412 VELOCITY OF LIGHT. 



distance from each other is 478,530,000 miles and if 

 we assume that the velocity of light is 190,000 miles 



per second, light would require = 2518'5 



seconds = 41m. 58*5s. for traversing the distance 

 between the two planets. If an eclipse of the satellite 

 takes place, say at 3 o'clock in the morning, while the 

 Earth is at the distance just stated, we should not 

 observe the occultation of the satellite until 3h. 41m. 

 58 '5s., because the last ray emitted by it before disap- 

 pearance requires just 41m. 58'5s. for reaching the 

 Earth. The next eclipse takes place 42h. 28m. 36s. 

 later, that is, on the following day at 9h. 28m. 36s. in 

 the evening. But during this interval the Earth has 

 moved towards Jupiter through 2,620,800 miles, and 

 the distance between the two planets is now only 

 475,929,200 miles ; hence the light requires now only 



475929200 



- onnrvn = 41m. 44' 7s. to reach the Earth, and we 

 1 JUUUU 



shall observe the beginning of the eclipse at lOh. 10m. 

 20'7s. ; or, between the first eclipse, which was observed 

 at 3h. 41m. 58'5s. in the morning, and the next, which 

 was observed at lOh. 10m. 20*7s. in the evening of the 

 following day, there is an interval of only 42h. 28m. 

 22*2s., while the actual interval between, one occultation 

 of the satellite and the succeeding one is 42h. 28m. 36s. 

 Thus for an observer on the Earth, when this planet 

 approaches Jupiter, the eclipses succeed each other in 

 shorter intervals than those which actually elapse. On 

 the other hand, when Jupiter is at c 1? and the Earth at 

 6', the latter planet recedes from the former, and in this 

 relative position light requires between each succeeding 



