io6 A Short History of Astronomy [CH. iv. 



earth Coppernicus discusses the chief objections which had 

 been urged by Ptolemy. To the objection that if the earth 

 a rapid motion of rotation about its axis, the earth 

 would be in danger of flying to pieces, and the air, as well 

 as loose objects on the surface, would be left behind, he 

 replies that if such a motion were dangerous to the solid 

 earth, it must be much more so to the celestial sphere, which, 

 on account of its vastly greater size, would have to move 

 enormously faster than the earth to complete its daily 

 rotation ; he enters also into an obscure discussion of 

 difference between a " natural " and an " artificial " motion, 

 of which the former might be expected not to disturb 

 anything on the earth. 



Coppernicus shews that the earth is very small compared 

 to the sphere of the stars, because wherever the observer 

 is on the earth the horizon appears to divide the celestial 

 sphere into two equal parts and the observer appears always 

 to be at the centre of the sphere, so that any distance 

 through which the observer moves on the earth is im- 

 perceptible as compared with the distance of the stars. 



8 1. He goes on to argue that the chief irregularity in the 

 motion of the planets, in virtue of which they move back- 

 wards at intervals (chapter I., 14, and chapter IL, 51), 

 can readily be explained in general by the motion of the 

 earth and by a motion of each planet round the sun, in its 

 own time and at its own distance. From the fact that 

 Venus and Mercury were never seen very far from the sun, 

 it could be inferred that their paths were nearer to the sun 

 than that of the earth, Mercury being the nearer to the sun 

 of the two, because never seen so far from it in the sky as 

 Venus. The other three planets, being seen at times in a 

 direction opposite to that of the sun, must necessarily 

 evolve round the sun in orbits larger than that of the 

 earth, a view confirmed by the fact that they were brightest 

 when opposite the sun (in which positions they would be 

 nearest to us). The order of their respective distances 

 from the sun could be at once inferred from the disturbing 

 effects produced on their apparent motions by the motion 

 of the earth ; Saturn being least affected must on the whole 

 be farthest from the earth, Jupiter next, and Mars next. 

 The earth thus became one of six planets revolving round 



