CHAPTER XVIII 



THE TRUE GRANDEUR OF THE SUN 



WHILE Descartes was busy with his air-castles, fabricating the 

 world from dreams, others less ambitious to construct a universe 

 de toutes pieces were endeavouring to build up a rational know- 

 ledge of that corner in which we live, from a foundation of solid 

 fact. Without the aid of the telescope, and merely from patient 

 observation by means of the two eyes with which he had been 

 endowed by nature, Coppernicus was able correctly to delineate 

 the relative positions of the planets and depict the order of 

 their revolutions % round about the central sun ; something of 

 their actual spacing as well. But the ideas which men still 

 might form of the true dimensions of the solar system the 

 relative grandeur of the earth to its companion bodies were of 

 the crudest. Even at the death of Galileo there remained one 

 great mystery. 



The simplest of considerations required that if the sun's 

 distance were not infinite, seen from two widely separated points 

 upon the earth it should show an apparent shift of position 

 with reference to any intervening body that is, a parallax. 

 Thus, if in the transit of the moon or a planet across the face 

 of the sun, to the eye of one observer the edge of the planet 

 just touched the edge of the sun, to that of an observer at a 

 sufficient distance a slight gap between the two should show. 

 And if the angle subtended by this gap could then be measured, 

 the true distance of the sun might be known. No such parallax 

 or apparent displacement could be found ; Galileo, as we have 

 seen, sought it in vain. The crude measures of Aristarchus, 

 open, as more accurate observations disclosed, to errors of 

 hundreds per cent., was still the best that men could find. 



The restless mind of Kepler busied with the problem. Turn- 

 ing it over and over, a simple geometrical construction sufficed 

 to show him that if the estimates of Hipparchus, not materially 

 changed, as we have seen, by Ptolemy or Coppernicus, were 



