THE PROBLEMS OF ASTRONOMY. 85 



of its end? Has it bounds outside of which nothing exists but the 

 black and starless depths of infinity itself? Or are the stars we see 

 simply such members of an infinite collection as happen to be the 

 nearest our system? A few such questions as these we are perhaps 

 beginning to answer; but hundreds, thousands, perhaps even millions 

 of years may elapse without our reaching a complete solution. Yet 

 the astronomer does not view them as Kantian antinomies, in the 

 nature of things insoluble, but as questions to which he may hopefully 

 look for at least a partial answer. 



The problem of the distances of the stars is of peculiar interest in con- 

 nection with the Copernican system. The greatest objection to this 

 system, which must have been more clearly seen by astronomers them- 

 selves than by any others, was found in the absence of any apparent 

 parallax of the stars. If the earth performed such an immeasurable 

 circle around the sun as Copernicus maintained, then, as it passed from 

 side to side of its orbit, the stars outside the solar system must appear 

 to have a corresponding motion in the other direction, and thus to 

 swing back and forth as the earth moved in one and the other direc- 

 tion. The fact that not the slightest swing of that sort could be seen 

 was, from the time of Ptolemy, the basis on which the doctrine of the 

 earth's immobility rested. The difficulty was simply ignored by Coper- 

 nicus and his immediate successors. The idea that nature would not 

 squander space by allowing immeasurable stretches of it to go unused 

 seems to have been one from which mediaeval thinkers could not entirely 

 break away. The consideration that there could be no need of any such 

 economy, because the supply was infinite, might have been theoretically 

 acknowledged, but was not practically felt. The fact is that magnifi- 

 cent as was the conception of Copernicus, it was dwarfed by the con- 

 ception of stretches from star to star so vast that the whole orbit of the 

 earth was only a point in comparison. 



An indication of the extent to which the difficulty thus arising was 

 felt is seen in the title of a book published by Horrebow, the Danish 

 astronomer, some two centuries ago. This industrious observer, one of 

 the first who used an instrument resembling our meridian transit of the 

 present day, determined to see if he could find the parallax of the 

 stars by observing the intervals at which a pair of stars in opposite 

 quarters of the heavens crossed his meridian at opposite seasons of 

 the year. When, as he thought, he had won success, he published his 

 observations and conclusions under the title of Copernicus Trium- 

 phans. But alas ! the keen criticism of his contemporaries showed that 

 what he supposed to be a swing of the stars from season to season 

 arose from a minute variation in the rate of his clock, due to the differ- 

 ent temperatures to which it was exposed during the day and the 

 night. The measurement of the distance even of the nearest stars 

 evaded astronomical research until Bessel and Struve arose in the 

 early part of the present century. 



