CHAPTER XXII 



ROEMER AND THE PROOF OF THE EARTH'S 

 MOTION 



WITH the demonstration of the law of attraction, the triumph 

 of the Coppernican theory seemed complete. Tested in every 

 direction in every land where observations were carried out, 

 the ideas of Newton were subjected to inquiry only to be con- 

 firmed, their application extended. There seemed no flaw, no 

 difficulty. And yet a difficulty remained. It was the same 

 that had met Aristarchus and Coppernicus, that had baffled 

 Galileo. It was left unsolved by Newton or by any contem- 

 porary. That was the unchangeable position of the fixed stars. 



To escape the difficulty, sometimes like the ancients men 

 had imagined a crystal sphere wherein the stars are fixed like 

 so many nails studded in a wall. Galileo considered this, but 

 in his third Dialogue he said : 



" Still I do not believe that all the stars are scattered over a 

 spherical surface at equal distances from a common centre ; but 

 I am of the opinion that their distances to us are so various 

 that some of them may be two or three times as remote as others, 

 so that when some minute star is discovered by the telescope 

 close to one of the larger, and the former is yet highest, it may 

 be that some sensible change may take place among them." l 



In the century that followed the invention of the telescope 

 no such change could be detected, and a truly marvellous pre- 

 cision had been attained. The accuracy of observations had 

 been carried down from limits of error of the fraction of a degree, 

 obtainable by the ancient instruments, first to a minute, finally 

 down to a few seconds. The telescope with which Galileo had 

 discovered the " little world " of Jupiter, magnified only seven 

 diameters. All his great discoveries, as we have seen, were 

 made with a telescope which did not magnify beyond thirty- two 

 diameters. Within fifty years Auzout had constructed colossal 



1 Dialogues on the Two Great World Systems, III. 

 281 



