THE GREEK ASTRONOMY. 



they could hardly avoid discovering the anomaly or unequal motion o' 

 the moon ; for in every revolution, her daily progression in the heavens 

 varies from about twenty-two to twenty-six times her own diameter. 

 But there is not, in their knowledge of this Period, any evidence that 

 they had measured the amount of this variation ; and Delambre 6 is 

 probably right in attributing all such observations to the Greeks. 



The sun's motion wouJd 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 equinoctial 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 mechanism of epicycles 

 might be applied so as to explain irregularities of this .kind. A wheel 

 travelling round the earth, while it revolved upon its centre, might 

 produce the effect of making the sun or moon fixed in its rim go some- 

 times 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. Ac- 

 cordingly, it is probable that, at the time of Plato and Aristotle, 

 philosophers were already endeavoring to apply the hypothesis to these 

 cases, though it does not appear that any one fully succeeded before 

 Hipparchus. 



The problem which was thus present to the minds of astronomers, 

 and which Plato is said to have proposed to them in a distinct form, 

 was, " To reconcile the celestial phenomena by the combination of 

 equable circular motions." That the circular motions should be equable 

 as well as circular, was a condition, which, if it had been merely tried 

 at first, as the most simple and definite conjecture, would have de- 

 served praise. But this condition, which is, in reality, inconsistent 

 with nature, was, in the sequel, adhered to with a pertinacity which 

 introduced endless complexity into the system. The history of this 

 assumption is one of the most marked instances of that love of sim- 

 plicity and symmetry which is the source of all general truths, though 

 it so often produces and perpetuates error. At present we can easily 

 see how fancifully the notion of simplicity and perfection was inter- 

 preted, in the arguments by which the opinion was defended, that the 



8 Astro nomie Ancienne, i. 212. 



