$25,26] Aristarchns : Plato 27 



to the subject in various places. He condemned any 

 careful study of the actual celestial motions as degrading 

 rather than elevating, and apparently regarded the subject 

 as worthy of attention chiefly on account of its connection 

 with geometry, and because the actual celestial motions 

 suggested ideal motions of greater beauty and interest. 

 This view of astronomy he contrasts with the popular 

 conception, according to which the subject was useful 

 chiefly for giving to the agriculturist, the navigator, and 

 others a knowledge of times and seasons.* At the end 

 of the same dialogue he gives a short account of the 

 celestial bodies, according to which the sun, moon, planets, 

 and fixed stars revolve on eight concentric and closely 

 fitting wheels or circles round an axis passing through the 

 earth. Beginning with the body nearest to the earth, the 

 order is Moon, Sun, Mercury, Venus, Mars, Jupiter, Saturn, 

 stars. The Sun, Mercury, and Venus are said to perform 

 their revolutions in the same time, while the other planets 

 move more slowly, statements which shew that Plato was at 

 any rate aware that the motions of Venus and Mercury are 

 different from those of the other planets. He also states 

 that the moon shines by reflected light received from 

 the sun. 



Plato is said to have suggested to his pupils as a worthy 

 problem the explanation of the celestial motions by means 

 of a combination of uniform circular or spherical motions. 

 Anything like an accurate theory of the celestial motions, 

 agreeing with actual observation, such as Hipparchus and 

 Ptolemy afterwards constructed with fair success, would 

 hardly seem to be in accordance with Plato's ideas of the 

 true astronomy, but he may well have wished to see 

 established some simple and harmonious geometrical 

 scheme which would not be altogether at variance with 

 known facts. 



26. Acting to some extent on this idea of Plato's, Eudoxus 

 of Cnidus (about 409-356 B.C.) attempted to explain the 

 most obvious peculiarities of the celestial motions by means 

 of a combination of uniform circular motions. He may be 

 regarded as representative of the transition from speculative 



* Republic, VII. 529, 530. 



