Thus the movements both of the sun and the moon, as well
as the diurnal rotation of the celestial sphere, seemed to justify
the notion that all celestial movements must be "perfect," that is to
say, described uniformly in those circles which were the only perfect
curves.
The simplest observations, however, show that the movements of the
planets cannot be explained in this simple fashion. Here the
geometrical genius of Ptolemy shone forth, and he devised a scheme by
which the apparent wanderings of the planets could be accounted for
without the introduction of aught save "perfect" movements.
To understand his reasoning, let us first set forth clearly those
facts of observation which require to be explained. I shall take, in
particular, two planets, Venus and Mars, as these illustrate, in the
most striking manner, the peculiarities of the inner and the outer
planets respectively. The simplest observations would show that
Venus did not move round the heavens in the same fashion as the sun
or the moon. Look at the evening star when brightest, as it appears
in the west after sunset. Instead of moving towards the east among
the stars, like the sun or the moon, we find, week after week, that
Venus is drawing in towards the sun, until it is lost in the
sunbeams.
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