is, and made persistent attempts to reduce the apparent motions of
celestial objects to geometrical laws. Some of the Pythagoreans, if not
Pythagoras himself, held that the earth is a sphere, and that the
apparent daily revolution of the sun and stars is really due to a motion
of the earth, though at first this motion of the earth was not supposed
to be one of rotation about an axis. These notions, and also that the
planets on the whole move round from west to east with reference to the
stars, were made known to a larger circle through the writings of Plato.
To Plato moreover is attributed the challenge to astronomers to
represent all the motions of the heavenly bodies by uniformly described
circles, a challenge generally held responsible for a vast amount of
wasted effort, and the postponement, for many centuries, of real
progress. Eudoxus of Cnidus, endeavouring to account for the fact that
the planets, during every apparent revolution round the earth, come to
rest twice, and in the shorter interval between these "stationary
points," move in the opposite direction, found that he could represent
the phenomena fairly well by a system of concentric spheres, each
rotating with its own velocity, and carrying its own particular planet
round its own equator, the outermost sphere carrying the fixed stars. It
was necessary to assume that the axes about which the various spheres
revolved should have circular motions also, and gradually an increased
number of spheres was evolved, the total number required by Aristotle
reaching fifty-five. It may be regarded as counting in Aristotle's
favour that he did consider the earth to be a sphere and not a flat
disc, but he seems to have thought that the mathematical spheres of
Eudoxus had a real solid existence, and that not only meteors, shooting
stars and aurora, but also comets and the milky way belong to the
atmosphere. His really great service to science in collating and
criticising all that was known of natural science would have been
greater if so much of the discussion had not been on the exact meaning
of words used to describe phenomena, instead of on the facts and causes
of the phenomena themselves.

Aristarchus of Samos seems to have been the first to suggest that the
planets revolved not about the earth but about the sun, but the idea
seemed so improbable that it was hardly noticed, especially as
Aristarchus himself did not expand it into a treatise.

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