210 THE GREEK ASTRONOMY. 



in use at this period, was such, their results could 

 not lead to any degree of conviction deserving to 

 be set in opposition to a theory which was so 

 satisfactory with regard to the more certain obser- 

 vations. 



The Eccentricity, or the Radius of the Epicycle, 

 which would satisfy the inequality of the motions 

 of the moon, would, in fact, double the inequality 

 of the distances. The Eccentricity of the moon's 

 orbit is determined by Ptolemy as T ^ of the radius 

 of the orbit ; but its real amount is only half as 

 great; this difference is a necessary consequence 

 of the supposition of uniform circular motions, on 

 which the Epicyclic Hypothesis proceeds. 



We see, therefore, that this part of the Hippar- 

 chian theory carries in itself the germ of its own 

 destruction. As soon as the art of celestial mea- 

 surement was so far perfected, that astronomers 

 could be sure of the apparent diameter of the moon 

 within sV or 5^ of the whole, the inconsistency 

 of the theory with itself would become manifest. 

 We shall see, hereafter, the way in which this 

 inconsistency operated; in reality, a very long 

 period elapsed before the methods of observing 

 were sufficiently good to bring it clearly into view. 



Sect. 3. Methods of Observation of the Greek 

 Astronomers. 



WE must now say a word concerning the Methods 

 above spoken of. Since one of the most important 



