208 A Short History of Astronomy [Cn. vm. 



a little less than 93,000,000. Though not really an accurate 

 result, this was an enormous improvement on anything 

 that had gone before, as Ptolemy's estimate of the sun's 

 distance, corresponding to a parallax of 3', had survived 

 up to the earlier part of the i7th century, and although 

 it was generally believed to be seriously wrong, most 

 corrections of it had been purely conjectural (chapter vii., 



I45)- 



162. Another famous discovery associated with the early 

 days of the Paris Observatory was that of the velocity 

 of light. In 1671 Picard paid a visit to Denmark to 

 examine what was left of Tycho Brahe's observatory at 

 Hveen, and brought back a young Danish astronomer, 

 Olaus Itoemer (1644-1710), to help him at Paris. Roemer, 

 in studying the motion of Jupiter's moons, observed (1675) 

 that the intervals between successive eclipses of a moon 

 (the eclipse being caused by the passage of the moon into 

 Jupiter's shadow) were regularly less when Jupiter and the 

 earth were approaching one another than when they were 

 receding. This he saw to be readily explained by the 

 supposition that light travels through space at a definite 

 though very great speed. Thus if Jupiter is approaching 

 the earth, the time which the light from one of his moons 

 takes to reach the earth is gradually decreasing, and con- 

 sequently the interval between successive eclipses as seen 

 by us is apparently diminished. From the difference of 

 the intervals thus observed and the known rates of motion 

 of Jupiter and of the earth, it was thus possible to form a 

 rough estimate of the rate at which light travels. Roemer 

 also made a number of instrumental improvements of 

 importance, but they are of too technical a character to 

 be discussed here. 



163. One great name belonging to the period dealt with 

 in this chapter remains to be mentioned, that of Rene 

 Descartes* (1596-1650). Although he ranks as a great 

 philosopher, and also made some extremely important 

 advances in pure mathematics, his astronomical writings 

 were of little value and in many respects positively harmful. 

 In his Principles of Philosophy (1644) he gave, among 

 some wholly erroneous propositions, a fuller and more 



* Also frequently referred to by the Latin name Cartesius. 



