The Copernicus of Antiquity Aristarchus of Samos
PIONEERS OF PROGRESS
MEN OF SCIENCE
Edited by S. CHAPMAN, M.A., D.Sc., F.R.S.
COPERNICUS OF ANTIQUITY
(ARISTARCHUS OF SAMOS)
Sir THOMAS HEATH
K.C.B., K.C.V.O., F.R.S.; Sc.D., Camb.; Hon. D.Sc., Oxford
SOCIETY FOR PROMOTING
NEW YORK: THE MACMILLAN COMPANY
|GREEK ASTRONOMY TO ARISTARCHUS|
|Œnopides of Chios||24|
|Eudoxus, Callippus, Aristotle||28|
|Heraclides of Pontus||33|
|ARISTARCHUS OF SAMOS|
|The Heliocentric Hypothesis||39|
|On the Apparent Diameter of the Sun||42|
|On the Sizes and Distances of the Sun and Moon||43|
|On the Year and “Great Year”||53|
|Later Improvements on Aristarchus’s Figures||54|
GREEK ASTRONOMY TO ARISTARCHUS.
The title-page of this book necessarily bears the name ofone man; but the reader will find in its pages the story, orpart of the story, of many other Pioneers of Progress.The crowning achievement of anticipating the hypothesisof Copernicus belongs to Aristarchus of Samos alone;but to see it in its proper setting it is necessary to havefollowed in the footsteps of the earlier pioneers who, byone bold speculation after another, brought the solutionof the problem nearer, though no one before Aristarchusactually hit upon the truth. This is why the writer hasthought it useful to prefix to his account of Aristarchusa short sketch of the history of the development ofastronomy in Greece down to Aristarchus’s time, whichis indeed the most fascinating portion of the story ofGreek astronomy.
The extraordinary advance in astronomy made by theGreeks in a period of little more than three centuriesis a worthy parallel to the rapid development, in theirhands, of pure geometry, which, created by them as atheoretical science about the same time, had by the timeof Aristarchus covered the ground of the Elements (includingsolid geometry and the geometry of the sphere), hadestablished the main properties of the three conic sections,had solved problems which were beyond the geometry ofthe straight line and circle, and finally, before the end of2the third century B.C., had been carried to its highestperfection by the genius of Archimedes, who measuredthe areas of curves and the surfaces and volumes ofcurved surfaces by geometrical methods practically anticipatingthe integral calculus.
To understand how all this was possible we have toremember that the Greeks, pre-eminently among all thenations of the world, possessed just those gifts which areessential to the initiation and development of philosophyand science. They had in the first place a remarkablepower of accurate observation; and to this were addedclearness of intellect to see things as they are, a passionatelove of knowledge for its own sake, and a genius forspeculation which stands unrivalled to this day. Nothingthat is perceptible to the senses seems to have escapedthem; and when the apparent facts had been accuratelyascertained, they wanted to know the why and thewherefore, never resting satisfied until they had given arational explanation, or what seemed to them to be such,of the phenomena observed. Observation or experimentand theory went hand in hand. So it was that theydeveloped such subjects as medicine and astronomy. Inastronomy their guiding principle was, in their own expressivewords, to “save the phenomena”. This meantthat, as more and more facts became known, their theorieswere continually revised to fit them.
It would be easy to multiply instances; it mustsuffice in this place to mention one, which illustrates notonly the certainty with which the Greeks detected theoccurrence of even the rarest phenomena, but also thepersistence with which they sought for the true explanation.
Cleomedes (second century A.D.) mentions that therewere stories of extraordinary eclipses which “the moreancient of the mathematicians” had vainly tried to3explain; the supposed “paradoxical” case was that inwhich, while the sun seems to be still above the westernhorizon, the eclipsed moon is seen to rise in the east.The phenomenon appeared to be inconsistent with theexplanation of lunar eclipses by the entry of the mooninto the earth’s shadow; how could this be if bothbodies were above the horizon at the same time? The“more ancient” mathematicians essayed a geometricalexplanation; they tried to argue that it was possiblethat a spectator standing on an eminence of the sphericalearth might see along the generators of a cone i.e. alittle downwards on all sides instead of merely in theplane of the horizon, and so might see both the sun andthe moon even when the latter was in the earth’sshadow. Cleomedes denies this and prefers to regardthe whole story of such cases as a fiction designed merelyfor the purpose of plaguing astronomers and philosophers;no Chaldæan, he says, no Egyptian, and no mathematicianor philosopher has recorded such a case. But thephenomenon is possible, and it is certain that it had beenobserved in Greece and that the Greek astronomers didnot rest until they had found out the solution of thepuzzle; for Cleomedes himself gives the explanation,namely that the phenomenon is due to atmospheric refraction.Observing that such cases of atmosphericrefraction were especially noticeable in the neighbourhoodof the Black Sea, Cleomedes goes on to say that itis possible that the visual rays going out from our eyesare refracted through falling on wet and damp air, and soreach the sun although it is already below the horizon;and he compares the well-known experiment of the ringat the bottom of a jug, where the ring, just out of sightwhen the jug is empty, is brought into view when wateris poured in.
The genius of the race being what it was, the Greeks4must from the earliest times have been in the habit ofscanning the heavens, and, as might be expected, wefind the beginnings of astronomical knowledge in theearliest Greek literature.
In the Homeric poems and in Hesiod the earth is aflat circular disc; round this disc runs the river Oceanus,encircling the earth and flowing back into itself. Theflat earth has above it the vault of heaven, like a sortof hemispherical dome exactly covering it; this vaultremains for ever in one position; the sun, moon andstars move round under it, rising from Oceanus in theeast and plunging into it again in the west.
Homer mentions, in addition to the sun and moon,the Morning Star, the Evening Star, the Pleiades, theHyades, Orion, the Great Bear (“which is also called bythe name of the Wain”), Sirius, the late-setting Boötes(the ploughman driving the Wain), i.e. Arcturus, as itwas first called by Hesiod. Of the Great Bear Homersays that it turns round on the same spot and watchesOrion; it alone is without lot in Oceanus’s bath (i.e.it never sets). With regard to the last statement it is tobe noted that some of the principal stars of the GreatBear do now set in the Mediterranean, e.g. in placesfurther south than Rhodes (lat. 36°), γ, the hind foot,and η, the tip of the tail, and at Alexandria all theseven stars except α, the head. It might be supposedthat here was a case of Homer “nodding”. But no; theold poet was perfectly right; the difference between thefacts as observed by him and as seen by us respectivelyis due to the Precession of the Equinoxes, the gradualmovement of the fixed stars themselves about the pole ofthe ecliptic, which was discovered by Hipparchus (secondcentury B.C.). We know from the original writings ofthe Greek astronomers that in Eudoxus’s time (say 380B.C.) the whole of the Great Bear remained always well5above the horizon, while in the time of Proclus (say A.D.460) the Great Bear “grazed” the horizon.
In Homer astronomical phenomena are only vaguelyused for such purposes as fixing localities or markingtimes of day or night. Sometimes constellations areused in giving sailing directions, as when Calypso directsOdysseus to sail in such a way as always to keep theGreat Bear on his left.
Hesiod mentions practically the same stars as Homer,but makes more use of celestial phenomena for determiningtimes and seasons. For example, he marked thetime for sowing at the beginning of winter by the settingof the Pleiades in the early twilight, or again by theearly setting of the Hyades or Orion, which means the3rd, 7th, or 15th November in the Julian calendaraccording to the particular stars taken; the time forharvest he fixed by the early rising of the Pleiades(19th May), threshing time by the early rising ofOrion (9th July), vintage time by the early rising ofArcturus (18th September), and so on. Hesiod makesspring begin sixty days after the winter solstice, and theearly summer fifty days after the summer solstice. Thushe knew about the solstices, though he says nothing ofthe equinoxes. He had an approximate notion of themoon’s period, which he put at thirty days.
But this use of astronomical facts for the purposeof determining times and seasons or deducing weatherindications is a very different thing from the scienceof astronomy, which seeks to explain the heavenlyphenomena and their causes. The history of thisscience, as of Greek philosophy in general, begins withThales.
The Ionian Greeks were in the most favourable positionfor initiating philosophy. Foremost among theGreeks in the love of adventure and the instinct of new6discovery (as is shown by their leaving their homes tofound settlements in distant lands), and fired, like allGreeks, with a passion for knowledge, they needed littleimpulse to set them on the road of independent thoughtand speculation. This impulse was furnished by theircontact with two ancient civilisations, the Egyptian andthe Babylonian. Acquiring from them certain elementaryfacts and rules in mathematics and astronomy whichhad been handed down through the priesthood from remoteantiquity, they built upon them the foundation of thescience, as distinct from the mere routine, of the subjectsin question.
Thales of Miletus (about 624–547 B.C.) was a man ofextraordinary versatility; philosopher, mathematician,astronomer, statesman, engineer, and man of business,he was declared one of the Seven Wise Men in 582–581B.C. His propensity to star-gazing is attested bythe story of his having fallen into a well while watchingthe stars, insomuch that (as Plato has it) he was rallied bya clever and pretty maidservant from Thrace for being so“eager to know what goes