THE TRUE GRANDEUR OF THE SUN 229 



astronomer Vendelinus. He took up again the theorem of 

 Aristarchus, but with the advantage of a weaponed eye, to 

 determine anew the exact moment of the dichotomy of the moon. 

 He had carefully considered the spots upon the moon, and 

 determined those which lie at a median point separating the 

 half that was illuminated from the half that was obscure. This 

 done, in order to detect the exact moment at which the moon 

 reached its quarter, it sufficed to mark the instant at which 

 these spots were illuminated. Better instructed than Aristarchus 

 or any of his successors could have been, he took into account 

 also the possible deviations of angle due to refraction by the 

 earth's atmosphere. It will be remembered that at the moment 

 the face of the moon was cut in twain, Aristarchus had cal- 

 culated the angle subtended by the moon and sun at about 

 eighty-seven degrees that is, within three degrees of a right 

 angle ; Vendelinus found that it varied but little more than 

 half a degree from a right angle. It followed from his measure- 

 ments that the sun must be not twenty times, but more than 

 two hundred times the distance of the moon that is to say, 

 not four or five millions, but forty or fifty million miles away. 

 The boundaries of the universe, which Galileo had boasted he 

 had pushed back hundreds of times from anything hitherto 

 imagined by men, were receding farther still. Vendelinus had 

 made half the step ; the true distance was soon to be disclosed. 



Between Galileo and Newton the most prominent figure 

 among observers of the heavens was Dominico Cassini. He 

 was a compatriot of Galileo, and acquired a considerable repu- 

 tation through his bold and somewhat fantastic projects for 

 the construction of telescopes of enormous size. He was a 

 tireless worker ; when he was not sweeping the heavens he 

 calculated and wrote. His discoveries were numerous ; among 

 others, the rotation of Jupiter and Mars, and four new satellites 

 of Saturn. He likewise constructed astronomical tables of great 

 value. 



It was at the instance of Cassini, who had come to Paris to 

 lake charge of the fine observatory then being erected there, 

 that the French king sent out the celebrated expedition to 

 Cayenne. One of the results of this expedition was to reveal 

 the fact that pendulums beat more slowly at the equator than 

 in the latitude of Paris ; by revealing the varying intensity of 

 gravitation, it suggested that the earth was not a perfect sphere ; 



