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EudoxusOf Cnidus (circa 408 -355 B.C.). He was a pupil of Plato, and a distinguished mathematician. held that the motion of the sun and moon involves in either case three spheres,For a full discussion of the theories of Eudoxus and Callipus see Dreyer, Planetary Systems 87-114; Heath,Aristarchus of Samos190-224. of which the outermost is that of the fixed stars,Not identical with that of the fixed stars, but having the same motion. the second revolves in the circle which bisects the zodiac,i.e., revolves with its equator in the ecliptic. and the third revolves in a circle which is inclined across the breadth of the zodiaci.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.; 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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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 sameNot the same, but having the same motion. 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.
