230 THE GREEK ASTRONOMY. 



soon proceeded to represent this inequality by a 

 combination of circular motions, uniting, for this 

 purpose, the hypothesis of an epicycle, already 

 employed to explain the first inequality, with the 

 hypothesis of an eccentric, in the circumference of 

 which the center of the epicycle was supposed to 

 move. The mode of combining these was some- 

 what complex; more complex we may, perhaps, 

 say, than was absolutely requisite 33 ; the apogee of 

 the eccentric moved backwards, or contrary to the 

 order of the signs, and the center of the epicycle 

 moved forwards nearly twice as fast upon the cir- 

 cumference of the eccentric, so as to reach a place 

 nearly, but not exactly, the same, as if it had 

 moved in a concentric instead of an eccentric path. 

 Thus the center of the epicycle went twice round 

 the eccentric in the course of one month : and in 

 this manner it satisfied the condition that it should 

 vanish at new and full moon, and be greatest 

 when the moon was in the quarters of her monthly 

 course (G). 



The discovery of the Evection, and the reduction 

 of it to the epicyclical theory, was, for several 

 reasons, an important step in astronomy; some 

 of these reasons may be stated. 



1. It obviously suggested, or confirmed, the 



33 If Ptolemy had used the hypothesis of an eccentric instead 

 of an epicycle for the first inequality of the moon, an epicycle 

 would have represented the second inequality more simply than 

 his method did. 



