ASTRONOMY 173 



also affirmed the materiality of the spheres by consider- 

 ing them to be composed of ether. This materialization 

 was generally abandoned later on, until the time when 

 the idea was revived by the Arabs. 



The system of homocentric spheres, perfected by 

 Calippus and Aristotle a hundred years before, sur- 

 vived until towards the end of the third century B.C. 

 It clashed, however, with weighty arguments based on 

 the noticeable variations of brightness shown by the 

 planets, especially Mars and Venus. These variations 

 of brightness indicate that the distances of the planets 

 from the observer change in a manner which is incom- 

 patible with a system of spheres concentric to the 

 earth, in which the planets are always equally distant 

 from the earth. 1 Further, the theory of Eudoxus 

 does not explain why Mercury and Venus are the only 

 planets which always remain in the neighbourhood of 

 the sun. 



To surmount these difficulties, a disciple of Plato, 

 namely Heraclides of Pontus, had recourse to two 

 hypotheses, of which one, which is quite original, 

 admits a partial heliocentrism. Like the Pythagorean 

 Ecphantus he first of all affirmed that the earth is at 

 the centre of an infinite universe and that it turns 

 on its axis in twenty-four hours, which explains the 

 apparent revolution of the starry heavens. This 

 being so, he supposed that Venus and Mercury revolve 

 round the sun, whilst the latter moves round the earth 

 as do the other planets. 



Aristarchus of Samos, the date of whose scientific 

 work is about 280 B.C., went farther still in the same 

 direction. He conceived a heliocentric system, the 

 essential ideas of which were reproduced by Copernicus 

 in the sixteenth century, and which may be described 

 as follows : the motionless sun is situated at the centre 

 of the universe which is bounded by the immobile 

 1 2 Bigourdan, Astronomie, p. 254. 



