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.

About this time the necessity for more accurate places of the sun and
moon, and the liberality of the Ptolemys who ruled Egypt, combined to
provide regular observations at Alexandria, so that, when Hipparchus
came upon the scene, there was a considerable amount of material for him
to use. His discoveries marked a great advance in the science of
astronomy. He noted the irregular motion of the sun, and, to explain it,
assumed that it revolved uniformly not exactly about the earth but
about a point some distance away, called the "excentric".[1] The line
joining the centre of the earth to the excentric passes through the
apses of the sun's orbit, where its distance from the earth is greatest