32 A Short History of Astronomy [Ca. n. 



argues from the circular form of the boundary of the shadow 

 as seen on the face of the moon during the progress of the 

 eclipse, or in a partial eclipse, that the earth must be 

 spherical ; for otherwise it would cast a shadow of a dif- 

 ferent shape. A second reason for the spherical form of 

 the earth is that when we move north and south the stars 

 change their positions with respect to the horizon, while 

 some even disappear and fresh ones take their place. This 

 shows that the direction of the stars has changed as com- 

 pared with the observer's horizon; hence, the actual direction 

 of the stars being imperceptibly affected by any motion of 

 the observer on the earth, the horizons at two places, north 

 and south of one another, are in different directions, and the 



earth is therefore curved. For 



B "^v^ v .5 example, if a star is visible to an 

 observer at A (fig. 12), while to 

 an observer at B it is at the same 

 time invisible, i.e. hidden by the 

 earth, the surface of the earth 

 FIG. 12. The curvature of at A must be in a different direc- 

 the earth. tion from that at B. Aristotle 



quotes further, in confirmation of 



the roundness of the earth, that travellers from the far 

 East and the far West (practically India and Morocco) 

 alike reported the presence of elephants, whence it may be 

 inferred that the two regions in question are not very far 

 apart. He also makes use of some rather obscure arguments 

 of an a priori character. 



There can be but little doubt that the readiness with 

 which Aristotle, as well as other Greeks, admitted the 

 spherical form of the earth and of the heavenly bodies, 

 was due to the affection which the Greeks always seem 

 to have had for the circle and sphere as being " perfect," 

 i.e. perfectly symmetrical figures. 



30. Aristotle argues against the possibility of the revo- 

 lution of the earth round the sun, on the ground that this 

 motion, if it existed, ought to produce a corresponding 

 apparent motion of the stars. We have here the first 

 appearance of one of the most serious of the many objections 

 ever brought against the belief in the motion of the earth, 

 an objection really only finally disposed of during the 



