History of Western Philosophy - Bertrand Russell [127]
To this various observations contributed. Oenopides, who was slightly later than Anaxagoras, discovered the obliquity of the ecliptic. It soon became clear that the sun must be much larger than the earth, which fact supported those who denied that the earth is the centre of the universe. The central fire and the counter-earth were dropped by the Pythagoreans soon after the time of Plato. Heraclides of Pontus (whose dates are about 388 to 315 B.C., contemporary with Aristotle) discovered that Venus and Mercury revolve about the sun, and adopted the view that the earth rotates on its own axis once every twenty-four hours. This last was a very important step, which no predecessor had taken. Heraclides was of Plato's school, and must have been a great man, but was not as much respected as one would expect; he is described as a fat dandy.
Aristarchus of Samos, who lived approximately from 310 to 230 B.C., and was thus about twenty-five years older than Archimedes, is the most interesting of all ancient astronomers, because he advanced the complete Copernican hypothesis, that all the planets, including the earth, revolve in circles round the sun, and that the earth, rotates on its axis once in twenty-four hours. It is a little disappointing to find that the only extant work of Aristarchus, On the Sizes and Distances of the Sun and the Moon, adheres to the geocentric view. It is true that, for the problems with which this book deals, it makes no difference which theory is adopted, and he may therefore have thought it unwise to burden his calculations with an unnecessary opposition to the general opinion of astronomers; or he may have only arrived at the Copernican hypothesis after writing this book. Sir Thomas Heath, in his work on Aristarchus,3 which contains the text of this book with a translation, inclines to the latter view. The evidence that Aristarchus suggested the Copernican view is, in any case, quite conclusive.
The first and best evidence is that of Archimedes, who, as we have seen, was a younger contemporary of Aristarchus. Writing to Gelon, King of Syracuse, he says that Aristarchus brought out 'a book consisting of certain hypotheses', and continues: 'His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit.' There is a passage
in Plutarch saying that Cleanthes 'thought it was the duty of the Greeks to indict Aristarchus of Samos on the charge of impiety for putting in motion the Hearth of the Universe (i.e. the earth), this being the effect of his attempt to save the phenomena by supposing the heaven to remain at rest and the earth to revolve in an oblique circle, while it rotates, at the same time, about its own axis'. Cleanthes was a contemporary of Aristarchus, and died about 232 B.C. In another passage, Plutarch says that Aristarchus advanced this view only as a hypothesis, but that his successor Seleucus maintained it as a definite opinion. (Seleucus flourished about 150 B.C.) Aëtius and Sextus Empiricus also assert that Aristarchus advanced the heliocentric hypothesis, but do not say that it was set forth by him only as a hypothesis. Even if he did so, it seems not unlikely that he, like Galileo two thousand years later, was influenced by the fear of offending religious prejudices, a fear which the attitude of Cleanthes (mentioned above) shows to have been well grounded.
The Copernican hypothesis, after being advanced, whether positively or tentatively, by Aristarchus, was definitely adopted by Seleucus, but by no other ancient astronomer. This general rejection was mainly due to Hipparchus, who flourished from 161 to 126 B.C. He is described by Heath as 'the greatest astronomer of antiquity'.4 He was the first to write systematically on trigonometry; he discovered the precession of the equinoxes; he estimated