CHAPTERS ON THE STABS. 653 



more likely of the two. From what we know of the constitution of the 

 stars, a change in the color of one of these bodies in so short a period 

 of time as that embraced by history is so improbable as to require 

 much stronger proofs than any that can be adduced from ancient 

 writers. In addition to the possible vagueness or errors of the original 

 writers, we have to bear in mind the possible mistakes or misinterpreta- 

 tions of the copyists who reproduced the manuscripts. 



THE PARALLAXES OF THE STARS. 



It needs only the most elementary conceptions of space, direction 

 and motion to see that, as the earth makes its vast swing from one 

 extremity of its orbit to the other, the stars, being fixed, must have 

 an apparent swing in the opposite direction. The seeming absence of 

 such a swing was in all ages before our own one of the great stumbling 

 blocks of astronomy. It was the base on which Ptolemy erected his 

 proof that the earth was immovable in the center of the celestial sphere. 

 It was felt by Copernicus to be a great difficulty in the reception of his 

 system. It led Tycho Brahe to suggest a grotesque combination of 

 the Ptolemaic and Copernican systems, in which the earth was the 

 center of motion, round which the sun revolved, carrying the planets 

 with it. 



With every improvement in their instruments, astronomers sought 

 to detect the annual swing of the stars. Each time that increased 

 accuracy in observations failed to show it, the difficulty in the way of 

 the Copernican system was heightened. How deep the feeling on the 

 subject is shown by the enthusiastic title, Copernicus Triumphans, given 

 by Horrebow to the paper in which, from observations by Koemer, he 

 claimed to have detected the swing. But, alas, critical examination 

 showed that the supposed inequality was produced by the varying effect 

 of the warmth of the day and the cold of the night upon the rate 

 of the clock used by the observer, and not by the motion of the earth. 



Hooke, a contemporary of Newton, published an attempt to deter- 

 mine the parallax of the stars, under the title of "An Attempt to Prove 

 the Motion of the Earth," but his work was as great a failure as that 

 of his predecessors. Had it not been that the proofs of the Copernican 

 system had accumulated until they became irresistible, these repeated 

 attempts might have led men to think that perhaps, after all, Ptolemy 

 and the ancients were somehow in the right. 



The difficulty was magnified by the philosophic views of the period. 

 It was supposed that Nature must economize in the use of space as a 

 farmer would in the use of valuable land. The ancient astronomers 

 correctly placed the sphere of the stars outside that of the planets, 

 but did not suppose it far outside. That Nature would squander her 

 resources by leaving a vacant space hundreds of thousands of times the 



