INDUCTIVE EPOCH OF HIPPARCHUS. 145 



CHAPTER III. 

 INDUCTIVE EPOCH OF HIPPARCHUS. 



Sect. 1. Establishment of the Theory of Epicycles and Eccentrics. 



A LTHOUGH, as we have already seer,, at the time of Plato, the 

 -^*- Idea of Epicycles had been suggested, and the problem of its gen- 

 eral application proposed, and solutions of this problem offered by his 

 followers; we still consider Hipparchus as the real discoverer and 

 founder of that theory ; inasmuch as he not only guessed that it might, 

 but showed that it must, account for the phenomena, both as to their 

 nature and as to their quantity. The assertion that " he only discovers 

 who proves," is just ; not only because, until a theory is proved to be 

 the true one, it has no pre-eminence over the numerous other guesses 

 among which it circulates, and above which the proof alone elevates 

 it ; but also because he who takes hold of the theory so as to apply 

 calculation to it, possesses it with a distinctness of conception- which 

 makes it peculiarly his. 



In order to establish the Theory of Epicycles, it was necessary to 

 assign the magnitudes, distances, and positions of the circles or spheres 

 in which the heavenly bodies were moved, in such a manner as to ac- 

 count for their apparently irregular motions. We may best under- 

 stand what was the problem to be solved, by calling to mind what we 

 now know to be the real motions of the heavens. The true motion of 

 the earth round the sun, and therefore the apparent annual motion of 

 the sun, is performed, not in a circle of which the earth is the centre, 

 but in an ellipse or oval, the earth being nearer to one end than to the 

 other ; and the motion is most rapid when the sun is at the nearer 

 end of this oval. But instead of an oval, we may suppose the sun to 

 move uniformly in a circle, the earth being now, not in the centre, 

 but nearer to one side ; for on this supposition, the sun will appear to 

 move most quickly when he is nearest to the earth, or in his Perigee, 

 as that point is called. Such an orbit is called an Eccentric, and the 

 distance of the earth from the centre of the circle is called the Eccen- 

 tricity. It may easily be shown by geometrical reasoning, that the 

 Inequality of apparent motion so produced, is exactly the same in 

 YOB." I. 10 



