That is one way of looking at the matter. It is an hypothesis that explains the observed facts--after a fashion, and indeed a very remarkable fashion. The idea of such an explanation did not originate with Hipparchus. The germs of the thought were as old as the Pythagorean doctrine that the earth revolves about a centre that we cannot see. Eudoxus gave the conception greater tangibility, and may be considered as the father of this doctrine of wheels--epicycles, as they came to be called. Two centuries before the time of Hipparchus he conceived a doctrine of spheres which Aristotle found most interesting, and which served to explain, along the lines we have just followed, the observed motions of the heavenly bodies. Calippus, the reformer of the calendar, is said to have carried an account of this theory to Aristotle. As new irregularities of motion of the sun, moon, and planetary bodies were pointed out, new epicycles were invented. There is no limit to the number of imaginary circles that may be inscribed about an imaginary centre, and if we conceive each one of these circles to have a proper motion of its own, and each one to carry the sun in the line of that motion, except as it is diverted by the other motions--if we can visualize this complex mingling of wheels--we shall certainly be able to imagine the heavenly body which lies at the juncture of all the rims, as being carried forward in as erratic and wobbly a manner as could be desired. In other words, the theory of epicycles will account for all the facts of the observed motions of all the heavenly bodies, but in so doing it fills the universe with a most bewildering network of intersecting circles. Even in the time of Calippus fifty-five of these spheres were computed.