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." * He was the first to write
systematically on trigonometry; he discovered the precession of the equinoxes; he estimated the
length of the lunar month with an error of less than one second; he improved Aristarchus's
estimates of the sizes and distances of the sun and moon; he made a catalogue of eight hundred
and fifty fixed stars, giving
* Greek Mathematics, Vol. II, p. 253.-215-their latitude and longitude. As against the heliocentric hypothesis of Aristarchus, he adopted and
improved the theory of epicycles which had been invented by Apollonius, who flourished about
220 B.C.; it was a development of this theory that came to be known, later, as the Ptolemaic
system, after the astronomer Ptolemy, who flourished in the middle of the second century A.D.
Copernicus came to know something, though not much, of the almost forgotten hypothesis of
Aristarchus, and was encouraged by finding ancient authority for his innovation. Otherwise, the
effect of this hypothesis on subsequent astronomy was practically nil.Ancient astronomers, in
estimating the sizes of the earth, moon, and sun, and the distances of the moon and sun, used
methods which were theoretically valid, but they were hampered by the lack of instruments of
precision. Many of their results, in view of this lack, were surprisingly good. Eratosthenes
estimated the earth's diameter at 7850 miles, which is only about fifty miles short of the truth.
Ptolemy estimated the mean distance of the moon at 29 ½ times the earth's diameter; the correct
figure is about 30.7. None of them got anywhere near the size and distance of the sun, which all
underestimated. Their estimates, in terms of the earth's diameter, were:
Aristarchus, 180;Hipparchus, 1245;