Homocentric Spheres in de caelo

Abstract

It is commonly assumed that Aristotle's starting point in the development of his cosmological ideas was the Platonic system described in the Timaeus, and this seems a natural assumption to make. But it is not clear at what stage Aristotle abandoned this system for the theory of homocentric spheres propounded by Eudoxus and Callippus,l nor is the development of this theory itself certain. We must not assume without reason that Aristotle had already adopted the theory at the time when he wrote De Caelo; but in one chapter at any rate, Bi 2, there is clear evidence that he had done so,2 and I wish first to consider the form which the theory takes in that chapter. In Metaphysics A 3Aristotle mentions three versions of the theory: that of Eudoxus, that of Callippus and his own. It is generally agreed that while the first two of these differ from one another only in detail, the third differs in principle from them both. The theory as originally formulated by Eudoxus was purely an abstract mathematical construction representing the paths followed by the heavenly bodies; Aristotle gave it an entirely new character by turning it into a mechanical system of physically existing spheres by means of which the celestial motions are produced.4