PRELUDE TO THE EPOCH OF HIPPARCHUS. 177 



her daily progression in the heavens varies from 

 about 22 to 26 times her own diameter. But there 

 is not, in the existence of this period, any evidence 

 that they had measured the amount of this varia- 

 tion ; and Delambre 6 is probably right in attribut- 

 ing all such observations to the Greeks. 



The sun's motion would also be seen to be 

 irregular as soon as men had any exact mode 

 of determining the lengths of the four seasons, by 

 means of the passage of the sun through the equi- 

 noctial and solstitial points. For spring, summer, 

 autumn, and winter, which would each consist of 

 an equal number of days if the motions were 

 uniform, are, in fact, found to be unequal in 

 length. 



It was not very difficult to see that the mecha- 

 nism of epicycles might be applied so as to explain 

 irregularities of this kind. A wheel travelling 

 round the earth, while it revolved upon its center, 

 might produce the effect of making the sun or 

 moon fixed in its rim go sometimes faster and 

 sometimes slower in appearance, just in the same 

 way as the same suppositions would account for a 

 planet going sometimes forwards and sometimes 

 backwards: the epicycles of the sun and moon 

 would, for this purpose, be less than those of the 

 planets. Accordingly, it is probable that, at the 

 time of Plato and Aristotle, philosophers were 

 already endeavouring to apply the hypothesis to 



6 Astronomic Ancienne, i. 212. 

 VOL. I. N 



