174 SCIENCE IN GRECO-ROMAN ANTIQUITY 



sphere of the fixed stars. The earth is animated by a 

 dual movement : the diurnal movement of rotation on 

 its axis, and the annual movement of revolution round 

 the sun. The planets also revolve round the sun. 

 According to Aristarchus it must also be supposed 

 that the sphere of the stars is very far away, otherwise 

 the existence of parallaxes would be ascertained, 

 which, in his opinion, is not the case. 



This conception, as ingenious as audacious, had no 

 renown in antiquity. The reasons for this failure are 

 diverse, religious as well as scientific. To liken the 

 earth to the planets, by making it, like them, revolve 

 round the sun, was to be guilty of impiety, for it 

 abolished the distinction between the corruptible 

 matter of the earth and the incorruptible essence of 

 the stars. The hypothesis of Aristarchus was also 

 contradictory to the then known laws of physics, since 

 the earth, being composed of the heaviest elements, must 

 necessarily occupy the centre of the universe. Lastly, 

 this hypothesis by its use of circular movements alone, 

 did not account for the inequality of the seasons. For 

 these reasons we can well understand why it was not 

 followed up. 



The solution of the difficulties which the system of 

 Eudoxus could not overcome was sought in another 

 direction. Hipparchus and Ptolemy, using the works 

 of Apollonius, had recourse to a combination of 

 eccentrics and epicycles. An eccentric movement 

 is that described by a circle turning round a point 

 within it other than its centre. A system of epicycles 

 is formed by an arrangement of successive circles 

 such that the centre of one is at a point on the circum- 

 ference of the other. It is therefore necessary first to 

 observe the stationary points, the retrogradations and 

 the variable brightness of a planet, and notice the 

 differences according to the region of the heaven it 

 traverses, and then find the combination of epicycles 



