128 



LECTURES 



determined witli considerable precision. But it was also found tli at 

 they presented remarkable inequalities. When the earth in her orbit 

 was receding from Jupiter, the periodic times were too long. On the 

 contrary, when she was approaching him they were too short. These 

 discrepancies were the occasion of much perplexity. 



Arago, in his popular astronomy, gives to Lord Bacon the credit of 

 having first suggested that the inequalities might be due to the fact 

 that time was occupied in the transmission of light ; that the effect of 

 light was not, as had been supposed, simultaneous at all distances. 



Roemer, an eminent Danish astronomer, was the first to urge this 

 fact as necessary in order to account lor the apparent irregularities. 

 Eesearches upon these satellites occupied the attention of eminent 

 astronomers for more than half a century. And it was not until after 

 the lapse of more than a hundred years from the time of their 

 discovery that their motions were fully explained ; and no explana- 

 tion was sufficient which did not involve the " equation of light." 

 The subject will be easily understood by reference to fig. 15. 



Let S be the sun, E E' the orbit of the earth, 1 1' a portion of the 

 orbit of Jupiter, s and s' the orbit of one of the satellites. The pri- 

 mary casts a shadow behind it in a direction opposite to the sun, by 

 which the satellites are often eclipsed. In some portions of the earth 

 the immersion and the emersion of the satellites can both be seen ; in 

 other positions only one of them. When the earth is at E and Jupi- 

 ter at I, the emersion of the satellite is seen at s. When the earth 

 has advanced in her orbit to E' and Jupiter to I', the emersion will be 

 seen at 6-'. While the earth is passing along the side of her orbit 

 approaching the planet the immersions are seen. While she is mo- 

 ving from the point of her orbit nearest to the planet the intervals 

 between the eclipse are too long ; and while returning from the most 

 remote to the nearest they are too short. And this difference, in the 

 course of a complete synodic revolution, amounted to more than 16| 

 minutes. It was at length found, by accurate calculation, that light 

 occupied 16 ra. 36 sec. in traversing the diameter of the earth's orbit, 

 or eight minutes and eighteen seconds in coming from the sun to us. 

 With this aUowance, the discrepancies between the computed and 

 observed times of tlie eclipse entirely disappeared. This result fixed 

 the velocity of light at about 192,000 miles per second. 



