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That the movers are substances, then, and that one of these is first
and another second according to the same order as the movements of the
stars, is evident. But in the number of the movements we reach a
problem which must be treated from the standpoint of that one of the
mathematical sciences which is most akin to philosophy-viz. of
astronomy; for this science speculates about substance which is
perceptible but eternal, but the other mathematical sciences, i.e.
arithmetic and geometry, treat of no substance. That the movements are
more numerous than the bodies that are moved is evident to those who
have given even moderate attention to the matter; for each of the
planets has more than one movement. But as to the actual number of
these movements, we now-to give some notion of the subject-quote
what some of the mathematicians say, that our thought may have some
definite number to grasp; but, for the rest, we must partly
investigate for ourselves, Partly learn from other investigators,
and if those who study this subject form an opinion contrary to what
we have now stated, we must esteem both parties indeed, but follow the
more accurate.
Eudoxus supposed that the motion of the sun or of the moon
involves, in either case, three spheres, of which the first is the
sphere of the fixed stars, and the second moves in the circle which
runs along the middle of the zodiac, and the third in the circle which
is inclined across the breadth of the zodiac; but the circle in
which the moon moves is inclined at a greater angle than that in which
the sun moves
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