ALEXANDRIAN OR HELLENISTIC PERIOD 



The idea is perhaps made clearer if we picture the 

 actual progress of the lantern attached to the rim of an 

 ordinary cart-wheel. When the cart is drawn forward 

 the lantern is made to revolve in a circle as regards the 

 hub of the wheel, but since that hub is constantly going 

 forward, the actual path described by the lantern is 

 not a circle at all but a waving line. It is precisely the 

 same with the imagined course of the sun in its orbit, 

 only that we view these lines just as we should view 

 the lantern on the wheel if we looked at it from di- 

 rectly above and not from the side. The proof that 

 the sun is describing this waving line, and therefore 

 must be considered as attached to an imaginary wheel, 

 is furnished, as it seemed to Hipparchus, by the ob- 

 served fact of the sun's varying speed. 



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 Hip- 

 parchus. 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 concep- 

 tion 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 ob- 

 served motions of the heavenly bodies. Calippus, the 

 reformer of the calendar, is said to have carried an ac- 

 count of this theory to Aristotle. As new irregularities 

 of motion of the sun, moon, and planetary bodies were 



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