464 HISTORY OF PHYSICAL ASTRONOMY. 



more striking. In 1676, a great number of observations of eclipses of 

 Jupiter's satellites were accumulated, and could be compared with Cas- 

 sini's Tables. Homer, a Danish astronomer, whom Picard had brought 

 to Paris, perceived that these eclipses happened constantly later than 

 the calculated time at one season of the year, and earlier at another 

 season; a difference for which astronomy could offer no account. 

 The error was the same for all the satellites ; if it had depended on a 

 defect in the Tables of Jupiter, it might have affected all, but the effect 

 would have had a reference to the velocities of the satellites. The 

 cause, then, was something extraneous to Jupiter. Romer had the 

 happy thought of comparing the error with the earth's distance from 

 Jupiter, and it was found that the eclipses happened later in proportion 

 as Jupiter was further off. 5 Thus we see the eclipse later, as it is more 

 remote ; and thus light, the messenger which brings us intelligence of 

 the occurrence, travels over its course in a measurable time. By this 

 evidence, light appeared to take about eleven minutes in describing 

 the diameter of the earth's orbit. 



This discovery, like so many others, once made, appears easy and 

 inevitable ; yet Dominic Cassini had entertained the idea for a moment," 

 and had rejected it ; and Fontenelle had congratulated himself pub- 

 licly on having narrowly escaped this seductive error. The objections 

 to the admission of the truth arose principally from the inaccuracy of 

 observation, and from the persuasion that the motions of the satellites 

 were circular and uniform. Their irregularities disguised the fact in 

 question. As these irregularities became clearly known, Homer's dis- 

 covery was finally established, and the " Equation of Light" took its 

 place in the Tables. 



Sect. 3. Discovery of Aberration. Bradley. 



IMPROVEMENTS in instruments, and in the art of observing, were re 

 quisite for making the next great step in tracing the effect of the laws 

 of light. It appears clear, on consideration, that since light and the 

 spectator on the earth are both in motion, the apparent direction of an 

 object will be determined by the composition of these motions. But yet 

 the effect of this composition of motions was (as is usual in such cases) 

 traced as a fact in observation, before it was clearly seen as a conse- 

 quence of reasoning. -This fact, the Aberration of Light, the greatest 

 astronomical discovery of the eighteenth century, belongs to Bradley, 



Bailly, ii. IT. Ib. ii. 419. 



