VELOCITY OF LIGHT. 



411 



-j 



lites, one of which is so near to the planet that during 

 each revolution it passes ^ 



once through the shadow 

 of the planet, and becomes, 

 for a time, invisible or 

 eclipsed. The period of re- 

 volution of this satellite is 

 only about 42h. 28m. 36s.; 

 these eclipses, therefore, take 

 place very frequently, and 

 they may be well observed 

 by means of a tolerably good 

 telescope. Since the motion 

 of the satellite is very uni- 

 form, the intervals between 

 the successive eclipses 

 should be equal ; but, as ob- 

 served from the Earth, these 

 intervals seem to be greater 

 at one time than at another. 



The Earth moves in its 

 orbit with a greater velo- 

 city than Jupiter. Let ab, 

 in fig. 238, represent that 

 portion of the orbit which 

 the Earth traverses while Jupiter moves from a x to b r 

 It will easily be seen that the Earth during this time 

 approaches Jupiter. When the Earth arrives at c, 

 Jupiter has moved to c x , and while the Earth passes from 

 c to c/, Jupiter moves from c x to d^ and the Earth 

 recedes from Jupiter. 



When the Earth is at a, and Jupiter at a x , their 



I 



FIG. 238. 



