PRELUDE TO THE EPOCH OF HIPPARCHUS. 175 



of C nidus introduced the hypothesis of revolving 

 circles or spheres. Calippus of Cyzicus, having 

 visited Polemarchus, an intimate friend of Eudoxus, 

 they went together to Athens, and communicated 

 to Aristotle the invention of Eudoxus, and with his 

 help improved and corrected it. 



Probably at first this hypothesis was applied 

 only to account for the general phenomena of the 

 progressions, retrogradations, and stations of the 

 planet ; but it was soon found that the motions of 

 the sun and moon, and the circular motions of 

 the planets, which the hypothesis supposed, had 

 other anomalies or irregularities, which made a 

 further extension of the hypothesis necessary. 



The defect of uniformity in these motions of the 

 sun and moon, though less apparent than in the 

 planets, is easily detected, as soon as men endea- 

 vour to obtain any accuracy in their observations. 

 We have already stated (Chap. I.) that the Chal- 

 deans were in possession of a period of about 18 

 years, which they used in the calculation of eclipses, 

 and which might have been discovered by close 

 observation of the moon's motions; although it 

 was probably rather hit upon by noting the recur- 

 rence of eclipses. The moon moves in a manner 

 which is not reducible to regularity without con- 

 siderable care and time. If we trace her path 

 among the stars, we find that, like the path of the 

 sun, it is oblique to the equator, but it does not, 

 like that of the sun, pass over the same stars in 



