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[1073b] [1]

Thus it is clear that the movers are substances, and that one of them is first and another second and so on in the same order as the spatial motions of the heavenly bodies.As regards the number of these motions, we have now reached a question which must be investigated by the aid of that branch of mathematical science which is most akin to philosophy, i.e. astronomy; for this has as its object a substance which is sensible but eternal, whereas the other mathematical sciences, e.g. arithmetic and geometry, do not deal with any substance. That there are more spatial motions than there are bodies which move in space is obvious to those who have even a moderate grasp of the subject, since each of the non-fixed stars has more than one spatial motion.As to how many these spatial motions actually are we shall now, to give some idea of the subject, quote what some of the mathematicians say, in order that there may be some definite number for the mind to grasp; but for the rest we must partly investigate for ourselves and partly learn from other investigators, and if those who apply themselves to these matters come to some conclusion which clashes with what we have just stated, we must appreciate both views, but follow the more accurate.

Eudoxus1 held that the motion of the sun and moon involves in either case three spheres,2 of which the outermost is that of the fixed stars,3 the second revolves in the circle which bisects the zodiac,4 [20] and the third revolves in a circle which is inclined across the breadth of the zodiac5; but the circle in which the moon moves is inclined at a greater angle than that in which the sun moves.And he held that the motion of the planets involved in each case four spheres; and that of these the first and second are the same6 as before (for the sphere of the fixed stars is that which carries round all the other spheres, and the sphere next in order, which has its motion in the circle which bisects the zodiac, is common to all the planets); the third sphere of all the planets has its poles in the circle which bisects the zodiac; and the fourth sphere moves in the circle inclined to the equator of the third. In the case of the third sphere, while the other planets have their own peculiar poles, those of Venus and Mercury are the same.

Callippus7 assumed the same arrangement of the spheres as did Eudoxus (that is, with respect to the order of their intervals), but as regards their number, whereas he assigned to Jupiter and Saturn the same number of spheres as Eudoxus, he considered that two further spheres should be added both for the sun and for the moon, if the phenomena are to be accounted for, and one for each of the other planets.

But if all the spheres in combination are to account for the phenomena,

1 Of Cnidus (circa 408 -355 B.C.). He was a pupil of Plato, and a distinguished mathematician.

2 For a full discussion of the theories of Eudoxus and Callipus see Dreyer, Planetary Systems 87-114; Heath,Aristarchus of Samos190-224.

3 Not identical with that of the fixed stars, but having the same motion.

4 i.e., revolves with its equator in the ecliptic.

5 i.e., has the plane of its equator inclined to the plane of the ecliptic. This sphere carries the sun (or moon) fixed to a point in its equator.

6 Not the same, but having the same motion.

7 of Cyzicus (fl. 380 B.C.). Simplicius says (Simplicius 493.5-8) that he corrected and elaborated Eudoxus's theory with Aristotle's help while on a visit to him at Athens.

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