23*2 THE GREEK ASTRONOMY. 



in an ellipse, so far as the central attraction is 

 undisturbed by any other. This first inequality 

 is called the Elliptic Inequality, or, more usually 

 the Equation of the Center (H). All the planets 

 have such inequalities, but the Evection is peculiar 

 to the moon. The discovery of other inequalities 

 of the moon's motion, the Variation and Annual 

 Equation, made an immediate sequel in the order 

 of the subject to the discoveries of Ptolemy, al- 

 though separated by a long interval of time ; for 

 these discoveries were only made by Tycho Brahe 

 in the sixteenth century. The imperfection of 

 astronomical instruments was the great cause of 

 this long delay. 



3. The Epicyclical Hypothesis was found 

 capable of accommodating itself to such new dis- 

 coveries. These new inequalities could be repre- 

 sented by new combinations of eccentrics and 

 epicycles: all the real and imaginary discoveries 

 of astronomers, up to Copernicus, were actually 

 embodied in these hypotheses; Copernicus, as we 

 have said, did not reject such hypotheses; the 

 lunar inequalities which Tycho detected might 

 have been similarly exhibited ; and even Newton 31 

 represents the motion of the moon's apogee by 

 means of an epicycle. As a mode of expressing 

 the law of the irregularity, and of calculating 

 its results in particular cases, the epicyclical 

 theory was capable of continuing to render groat 

 31 Principin, lib. iii. prop. xxxv. 



