96 



SCENEUY OF THE I1EAVENS. 



All the satellites are eclipsed once in every revolution by passing through the shadow 

 of the planet, with the exception of the fourth, which sometimes escapes, because of the 

 greater magnitude and inclination of its orbit. But with reference to the first satellite, it 

 is never the case that both the immersion and the emersion can be observed, owing to its 

 being so near the planet, and this is generally true of the second. On the other hand, 

 the satellites pass between Jupiter and the sun in their revolution round him, and then 

 project their shadows upon the enlightened portion of his disk, causing a solar eclipse to 

 that part of his surface. They pass also between the planet and the earth, and then 

 exhibit the phenomena of transits ; and, lastly, they pass directly behind the body of the 

 planet in opposition, and are then occulted. At A B c D the earth is represented in 

 different parts of its orbit ; and at J Jupiter is seen surrounded by his four satellites, 

 whose orbits are marked 1, 2, 3, 4. The first satellite is shown entering the shadow of 

 the planet at a ; b is the point of emergence from it ; and clearly, the interposing body 

 of the planet will prevent the emersion from being visible to a spectator on the earth at 



D, who sees the immersion, and that will be invisible from the same cause to a terrestrial 

 observer at B, who sees the emergence. The satellite at c and e has its greatest eastern 

 and western elongations to a spectator on the earth at A ; and it appears projected as a 

 small dark spot upon the disk of Jupiter at d. The following table expresses the general 

 elements of the satellites, their distances and dimensions, times of revolution, and the 

 duration of their eclipses : 



Satellite. 



1 



2 



a 



4 



Diameter 

 in Miles. 

 250S 

 2068 

 3377 

 2890 



Distance. 



260,000 



420,000 



670,000 



1,180,000 



Time of Revolution. 



Id 18h 28m 

 3 13 14 

 7 3 43 

 16 16 32 



Duration 

 of Eclipse. 



2 hours. 

 3 



N 



5 



Owing to the eclipses of the satellites taking place so frequently, they are of great use 

 in the determination of terrestrial longitudes, those of the first being of the most import 

 ance on account of their more rapid recurrence. For this purpose they are carefully 

 calculated, predicted, and registered in the Nautical Almanack, a book which forms one 

 of the finest examples we have of the power of the human mind. There is nothing more 

 simple than this method of finding the longitude. Let us suppose an individual to observe 

 the immersion of the first satellite to take place where he is situated at ten o'clock at 

 night, and that the Nautical Almanack registers its occurrence at Greenwich at six, it 

 follows, that, as the sun apparently travels fifteen degrees an hour, the longitude of the 

 observer is 4 h =60 east of Greenwich; for faster time shows that he is east, and slower 

 that he is west. At sea, great practical difficulty besets this method of finding the longi- 



