers who had preceded him. He incorporated this withthe results of his own observations, and illumined it withhis theories. His speculations, even when they were, aswe now know, quite erroneous, had such an astonishingverisimilitude to the actual facts of nature that theycommanded universal assent. Even in these modern dayswe not unfrequently find lovers of paradox who maintainthat Ptolemy's doctrines not only seem true, but actuallyare true.

In the absence of any accurate knowledge of the scienceof mechanics, philosophers in early times were forced to


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PTOLEMY.fall back on certain principles of more or less validity,which they derived from their imagination as to what thenatural fitness of things ought to be. There was no geo-metrical figure so simple and so symmetrical as a circle,

Ptolemy.

and as it was apparent that the heavenly bodies pursuedtracks whichwerenot straight lines, the conclusion obviouslyfollowed that their movements ought to be circular. Therewas no argument in favour of this notion, other than the


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10 GREAT ASTRONOMERS.merely imaginary reflection that circular movement, andcircular movement alone, was "perfect/^ whatever *' per-fect " may have meant. It was further believed to beimpossible that the heavenly bodies could have any othermovements save those which were perfect. Assumingthis, it followed, in Ptolemy's opinion, and in that ofthose who came after him for fourteen centuries, that allthe tracks of the heavenly bodies were in some way orother to be reduced to circles.Ptolemy succeeded in devising a scheme by which theapparent changes that take place in the heavens coidd, sofar as he knew them, be explained by certain combina-tions of circular movement. This seemed to reconcileso completely the scheme of things celestial with thegeometrical instincts which pointed to the circle as thetype of perfect movement, that we can hardly wonderPtolemy's theory met with the astonishing success thatattended it. We shall, therefore, set forth with sufficientdetail the various steps of this famous doctrine.Ptolemy commences with laying down the undoubted

truth that the shape of the earth is globular. The proofswhich he gives of this fundamental fact are quite satis-factory ; they are indeed the same proofs as we give to-day. There is, first of all, the well-known circumstanceof which our books on geography remind us, that whenan object is viewed at a distance across the sea, the lowerpart of the object appears cut off by the interposingcurved mass of water.The sagacity of Ptolemy enabled him to adduce another

argument, which, though not quite so obvious as thatjust mentioned, demonstrates the curvature of the earth


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PTOLEMY. IIin a very impressive manner to anyone wlio will take thetrouble to understand it. Ptolemy mentions that travel-lers who went to the south reported, that, as they did so,the appearance of the heavens at night underwent agradual change. Stars that they were familiar with in thenorthern skies gradually sank lower in the heavens. Theconstellation of the Great Bear, which in our skies neversets during its revolution round the pole, did set and risewhen a sufficient southern latitude had been attained.On the other hand, constellations new to the inhabitantsof northern climes were seen to rise above the southernhorizon. These circumstances would be quite incom-patible with the supposition that the earth was a flatsurface. Had this been so a little reflection will showthat no such changes in the apparent movements of thestars would be the consequence of a voyage to the south.Ptolemy set forth with much insight the significance ofthis reasoning, and even now, with the resources of moderndiscoveries to help us, we can hardly improve upon hisarguments.

Ptolemy, like a true philosopher disclosing a new truthto the world, illustrated and enforced his subject by avariety of happy demonstrations. I must add one ofthem, not only on account of its striking nature, but alsobecause it exemplifies Ptolemy's acuteness. If the earthwere flat, said this ingenious reasoner, sunset must neces-sarily take place at the same instant, no matter in whatcountry the observer may happen to be placed. Ptolemy,however, proved that the time of sunset did vary greatlyas the observer's longitude was altered. To us, of course,this is quite obvious ; everybody knows that the hour of


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12 GREAT ASTRONOMERS.sunset may haye been reached in Great Britain while it isstill noon on the western coast of America. Ptolemyhad, however, few of those sources of knowledge whichare now accessible. How was he to show that the sunactually did set earlier at Alexandria than it would in acity which lay a hundred miles to the west ? There wasno telegraph wire by which astronomers at the two placescould communicate. There was no chronometer or watchwhich could be transported from place to place ; therewas not any other reliable contrivance for the keepingof time. Ptolemy's ingenuity, however, pointed out athoroughly satisfactory method by which the times ofsunset at two places could be compared. He was ac-quainted with the fact, which must indeed have beenknown from the very earliest times, that the illuminationof the moon is derived entirely from the sun. He knewthat an eclipse of the moon was due to the interpositionof the earth which cuts off the light of the sun. It was,therefore, plain that an eclipse of the moon must be aphenomenon which would begin at the same instant fromwhatever part of the earth the moon could be seen atthe time. Ptolemy, therefore, brought together fromvarious quarters the local times at which different ob-servers had recorded the beginning of a lunar eclij)se.He found that the observers to the west made the timeearlier and earlier the further away their stations werefrom Alexandria. On the other hand, the eastern ob-servers set down the hour as later than that at which thephenomenon appeared at Alexandria. As these observersall recorded something which indeed appeared to themsimultaneously, the only interpretation was, that the more


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PTOLEMY. 13easterly a place the later its time. Suppose there were anumber of observers along a parallel of latitude, and eachnoted the hour of sunset to be six o'clock, then, since theeastern times are earlier than western times, 6 p.m. atone station a will correspond to 5 i*.M. at a station esufficiently to the west. If, therefore, it is sunset to theobserver at a, the hour of sunset will not yet be reachedfor the observer at b. This proves conclusively that thetime of sunset is not the same all over the earth. Wehave, however, already seen that the apparent time ofsunset would be the same from all stations if the earthwere flat. When Ptolemy, therefore, demonstrated thatthe time of sunset was not the same at various places, heshowed conclusively that the earth was not flat.As the same arguments applied to all parts of the

earth where Ptolemy had either been himself, or fromwhich he could gain the necessary information, it fol-lowed that the earth, instead of being the flat plain,girdled with an illimitable ocean, as was generally sup-posed, must be in reality globular. This led at once to astartling consequence. It was obvious that there couldbe no supports of any kind by which this globe was sus-tained ; it therefore followed that the mighty object mustbe simply poised in space. This is indeed an astonish-ing doctrine to anyone who relies on what merely seemsthe evidence of the senses, without giving to that evidenceits due intellectual interpretation. According to ourordinary experience, the very idea of an object poisedwithout support in space, appears preposterous. Would itnot fall? we are immediately asked. Yes, doubtless itcould not remain poised in any way in which we try the


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14 GREA7 ASTRONOMERS,experiment. "We must, however, observe that there areno such ideas as upwards or downwards in relation toopen space. To say that a body falls downwards, merelymeans that it tries to fall as nearly as possible towardsthe centre of the earth. There is no one direction alongwhich a body will tend to move in space, in preference toany other. This may be illustrated by the fact that astone let fall at New Zealand will, in its approach towardsthe earth's centre, be actually moving upwards as far asany locality in our hemisphere is concerned. Why, then,argued Ptolemy, may not the earth remain poised inspace, for as all directions are equally upward or equallydownward, there seems no reason why the earth shouldrequire any support ? By this reasoning he arrives at thefundamental conclusion that the earth is a globular bodyfreely lying in space, and surrounded above, below, andon all sides by the glittering stars of heaven.The perception of this sublime truth marks a notable

epoch in the history of the gradual development of thehuman intellect. No doubt, other philosophers, in grop-ing after knowledge, may have set forth certain assertionsthat are more or less equivalent to this fundamental truth.It is to Ptolemy we must give credit, however, not onlyfor announcing this doctrine, but for demonstrating it byclear and logical argument. We cannot easily projectour minds back to the conception of an intellectual statein which this truth was unfamiliar. It maj^ however, bewell imagined that, to one who thought the earth was aflat plain of indefinite extent, it would be nothing lessthan an intellectual convulsion for him to be forced tobelieve that he stood upon a spherical earth, forming


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PTOLEMY. 15merely a particle relatively to the immense sphere of theheavens.What Ptolemy saw in the movements of the stars ledhim to the conclusion that they were bright points attached

to the inside of a tremendous globe. The movements ofthis globe which carried the stars were only compatiblewith the supposition that the earth occupied its centre.The imperceptible effect produced by a change in thelocality of the observer on the apj)arent brightness ofthe stars made it plain that the dimensions of theterrestrial globe must be quite insignificant in com-parison with those of the celestial sphere. The earthmight, in fact, be regarded as a grain of sand, while thestars lay upon a globe many yards in diameter.So tremendous was the revolution in human knowledo-e

implied by this discovery, that we can well imagine howPtolemy, dazzled as it were by the fame which had sojustly accrued to him, failed to make one further step.Had he made that step, it would have emancipated thehuman intellect from the bondage of fourteen centuries ofservitude to a wholly monstrous notion of this earth's im-portance in the scheme of the heavens. The obvious factthat the sun, the moon, and the stars rose day by dav,moved across the sky in a glorious never-ending proces-sion, and duly set when their appointed courses had beenrun, demanded some explanation. The circumstance thatthe fixed stars preserved their mutual distances from yearto year, and from age to age, appeared to Ptolemy toprove that the sphere which contained those stars, and onwhose surface they were believed by him to be fixed, re-volved completely around the earth once every day. He


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l6 GREAT ASTRONOMERS.would thus account for all the phenomena of rising andsetting consistently with the supposition that our globewas stationary. Probably this supposition must haveappeared monstrous, even to Ptolemy. He knew that theearth was a gigantic object, but, large as it may havebeen, he knew that it was only a particle in comparisonwith the celestial sphere, yet he apparently believed, andcertainly succeeded in persuading other men to believe,that the celestial sphere did actually perform these move-ments.

Ptolemy was an excellent geometer. I He knew thatthe rising and the setting of the sun, the moon, and themyriad stars, could have been accounted for in a differentway. If the earth turned round uniforml}^ once a daywhile poised at the centre of the sphere of the heavens, allthe phenomena of rising and setting could be completelyexplained. This is, indeed, obvious after a moment's re-flection. Consider yourself to be standing on the earthat the centre of the heavens. There are stars over yourhead, and half the contents of the heavens are visible, whilethe other half are below jouv horizon. As the earth turnsround, the stars over your head will change, and unless itshould happen that you have taken up your position ateither of the poles, new stars will pass into your view,and others will disappear, for at no time can you havemore than half of the whole sphere visible. The observeron the earth would, therefore, say that some stars wererising, and that some stars were setting. We have,therefore, two totally distinct methods, each of whichwould completely explain all the observed facts of thediurnal movement. One of these suppositions requires


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PTOLEMY. 17that the celestial sphere, bearing with it the stars and othercelestial bodies, turns uniformly around an invisible axis,while the earth remains stationary at the centre. Theother supposition would be, that it is the stupendouscelestial sphere which remains stationary, while the earthat the centre rotates about the same axis as the celestialsphere did before, but in an. opposite direction, and witha uniform velocity which would enable it to complete oneturn in twenty-four hours. Ptolemy was mathematicianenough to know that either of these suppositionswould suffice for the explanation of the observed facts.Indeed, the phenomena of the movements of the stars, sofar as he could observe them, could not be called upon topronounce which of these views was true, and which wasfalse.Ptolemy had, therefore, to resort for guidance to indirect

lines of reasoning. One of these suppositions must betrue, and yet it appeared that the adoption of either wasaccompanied by a great difficulty. It is one of his chiefmerits to have demonstrated that the celestial sphere wasso stupendous that the earth itself was absolutely insigni-ficant in comparison therewith. If, then, this stupendoussphere rotated once in twenty-four hours, the speed withwhich the movement of some of the stars must be executedwould be so portentous as to seem well-nigh impossible.It would, therefore, seem much simpler on this ground toadopt the other alternative, and to suppose the diurnalmovements were due to the rotation of the earth. HerePtolemy saw, or at all events fancied he saw, objections ofthe weightiest description. The evidence of the sensesappeared directly to controvert the supposition that this


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1 GREAT ASTRONOMERS.earth is anythiug but stationary. Ptolemy might, per-haps, have dismissed this objection on the ground that thetestimony of the senses on such a matter should be en-tirely subordinated to the interpretation which our intelli-gence would place upon the facts to which the sensesdeposed. Another objection, however, appeared to himto possess the gravest moment. It was argued that if theearth were rotating, there is nothing to make the air par-ticipate in this motion, mankind would therefore be sweptfrom the earth by the furious blasts which would arisefrom the movement of the earth through an atmosphereat rest. Even if we could imagine that the air werecarried round with the earth, the same would not apply,so thought Ptolemy, to any object suspended in the air.So long as a bird was perched on a tree, he might very wellbe carried onward by the moving earth, but the momenthe took wing, the ground would slip from under him at afrightful pace, so that when he dropped down again hewould find himself at a distance perhaps ten times asgreat as that which a carrier-pigeon or a swallow couldhave traversed in the same time. Some vague delusion ofthis description seems even still to crop up occasionall}^I remember hearing of a proposition for balloon travellingof a very remarkable kind. The voyager who wanted toreach any other place in the same latitude was simply toascend in a balloon, and wait there till the rotation of theearth conveyed the locality which happened to be hisdestination directly beneath him, whereupon he was to letout the gas and drop down ! Ptolemy knew quite enoughnatural philosophy to be aware that such a proposal forlocomotion would be an utter absurdity ; he knew that


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PTOLEMY. 19there was no such relative shift between the air and theearth as this motion would imply. It appeared to him tobe necessary that the air should lag behind, if theearth had been animated by a movement of rotation. Inthis he was, as we know, entirely wrong. There were,however, in his days no accurate notions on the subject ofthe laws of motion.

Assiduous as Ptolemy may have been in the study ofthe heavenly bodies, it seems evident that he cannot havedevoted much thought to the phenomena of motion ofterrestrial objects. Simple, indeed, are the experimentswhich might have convinced a philosopher much lessacute than Ptolemy, that, if the earth did revolve, the airmust necessarily accompany it. If a rider galloping onhorseback tosses a ball into the air, it drops again intohis hand, just as it would have done had he been re-maining at rest during the ball's flight ; the ball in factparticipates in the horizontal motion, so that though itreally describes a curve as any passer-by would observe,yet it appears to the rider himself merely to move up anddown in a straight line. This fact, and many others simi-lar to it, demonstrate clearly that if the earth were endowedwith a movement of rotation, the atmosphere surroundingit must participate in that movement. Ptolemy did notknow this, and consequently he came to the conclusionthat the earth did not rotate, and that, therefore, notwith-standing the tremendous improbability of so mighty anobject as the celestial sphere spinning round once inevery twenty-four hours, there was no course open exceptto believe that this very improbable thing did reallyhappen. Thus it came to pass that Ptolemy adopted as


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20 GREAT ASTRONOMERS,the cardinal doctrine of his system a stationary earthpoised at the centre of the celestial sphere, which stretchedaround on all sides at a distance so vast that the diameterof the earth was an inappreciable point in comparisontherewith.Ptolemy having thus deliberately rejected the doctrine

of the earth's rotation, had to make certain other entirelyerroneous suppositions. It was easily seen that each starrequired exactly the same period for the performance ofa complete revolution of the heavens. Ptolemy knewthat the stars were at enormous distances from the earth,though no doubt his notions on this point came very farshort of what we know to be the reality. If the starshad been at very varied distances, then it would be sowildly improbable that they should all accomplish theirrevolutions in the same time, that Ptolemy came to theconclusion that they must be all at the same distance,that is, that they must be all on the surface of a sphere.This view, however erroneous, was corroborated by theobvious fact that the stars in the constellations preservedtheir relative places unaltered for centuries. Thus it wasthat Ptolemy came to the conclusion that they were allfixed on one spherical surface, though we are not informedas to the material of this marvellous setting which sus-tained the stars like jewels.

Nor should w^e hastily pronounce this doctrine to beabsurd. The stars do appear to lie on the surface ofa sphere, of which the observer is at the centre ; not onlyis this the aspect which the skies present to the untechni-cal observer, but it is the aspect in which the skies arepresented to the most experienced astronomer of modern


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PTOLEMY, 21days. l^Q doubt he knows well that the stars are at themost varied distances from him ; he knows that certainstars are ten times, or a hundred times, or a thousand times,as far as other stars. Nevertheless, to his eye the starsappear on the surface of the sphere, it is on that surfacethat his measurements of the relative places of the starsare made ; indeed, it may be said that almost all the accu-rate observations in the Observatory relate to the placesof the stars, not as they really are, but as they appear tobe projected on that celestial sphere whose conception weowe to the genius of Ptolemy.

This great philosopher shows very ingeniously that theearth must be at the centre of the sphere. He provesthat, unless this were the case, each star would not appearto move with the absolute uniformity which does, as amatter of fact, characterise it. In all these reasonings wecannot but have the most profound admiration for thegenius of Ptolemy, even though he had made an errorso enormous in the fundamental point of the stability ofthe earth. Another error of a somewhat similar kindseemed to Ptolemy to be demonstrated. He had shownthat the earth was an isolated object in space, and beingsuch was, of course, capable of movement. It could eitherbe turned round, or it could be moved from one place toanother. We know that Ptolemy deliberately adopted theview that the earth did not turn round; he had then to in-vestigate the other question, as to whether the earth wasanimated by any movement of translation. He came tothe conclusion that to attribute any motion to the earthwould be incompatible with the truths at which he hadalready arrived. The earth, argued Ptolemy, lies at the


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22 GREAT ASTRONOMERS.centre of the celestial sphere. If the earth were to beendowed with movement, it would not lie alwaj^s at thispoint, it must, therefore, shift to some other part of thesphere. The movements of the stars, however, precludethe possibility of this ; and, therefore, the earth must beas devoid of any movement of translation as it is devoidof rotation. Thus it was that Ptolemy convinced himselfthat the stability of the earth, as it appeared to the ordi-nary senses, had a rational philosophical foundation.Not unfrequently it is the lot of the philosophers tocontend against the doctrines of the vulgar, but when ithappens, as in the case of Ptolemy's researches, that thedoctrines of the vulgar are corroborated by philosophicalinvestigation which bear the stamp of the highestauthority, it is not to be wondered at that such doctrinesshould be deemed well-nigh impregnable. In this way wemay, perhaps, account for the remarkable fact that thetheories of Ptolemy held unchallenged sway over thehuman intellect for the vast period already mentioned.Up to the present we have been speaking only of those

primary motions of the heavens, by which the wholesphere appeared to revolve once every twenty-four hours.We have now to discuss the remarkable theories by whichPtolemy endeavoured to account for the monthly move-ment of the moon, for the annual movement of the sun,and for the periodic movements of the planets which hadgained for them the titles of the wandering stars.

Possessed with the idea that these movements mustbe circular, or must be capable, directly or indirectly, ofbeing explained by circular movements, it seemed obviousto Ptolemy, as indeed it had done to previous astronomers,


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PTOLEMY. 23that the track of the moon throusrh the stars was a circleof which the earth is the centre. A similar movementwith a yearly period must also be attributed to the sun,for the changes in the positions of the constellations, inaccordance with the progress of the seasons, placed itbeyond doubt that the sun made a circuit of the celestialsphere, even though the bright light of the sun pre-vented the stars in its vicinity from being seen in day-light. Thus the movements both of the sun and themoon, as well as the diurnal rotation of the celestialsphere, seemed to justify the notion that all celestialmovements must be " perfect," that is to say, describeduniformly in those circles which were the only perfectcurves.The simplest observations, however, show that the

movements of the planets cannot be explained in thissimple fashion. Here the geometrical genius of Ptolemyshone forth, and he devised a scheme by which the appa-rent wanderings of the planets could be accounted forwithout the introduction of aught save *' perfect" move-ments.

To understand his reasoning, let us first set forthclearly those facts of observation which require to beexplained. I shall take, in particular, two planets, Venusand Mars, as these illustrate, in the most striking manner,the peculiarities of the inner and the outer planets respec-tively. The simplest observations would show that Venusdid not move round the heavens in the same fashion asthe sun or the moon. Look at the evening star whenbrightest, as it appears in the west after sunset. Insteadof moving towards the east among the stars, like the sun


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24 GREAT ASTRONOMERS,or the moon, we find, week after week, tkat Venus isdrawing in towards the sun, until it is lost in the sun-beams. Then the planet emerges on the other side, notto be seen as an. evening star, but as a morning star. In.fact, it was plain that in some ways Yenus accompaniedthe sun in its annual movement. Now it is found advanc-ing in front of the sun to a certain limited distance, andnow it is lagging to an equal extent behind the sun.

,t-- -...,

,\

/EARTH

FIG. I.

These movements were wholly incompatible with the sup-position that the journeys of Yenus were described by asingle motion of the kind regarded as perfect. It wasobvious that the movement was connected in some strangemanner with the revolution of the sun, and here was the in-genious method by which Ptolemy sought to render accountof it. Imagine a fixed arm to extend from the earth tothe sun, as shown in the accompanying figure (Fig. ]),


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PTOLEMY. 25then this arm will move round uniformly, in consequenceof the sun's movement. At a point p on this arm let asmall circle be described. Yenus is supposed to revolveuniformly in this small circle, while the circle itself iscarried round continuously by the movement of the sun.In this way it was possible to account for the chief pecu-liarities in the movement of Yenus. It will be seen that,in consequence of the revolution around p, the spectatoron the earth will sometimes see Yenus on one side of thesun, and sometimes on the other side, so that the planetalways remains in the sun's vicinity. By properly pro-portioning the movements, this little contrivance simulatedthe transitions from the morning star to the evening star.Thus the changes of Yenus could be accounted for by acombination of the " perfect" movement of p in the circlewhich it described uniformly round the earth, combinedwith the *' perfect" motion of Yenus in the circle whichit described uniformly around the moving centre.

In a precisely similar manner Ptolemy rendered anexplanation of the fitful apparitions of Mercury. Nowjust on one side of the sun, and now just on the other,this rarely-seen planet moved like Yenus on a circlewhereof the centre was also carried by the line joiningthe sun and the earth. The circle, however, in whichMercury actually revolved had to be smaller than that ofYenus, in order to account for the fact that Mercury liesalways much closer to the sun than the better-knownplanet.

The explanation of the movement of an outer planetlike Mars could also be deduced from the joint effect of twoperfect motions. The changes through which Mars goes


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26 GREAT ASTRONOMERS.

a MARS

M ti'.--^

EARTH

TJG 2

are, however, so different from the movements of Yenusthat quite a different disposition of the circles is necessary.For consider the facts which characterise the movementsof an outer planet such as Mars. In the first place. Marsaccomplishes an entire circuit of the heaven. In thisrespect, no doubt, it may be said to resemble the sun orthe moon. A little attention will, however, show thatthere are extraordinary irregularities in the movement ofthe planet. Generally speaking, it speeds its way fromwest to east among the stars, but sometimes the attentiveobserver will note that the speed with which the planetadvances is slackening, and then it will seem to becomestationary. Some days later the direction of the planet'smovement will be reversed, and it will be found movingfrom the east towards the west. At first it proceeds


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PTOLEMY. 27slowly, and then quickens its pace, until a certain speed isattained, which afterwards declines until a second sta-tionary position is reached. After a due pause the originalmotion from west to east is resumed, and is continueduntil a similar cycle of changes again commences. Suchmovements as these were obviously quite at variance wdthany perfect movement in a single circle round the earth.Here, again, the geometrical sagacity of Ptolemy providedhim with the means of representing the apparent move-ments of Mars, and, at the same time, restricting theexplanation to those perfect movements which he deemedso essential. In Fig. 2 we exhibit Ptolemy's theory as tothe movement of Mars. We have, as before, the earth atthe centre, and the sun describing its circular orbit aroundthat centre. The path of Mars is to be taken as exteriorto that of the sun. We are to suppose that at a pointmarked m there is a fictitious planet, which revolvesaround the earth uniformly, in a circle called the deferent.This point m, which is thus animated by a perfect move-ment, is the centre of a circle which is carried onwardswith M, and around the circumference of which Marsrevolves uniformly. It is easy to show that the combinedeffect of these two perfect movements is to produce exactlythat displacement of Mars in the heavens which observa-tion discloses. In the position represented in the figure,Mars is obviously pursuing a course which will appear tothe observer as a movement from west to east. When, how-ever, the planet gets round to such a position as R, it isthen moving from east to west in consequence of its revo-lution in the moving circle, as indicated by the arrow-head. On the other hand, the whole circle is carried


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28 GREAT ASTRONOMERS.forward in the opposite direction. If the latter move-ment be less rapid than the former, then we shall havethe backward movement of Mars on the heavens which itwas desired to explain. By a proper adjustment of therelative lengths of these arms the movements of theplanet as actually observed could be completely accountedfor.The other outer planets with which Ptolemy was ac-

quainted, namely, Jupiter and Saturn, had movements ofthe same general character as those of Mars. Ptolemywas equally successful in explaining the movementsthey performed by the , supposition that each planet hadperfect rotation in a circle of its own, which circleitself had perfect movement around the earth in thecentre.

It is somewhat strange that Ptolemy did not advanceone step further, as by so doing he would have givengreat simplicity to his system. He might, for instance,have represented the movements of Yenus equally well byputting the centre of the moving circle at the sun itself,and correspondingly enlarging the circle in which Yenusrevolved. He might, too, have arranged that the severalcircles which the outer planets traversed should also havehad their centres at the sun. The planetary system wouldthen have consisted of an earth fixed at the centre, of a sunrevolving uniformly around it, and of a system of planetseach describing its own circle around a moving centreplaced in the sun. Perhaps Ptolemy had not thought ofthis, or perhaps he may have seen arguments against it.This important step was, however, taken by Tycho. Heconsidered that all the planets revolved around the sun in


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PTOLEMY. 29circles, and that the sun itself, bearing all these orbits,described a mighty circle around the earth. This pointhaving been reached, only one more step would have beennecessary to reach the glorious truths that revealed thestructure of the solar system. That last step was takenby Co|; amicus.


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COPEUNICUS.The quaint town of Thorn, on the Vistula, was more thantwo centuries old when Copernicus was born there on the19th of February, 1473. The situation of this town onthe frontier between Prussia and Poland, with the com-modious waterway offered by the river, made it a placeof considerable trade. A view of the town, as it was atthe time of the birth of Copernicus, is here given. Thewalls, with their watch-towers, will be noted, and thestrategic importance which the situation of Thorn gaveto it in the fifteenth century still belongs thereto, somuch so that the German Government recently consti-tuted the town a fortress of the first class.

Copernicus, the astronomer, whose discoveries makehim the great predecessor of Kepler and IN'ewton, did notcome from a noble family, as certain other early astro-nomers have done, for his father was a tradesman.Chroniclers are, however, careful to tell us that one ofhis uncles was a bishop. We are not acquainted withany of those details of his childhood or youth which areoften of such interest in other cases where men have risento exalted fame. It would appear that the young Nico-laus, for such was his Christian name, received his educa-


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COPERNICUS.tion at home uutil such time as lie was deemed sufficientlyadvanced to be sent to the University at Cracow. Theeducation that hethere obtained musthave been in thosedays of a very primi-tive description, butCopernicus seems tohave availed himselfof it to the utmost.He devoted himselfmore particularly tothe study of medi-cine, with the viewof adopting its prac-tice as the professionof his life. The ten-dencies of the futureastronomer were,however, revealed inthe fact that heworked hard at ma-thematics, and, likeone of his illustrioussuccessors, Galileo,the practice of theart of painting hadfor him a very greatinterest, and in it he obtained some measure of success.By the time he was twenty -seven years old, it would

seem that Copernicus had given up the notion of becoming


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Z2 GREA2 ASTRONOMERS.a medical practitioner, and had resolved to devote him-self to science. He was engaged in teaching mathe-matics, and appears to have acquired some reputation.His growing fame attracted the notice of his uncle thebishop, at whose suggestion Copernicus took holy orders,and he was presently appointed to a canonry in thecathedral of Frauenburg, near the mouth of the Vistula.To Frauenburg, accordingly, this man of varied gifts

retired. Possessing somewhat of the ascetic spirit, heresolved to devote his life to work of the most seriousdescription. He eschewed all ordinary society, restrict-ing his intimacies to very grave and learned companions,and refusing to engage in conversation of any uselesskind. It would seem as if his gifts for painting werecondemned as frivolous; at all events, we do not learnthat he continued to practise them. In addition to thedischarge of his theological duties, his life was occupiedpartly in ministering medically to the wants of the poor,and partly with his researches in astronomy and mathe-matics. His equipment in the matter of instruments forthe study of the heavens seems to have been of a verymeagre description. He arranged apertures in the wallsof his house at Allenstein, so that he could observe insome fashion the passage of the stars across the meridian.That he possessed some talent for practical mechanics isproved by his construction of a contrivance for raisingwater from a stream, for the use of the inhabitants ofFrauenburg. Relics of this machine are still to beseen.The intellectual slumber of the Middle Ages was des-

tined to be awakened by the revolutionary doctrines of


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COPERNICUS. 33

Copernicus.

Copernicus. It may be noted, as an interesting circum-stance, that tlie time at which he discovered the scheme ofthe solar system has coincided with a remarkable epoch inthe world's history. The great astronomer had just reachedmanhood at the time when Columbus discovered the newworld.

Before the publication of the researches of Copernicus,the orthodox scientific creed averred that the earth wasstationary, and that the apparent movements of the

i)


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^4 GREAT ASTRONOMERS.heavenly bodies were indeed real movements. Ptolemyhad laid down this doctrine 1,400 years before. In histheory this huge error was associated with so much im-portant truth, and the whole presented such a coherentscheme for the explanation of the heavenly movements,that the Ptolemaic theory was not seriously questioneduntil the great work of Copernicus appeared. No doubtothers, before Copernicus, had from time to time in somevague fashion surmised, with more or less plausibility,that the sun, and not the earth, was . the centre aboutwhich the system really revolved. It is, however, onething to state a scientific fact ; it is quite another thingto be in possession of the train of reasoning, founded onobservation or experiment, by which that fact may beestablished. Pythagoras, it appears, had indeed told hisdisciples that it was the sun, and not the earth, whichwas the centre of movement, but it does not seem at allcertain that Pythagoras had any grounds which sciencecould recognise for the belief which is attributed to him.So far as information is available to us, it would seemthat Pythagoras associated his scheme of things celestialwith a number of preposterous notions in natural philo-sophy. He may certainly have made a correct statementas to which was the most important body in the solarsystem, but he certainly did not provide any rationaldemonstration of the fact. Copernicus, by a strict trainof reasoning, convinced those who would listen to himthat the sun was the centre of the system. It is usefulfor us to consider the arguments which he urged, and bywhich he effected that intellectual revolution which isalways connected with his name.


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COPERNICUS. 35The first of the great discoveries which Copernicus made

relates to the rotation of the earth on its axis. Thatgeneral diurnal movement, by which tlie stars and all othercelestial bodies appear to be carried completely round theheavens once every twenty-four hours, had been accountedfor by Ptolemy on the supposition that the apparentmovements were the real movements. As we bave alreadyseen, Ptolemy himself felt the extraordinary difficultyinvolved in the supposition that so stupendous a fabricas the celestial sphere should spin in the way supposed.{Such movements required that many of the stars shouldtravel with almost inconceivable velocity. Copernicus alsosaw that the daily rising and setting of the heavenlybodies could be accounted for either by the suj^positionthat the celestial sphere moved round and that the earthremained at rest, or by the supposition that the celestialsphere was at rest while the earth turned round in the oppo-site direction. He weighed the arguments on both sidesas Ptolemy had done, and, as the result of his delibera-tions, Copernicus came to an opposite conclusion fromPtolemy. To Copernicus it appeared that the difficultiesattending the supposition that the celestial sphere re-volved, were vastly greater than those which appearedso weighty to Ptolemy as to force him to deny the earth'srotation.

Copernicus shows clearly how the observed phenomenacould be accounted for j ust as completely by a rotation ofthe earth as by a rotation of the heavens. He alludes tothe fact that, to those on board a vessel which is movingthrough smooth water, the vessel itself appears to be atrest, while the objects on shore seem to be moving past.


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36 GREAT ASTRONOMERS.If, therefore, tlie eartli were rotating tiniformly, wedwellers upon the eartli, oblivious of our own move-ment, would wrongly attribute to the stars the dis-placement which was actually the consequence of our ownmotion.

Copernicus saw the futility of the arguments by whichPtolemy had endeavoured to demonstrate that a revolu-tion of the earth was impossible. It was plain to himthat there was nothing whatever to warrant refusal tobelieve in the rotation of the earth. In his clear-sighted-ness on this matter we have specially to admire the saga-city of Copernicus as a natural philosopher. It had beenurged that, if the earth moved round, its motion wouldnot be imparted to the air, and that therefore the earthwould be uninhabitable by the terrific winds which wouldbe the result of our being carried through the air. Coj)er-nicus convinced himself that this deduction was prepos-terous. He proved that the air must accompany theearth, just as his coat remains round him, notwithstand-ing the fact that he is walking down the street. In thisway he was able to show that all a priori objections to theearth's movements were absurd, and therefore he was ableto compare together the plausibilities of the two rivalschemes for explaining the diurnal movement.Once the issue had been j)laced in this form, the result

could not be long in doubt. Here is the question : Whichis it more likelythat the earth, like a grain of sand atthe centre of a mighty globe, should turn round once intwenty-four hours, orthat the whole of that vast globeshould complete a rotation in the opposite direction in thesame time ? Obviously, the former is far the more simple


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COPERNICUS. nsupposition. But the case is really much stronger thanthis. Ptolemy had supposed that all the stars were attached

Pcr'

PO

He had no ground whatevero the surface of a sphere.for this supposition, except that otherwise it would havebeen well-nigh impossible to have devised a scheme by


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38 GREA2 ASTRONOMERS.whicli the rotation of the heavens around a fixed earthcould have been arranged. Copernicus, however, withthe just instinct of a philosopher, considered that thecelestial sphere, however convenient from a geometricalpoint of view, as a means of representing apparent pheno-mena, could not actually have a material existence. Inthe first place, the existence of a material celestial spherewould require that all the myriad stars should be atexactly the same distances from the earth. Of course,no one will say that this or any other arbitrary disposi-tion of the stars is actually impossible, but as there was noconceivable physical reason why the distances of all thestars from the earth should be identical, it seemed in thevery highest degree improbable that the stars should beso placed.

Doubtless, also, Copernicus felt a considerable difficultyas to the nature of the materials from which Ptolemy'swonderful sphere was to be constructed. J^or could aphilosopher of his penetration have failed to observe that,unless that sphere were infinitely large, there must havebeen space outside it, a consideration which would open upother difficult questions. Whether infinite or not, it wasobvious that the celestial sphere must have a diameterat least many thousands of times as great as that ofthe earth. From these considerations Copernicus deducedthe important fact that the stars and the other celestialbodies must all be vast objects. He was thus enabled toput the question in such a form that it could hardlyreceive any answer but the correct one. Which is itmore rational to suppose, that the earth should turn roundon its axis once in twenty-four hours, or that thousands of


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COPERNICUS. 39mighty stars should circle round the earth in the sametime, many of them having to describe circles manythousands of times greater in circumference than thecircuit of the earth at the equator ? The obvious answerpressed upon Copernicus with so much force that he wascompelled to reject Ptolemy's theory of the stationaryearth, and to attribute the diurnal rotation of the heavensto the revolution of the earth on its axis.

Once this tremendous step had been taken, the greatdifficulties which beset the monstrous conception of thecelestial sphere vanished, for the stars need no longerbe regarded as situated at equal distances from theearth. Copernicus saw that they might lie at the mostvaried degrees of remoteness, some being hundreds orthousands of times farther away than others. The com-plicated structure of the celestial sphere as a materialobject disappeared altogether ; it remained only as ageometrical conception, whereon we find it convenient toindicate the places of the stars. Once the Copernicandoctrine had been fully set forth, it was impossible foranyone, who had both the inclination and the capacity tounderstand it, to withhold acceptance of its truth. Thedoctrine of a stationary earth had gone for ever.

Copernicus having established a theory of the celestialmovements which deliberately set aside the stability of theearth, it seemed natural that he should inquire whetherthe doctrine of a moving earth might not remove thedifficulties presented in other celestial phenomena. Ithad been universally admitted that the earth lay un-supported in space. Copernicus had further shown thatit possessed a movement of rotation. Its want of stability


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40 GREAT ASTRONOMERS.being thus recognised, it seemed reasonable to supposethat the earth might also have some other kinds of move-ments as well. In this, Copernicus essayed to solve aproblem far more difficult than that which had hithertooccupied his attention. It was a comparatively easy taskto show how the diurnal rising and setting could beaccounted for by the rotation of the earth. It was amuch more difficult undertaking to demonstrate that theplanetary movements, which Ptolemy had representedwith so much success, could be completely explained bythe supposition that each of those planets revolved uni-formly round the sun, and that the earth was also aplanet, accomplishing a complete circuit of the sun oncein the course of a year.

It would be impossible in a sketch like the present toenter into any detail as to the geometrical propositions onwhich this beautiful investigation of Copernicus depended.We can only mention a few of the leading principles.It may be laid down in general that, if an observer is inmovement, he will, if unconscious of the fact, attribute tothe iixed objects around him a movement equal and oppo-site to that which he actually possesses. A passenger ona canal-boat sees the objects on the banks apparentlymoving backward with a speed equal to that by which heis himself advancing forwards. By an application of thisprinciple, we can account for all the phenomena of themovements of the planets, which Ptolemy had so in-geniously represented by his circles. Let us take, forinstance, the most characteristic feature in the irregu-larities of the outer planeta. We have already remarkedthat Mars, though generally advancing from west to east


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COPERNICUS. 41among the stars, occasionally pauses, retraces Ms steps fora while, again pauses, and then resumes his ordinaryonward progress. Copernicus showed clearly how thiseffect was produced by the real motion of the earth, com-bined with the real motion of Mars. In the adjoiningfigure we represent a portion of the circular tracksin which the earth and Mars move in accordance with

MARS.

EARTH,

\

the Copernican doctrine. I show particularly the casewhere the earth comes directly between the planet andthe sun, because it is on such occasions that the retro-grade movement (for so this backward movement of Marsis termed) is at its highest. Mars is then advancing inthe direction shown by the arrow-head, and the earth isalso advancing in the same direction. We, on the earth,however, being unconscious of our owm motion, attribute,


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4^ GREAT ASTRONOMERS,by tlie principle I liave already explained, an equal andopposite motion to Mars. The visible effect upon theplanet is, that Mars bas two movements, a real onwardmovement in one direction, and an apparent movement inthe opposite direction. If it so happened that the earthwas moving with the same speed as Mars, then theapparent movement would exactly neutralise the realmovement, and Mars would seem to be at rest relativelyto the surrounding stars. Under the actual circumstancesrepresented, however, the earth is moving faster thanMars, and the consequence is, that the apparent move-ment of the planet backwards exceeds the real movementforwards, the net result being an apparent retrogrademovement.With consummate skill, Copernicus showed how theapplications of the same principles could account for the

characteristic movements of the planets. His reasoningin due time bore down all opposition. The supreme im-portance of the earth in the system vanished. It hadnow merely to take rank as one of the planets.The same great astronomer now, for the first time,rendered something like a rational account of the changes

of the seasons. Nor did certain of the more obscureastronomical phenomena escape his attention.He delayed publishing his wonderful discoveries to the

world until he was quite an old man. He had a well-founded apprehension of the storm of opposition whichthey would arouse. However, he yielded at last to theentreaties of his friends, and his book was sent to thepress. But ere it made its appearance to the world,Copernicus was seized by mortal illness. A copy of the


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COPERNICUS. ^ 43book was brought to him on May 23, 1543. We aretold tbat be was able to see it and to touch it, butno more, and he died a few hours afterwards. He wasburied in that Cathedral of Frauenburg, with which hislife had been so closely associated.


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TYCHO BRAHE.The most picturesque figure in the history of astronomyis undoubtedly that of the famous old Danish astronomerwhose name stands at the head of this chapter. TychoBrahe was alike notable for his astronomical genius andfor the extraordinary vehemence of a character whichwas by no means perfect. His romantic career as a philo-sopher, and his taste for splendour as a Danish noble, hisardent friendships and his furious quarrels, make him anideal subject for a biographer, while the magnificentastronomical work which he accomplished has given himimperishable fame.The history of Tycho Brahe has been admirably told by

Dr. Dreyer, the accomplished astronomer who now directsthe observatory at Armagh, though himself a countrymanof Tycho. Every student of the career of the great Danemust necessarily look on Dr. Dreyer's work as the chiefauthority on the subject. Tycho sprang from an illustriousstock. His family had flourished for centuries, both inSweden and in Denmark, where his descendants are to bemet with at the present day. The astronomer's fatherwas a privy councillor, and having filled important posi-tions in the Danish government, he was ultimately pro-moted to be governor of Helsingborg Castle, where he


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TYCHO BRAHE. 45spent the last years of his life. His illustrious son Tychowas born in 1546, and was the second child and eldestboy in a family of ten.

It appears that Otto, the father of Tycho, had a brothernamed George, who was childless. George, howeyer,desired to adopt a boy on whom he could lavish his affec-tion and to whom he could bequeath his wealth. A some-what singular arrangement was accordingly entered intoby the brothers at the time when Otto was married. Itwas agreed that the first son who might be born to Ottoshould be forthwith handed over by the parents to Georgeto be reared and adopted by him. In due time littleTycho appeared, and was immediately claimed by Georgein pursuance of the compact. But it was not unnaturalthat the parental instinct, which had been dormant whenthe agreement was made, should here interpose. Tj^cho'sfather and mother receded from the bargain, and refusedto part with their son. George thought he was badlytreated. However, he took no violent steps until a yearlater, when a brother was born to Tycho. The uncle thenfelt no scruple in asserting what he believed to be hisrights by the simple process of stealing the first-bornnephew, which the original bargain had promised him.After a little time it would seem that the parents acquiescedin the loss, and thus it was in Uncle George's home thatthe future astronomer passed his childhood.When we read that Tycho was no more than thirteen

years old at the time he entered the University of Copen-hagen, it might be at first supposed that even in his boyishyears he must have exhibited some of those remarkabletalents with which he was afterwards to astonish the


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46 GREAT ASTRONOMERS.world. Such an inference should not, however, be drawn.The fact is that in those days it was customary for studentsto enter the universities at a much earlier age than is nowthe case. Not, indeed, that the boys of thirteen knew morethen than the boys of thirteen know now. But the edu-cation imparted in the universities at that time was of amuch more rudimentary kind than that which we under-stand by university education at present. In illustrationof this Dr. Dreyer tells us how, in the University ofWittenberg, one of the professors, in his opening address,was accustomed to point out that even the processes ofmultiplication and division in arithmetic might be learnedby any student who possessed the necessary diligence.

It was the wish and the intention of his uncle that Tycho'seducation should be specially directed to those branchesof rhetoric and philosophy which were then supposed tobe a necessary preparation for the career of a statesman.Tycho, however, speedily made it plain to his teachersthat though he was an ardent student, yet the thingswhich interested him were the movements of the heavenlybodies and not the subtleties of metaphysics.On the 21st October, 1560, an eclipse of the sunoccurred, which was partially visible at Copenhagen.Tycho, boy though he was, took the utmost interest inthis event. His ardour and astonishment in connectionwith the circumstance were chiefly excited by the fact thatthe time of the occurrence of the phenomenon could bepredicted with so much accuracy. Urged by his desire tounderstand the matter thoroughly, Tycho sought to pro-cure some book which might explain what he so greatlywanted to know. In those days books of any kind were


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TYCHO BRAHE.

Tycho Brah.e.

but few and scarce, and scientific books were especiallyunattainable. It so happened, however, that a Latinversion of Ptolemy^s astronomical works had appeared afew years before the eclipse took place, and T3xho managedto buy a copy of this book, Vv^hich was then the chief


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48 GREAT ASTRONOMERS.authority on celestial matters. Young as the boy astro-nomer was, lie studied hard, although perhaps not alwayssuccessfully, to understand. Ptolemy, and to this day hiscopy of the great work, copiously annotated and markedby the schoolboy hand, is preserved as one of the chieftreasures in the library of the University at Prague.

After Tycho had studied for about three years at theUniversity of Copenhagen, his uncle thought it would bebetter to send him, as was usual in those days, to com-plete his education by a course of study in some foreignuniversity. The uncle cherished the hope that in thisway the attention of the young astronomer might bewithdrawn from the study of the stars and directed inwhat appeared to him a more useful way. Indeed, to thewise heads of those days, the pursuit of natural scienceseemed "so much waste of good time which might other-wise be devoted to logic or rhetoric or some other .branchof study more in vogue at that time. To assist in thisattempt to wean Tycho from his scientific tastes, his unclechose as a tutor to accompany him an intelligent andupright young man named Yedel, who was four yearssenior to*his pupil, and accordingly, in 1562, we find thepair taking up their abode at the University of Leipzig.

The tutor, however, soon found that he had undertakena most hopeless task. He could not succeed in imbuingTycho with the slightest taste for the study of the law orthe other branches of knowledge which were then thoughtso desirable. The stars, and nothing but the stars, en-oTossed the attention of his pupil. We are told that allthe money he could obtain was spent secretly in buyingastronomical books and instruments. He learned the


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TYCHO BRAHE. 49name of the stars from a little globe, wliicli lie kept hiddenfrom Yedel, and only ventured to use during the latter'sabsence. JS'o little friction was at first caused by all this,but in after years a fast and enduring friendship grew upbetween Tycho and his tutor, each of whom learned torespect and to love the other.

Before Tycho was seventeen he had commenced thedifficult task of calculating the movements of the planetsand the places which they occupied on the sky from timeto time. He was not a little surprised to find that theactual positions of the planets differed very widely fromthose which were assigned to them by calculations fromthe best existing works of astronomers. With the insio-htof genius he saw that the only true method of investiga-ting the movements of the heavenly bodies would be tocarry on a protracted series of measurements of theirplaces. This, which now seems to us so obvious, was thenan entirely new doctrine. Tycho at once commencedregular observations in such fashion as he could. Hisfirst instrument was, indeed, a very primitive one, consist-ing of a simple pair of compasses, which he used in thiswa}^ He placed his eye at the hinge, and then openedthe legs of the compass so that one leg pointed to onestar and the other leg to the other star. The compasswas then brought down to a divided circle, by whichmeans the number of degrees in the apparent angular dis-tance of the two stars was determined.

His next advance in instrumental equipment was toprovide himself with the contrivance known as the '^ cross-staff," with which he used to observe the stars wheneveropportunity offered. It must, of course, be remembered

E


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so GREAT ASTRONOMERS.that in those daj^s there were no telescopes. In theabsence of optical aid, such as lenses afford the modernobservers, astronomers had to rely on mechanical ap-pliances alone to measure the places of the stars. Ofsuch appliances, perhaps the most ingenious was oneknown before Tycho's time, which we have representedin the adjoining figure.

Let us suppose that it be desired to measure the angleA

TO .STAp_ \D EYE HERE

'e J3THE CROSS STAFF

between two stars, then if the angle be not too large itcan be determined in the following manner. Let the rodA B be divided into inches and parts of an inch, and letanother rod, c d, slide up and down along a b in such away that the two always remain perpendicular to eachother. " Sights,'* like those on a rifle, are placed at aand c, and there is a pin at d. It will easily be seen that,by sliding the movable bar along the fixed one, it mustalways be possible when the stars are not tPQ far apart tQ


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TYCHO BRAHE, 51bring the sights into such positions that one star can beseen along d c and the other along d a. This having beenaccomplished, the length from a to the cross-bar is readoff on the scale, and then, by means of a table previouslj^prepared, the value of the required angular distance isobtained. If the angle between the two stars weregreater than it would be possible to measure in the wayalready described, then there was a provision by whichthe pin at d might be moved along c d into some otherposition, so as to bring the angular distance of the starswithin the range of the instrument.No doubt the cross-staff is a very primitive contrivance,

but when handled byone so skilful as Tychoit afforded results ofconsiderable accuracy.I would recommendany reader who mayhave a taste for suchpursuits to constructa cross-staff for him-self, and see what mea-surements he can ac-complish with its aid.To employ this little

instrument Tycho hadto evade the vigilanceof his conscientioustutor, who felt it hisduty to interdict all

^ . Tycho's " New Star " Sextant of 1572.PUCh OCCUpatlOllS as ^Tj^g^yp^p^of^aluut-wooa, are about 6f ft. long.


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52 GREAT ASTRONOMERS.being a frivolous wasteof time. It was whenYedel was asleep tliatTycho managed toescape with his crossstaff and measure theplaces of the heavenly-bodies. Even at thisearly age Tycho usedto conduct his obser-vations on those tho-roughly sound prin-ciples which lie at thefoundation of all ac-curate modern astro-nomy. Recognisingthe inevitable errorsof workmanship in hislittle instrument, heascertained their

amount and allowed for their influence on the resultswhich he deduced. This principle, employed by the boywith his cross-staff in 1564, is employed at the presentday by the Astronomer E,oyal at Greenwich with themost superb instruments that the skill of modern opticianshas been able to construct.

After the death of his uncle, when Tycho was nineteenyears of age, it appears that the young philosopher wasno longer interfered with in so far as the line which hisstudies were to take was concerned. Always of a some-what restless temperament, we now find that he shifted

Tycho's TrigOBic Sextant.(The arms, AB and A C, are about 5J ft. long.)


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TYCHO BRAHE, 53his abode to tlie University of Rostock, where he speedilymade himself notable in connection with an eclipse ofthe moon on 28th October, 1566. Like every otherastronomer of those days, Tycho had always associatedastronomy with astrology. He considered that thephenomena of the heavenly bodies always had somesignificance in connection with human affairs. Tycho wasalso a poet, and in the united capacity of poet, astrologer,and astronomer, he posted up some verses in the collegeat Rostock announcing that the lunar eclipse was a prog-nostication of the death of the great Turkish Sultan,whose mighty deedsat that time filledmen's minds. Pre-sently news didarrive of the deathof the Sultan, andTycho was accord-ingly triumphantbut a little later itappeared that the de-cease had taken placebefore the eclipse, acircumstance whichcaused many a laughat Tycho's expense.

Tvcho beinff of asomewhat turbulentdisposition, it appearsthat, while at the Uni- Tycho' s Astronomic Sextant.VerSlty of Rostock, (Madeofeteel; thearms,^5,^C,measure4ft.)


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54 GREAT ASTRONOMERS,lie had a seriousquarrel with, anotherDanish nobleman. "Weare not told for cer-tain what was thecause of the dispute.It does not, however,seem to have had anymore romantic originthan a difference ofopinion as to whichof them knew the moremathematics. Theyfought, as perhaps itwas becoming for twoastronomers to fight,under the canopy ofheaven in utter dark-ness, at the dead ofnight, and the duelwas honourably ter-

minated when a slice was taken off Tycho's nose by theinsinuating sword of his antagonist. For the repair ofthis injury the ingenuity of the great instrument-makerwas here again useful, and he made a substitute for hisnose "with a composition of gold and silver." Theimitation was so good that it is declared to have beenquite equal to the original. Dr. Lodge, however,pointedly observes that it does not appear whether thisremark was made by a friend or an enemy.

The next few years Tycho spent in various places


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TYCHO BRAHE, 55ardently pursuing somewliat varied branches of scientificstudy. At one time we hear of him assisting an astro-nomical alderman, in the ancient city of Augsburg, toerect a tremendous wooden machinea quadrant of19-feet radiusto be used in observing the heavens.At another time we learn that the King of Denmark hadrecognised the talents of his illustrious subject, andpromised to confer on him a pleasant sinecure in the shapeof a canonry, which would assist him with the means forindulging his scientific pursuits. Again we are told thatTj^cho is pursuing experiments in chemistry with thegreatest energy, noris this so incompa-tible as might atfirst be thought withhis devotion to astro-nomy. In those earlydays of knowledgethe different sciencesseemed bound to-gether by mysteriousbonds. Alchemistsand astrologerstaught that theseveral planets werecorrelated in somemysterious mannerwith the severalmetals. It was, there-

' J P ' rpj^g ^g^^ Augsburg Quadrant,ing that Tycho (Built of heart of oak; the radii about 19 ft.]


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56 GREAT ASTRONOMERS.

Tycho's "New Scheme of tlie Terrestrial System," 1577.(should have included a study of the properties of themetals in the programme of his astronomical work.An event, however, occurred in 1572 which stimulated

Tycho's astronomical labours, and started him on his life'swork. On the 11th of November in that year, he wasreturning home to supper after a day's work in hislaboratory, when he happened to lift his face to the sky,and there he beheld a brilliant new star. It was in theconstellation of Cassiopeia, and occupied a position inwhich there had certainly been no bright star visiblewhen his attention had last been directed to that part of


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TYCHO BRAHE, srthe heavens. Such a phenomenon was so startling thathe found it hard to trust the evidence of his senses. Hethought he must be the subject of some hallucination.He therefore called to the servants who were accompany-ing him, and asked them whether they, too, could see abrilliant object in the direction in which he pointed.They certainly could, and thus he became convinced thatthis marvellous object was no mere creation of the fancy,but a veritable celestial bodya new star of surpassingsplendour which had suddenly burst forth. In these daysof careful scrutiny of the heavens, we are accustomed to

OCClDEJT^

Uraniborg and its grounds


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58 GREAT ASTRONOMERSOCCIDENS

ORIENSGround-plan of the Observatory.

the occasional outbreak of new stars. It is not, however,believed that any new star wbicb has ever appeared hasdisplaj^ed the same phenomenal brilliance as was exhibitedby the star of 1572.

This object has a value in astronomy far greater thanmight at first appear. It is true, in one sense, that Tychodiscovered the new star, but it is equally true, in a dif-ferent sense, that it was the new star which discoveredTycho. Had it not been for this opportune apparition,it is quite possible that Tycho might have found a careerin some direction less beneficial to science than that whichhe ultimately pursued.When he reached his home on this memorable evening,

Tycho immediately applied his great quadrant to themeasurement of the place of the new star. His observa-tions were specially directed to the determination of thedistance of the object. He rightly conjectured that if itwere very much nearer to us than the stars in its vicinity,


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TYCHO BRAHE. 59the distance of the brilliant body might be determined ina short time by the apparent changes in its distance fromthe surrounding points. It was speedily demonstratedthat the new star could not be as near as the moon, bythe simple fact that its apparent place, as compared withthe stars in its neighbourhood, was not appreciably alteredwhen it was observed below the pole, and again abovethe pole at an interval of twelve hours. Such observa-tions were possible, inasmuch as the star was brightenough to be seen in full daylight. Tycho thus showedconclusively that the body was so remote that the diameterof the earth bore an insignificant ratio to the star's dis-tance. His success in this respect is the more noteworthywhen we find that many other observers, who studied thesame object, came to the erroneous conclusion that thenew star was quite as near as the moon, or even much

The Observatory of Uraniborg, Island of Hven.


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6o GREAT ASTRONOMERS.nearer. In fact, it may be said, that with regard to thisobject Tycho discovered everything which could possiblyhave been discovered in the days before telescopes wereinvented. He not only proved that the star's distancewas too great for measurement, but he showed that ithad no proper motion on the heavens. He recorded thesuccessive changes in its brightness from week to week,as well as the fluctuations in hue with which the altera-tions in lustre were accompanied.

It seems, nowadays, strange to find that such tho-roughly scientific observations of the new star as thosewhich Tycho made, possessed, even in the eyes of thegreat astronomer himself, a profound astrological signi-ficance. We learn from Dr. Dreyer that, in Tycho' s opinion,"the star was at first like Yenus and Jupiter, and itsefi'ects will therefore, first, be pleasant ; but as it thenbecame like Mars, there will next come a period of wars,seditions, captivity, and death of princes, and destructionof cities, together with dryness and fiery meteors in theair, pestilence, and venomous snakes. Lastly, the starbecame like Saturn, and thus will finally come a time ofwant, death, imprisonment, and all kinds of sad things !Ideas of this kind were, however, universally entertained.It seemed, indeed, obvious to learned men of that periodthat such an apparition must forebode startling events.One of the chief theories then held was, that just as theStar of Bethlehem announced the first coming of Christ,so the second coming, and the end of the world, washeralded by the new star of 1572.

The researches of Tycho on this object were the occa-sion of his first appearance as an author. The publication


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TYCHO B

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