ARISTOTLE AND THE EARLIER PERIPATETICS vol.I by Eduard Zeller, B.F.C.Costelloe 1897

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PHYSICS * 409
specified circumstances, to determine both the number
of the spheres and also the direction and rapidity of
their rotation, so as to explain the motions of the stars
revealed to us by observation. 1
For this purpose Eudoxus, the famous astronomer
of Cnidos, who may be regarded as the first founder of
a complete theory of the spheres based upon accurate
observation, 2 sketched out a system of twenty-seven
spheres, twenty-six of which belong to the planets.
Considering the simple nature of its motion, he thought
one sphere enough for the heaven of the fixed stars, and
in this sphere the whole assemblage of the stars was
fastened. On the other hand, he assigned four spheres
to each of the five upper planets, and three apiece to
the sun and moon, which, in agreement with Plato, he
placed lowest in the planetary scale. The first sphere
of each planet was intended to explain its
daily revolution
in concert with the heaven of the fixed stars,
since it accomplished every day a rotation from east
to the same planet. Aristotle, on immediate contact with one an -
the contrary, extended Plato's other (see p. 496, n.l.swpra), each
doctrine to the relation of all the can communicate its motion to the
upper spheres to those that are one next below it. This relation
contained within them, as is need not apply with equal strict
clear from his hypothesis of ness to the elementary spheres as
retrogressive spheres (see infra), to the heavenly, seeing that they
(Cf. also De Casio, ii. 12, 293, a, do not, like the latter, consist of
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