lived at the end of the fifth century B.C. Although it is fanciful and in part quite unscientific, it
is very important, since it involves the greater part of the imaginative effort required for
conceiving the Copernican hypothesis. To conceive of the earth, not as the centre of the
universe, but as one among the planets, not as eternally fixed, but as wandering through space,
showed an extraordinary emancipation from anthropocentric thinking. When once this jolt had
been given to men's natural picture of the universe, it was not so very difficult to be led by
scientific arguments to a more accurate theory.
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, * which contains the text
of this book
* Aristarchus of Samos, the Ancient Copernicus. By Sir Thomas Heath. Oxford 1913. What
follows is based on this book.