30 ECLIPSES OF THE SATELLITES. SECT. IV. 



(N. 92) of the satellites ; consequently they affect their eclipses, 

 and have a very great influence on their whole theory. The 

 satellites move so nearly in the plane of Jupiter's equator, which 

 has a very small inclination to his orbit, that the first three are 

 eclipsed at each revolution by the shadow of the planet, which 

 is much larger than the shadow of the moon : the fourth satellite 

 is not eclipsed so frequently as the others. The eclipses take 

 place close to the disc of Jupiter when he is near opposition 

 (N. 93) ; but at times his shadow is so projected with regard to 

 the earth, that the third and fourth satellites vanish and reappear 

 on the same side of the disc (N. 94). These eclipses are in all 

 respects similar to those of the moon : but, occasionally, the 

 satellites eclipse Jupiter, sometimes passing like obscure spots 

 across his surface, resembling annular eclipses of the sun, and 

 sometimes like a bright spot traversing one of his dark belts. 

 Before opposition, the shadow of the satellite, like a round black 

 spot, precedes its passage over the disc of the planet ; and, after 

 opposition, the shadow follows the satellite. 



In consequence of the relations already mentioned in the mean 

 motions and mean longitudes of the first three satellites, they 

 never can be all eclipsed at the same time : for, when the second 

 and third are in one direction, the first is in the opposite direc- 

 tion ; consequently, when the first is eclipsed, the other two must 

 be between the sun and Jupiter. The instant of the beginning 

 or end of an eclipse of a satellite marks the same instant of 

 absolute time to all the inhabitants of the earth ; therefore, the 

 time of these eclipses observed by a traveller, when compared 

 with the time of the eclipse computed for Greenwich, or any 

 other fixed meridian (N. 95), gives the difference of the meri- 

 dians in time, and, consequently, the longitude of the place of 

 observation. The longitude is determined with extreme preci- 

 sion whenever it is possible to convey the time instantaneously 

 by means of electricity from one place to another, since it 

 obviates the errors of clocks and chronometers. The eclipses of 

 Jupiter's satellites have been the means of a discovery which, 

 though not so immediately applicable to the wants of man, 

 unfolds one of the properties of light that medium without 

 whose cheering influence all the beauties of the creation would 

 have been to us a blank. It is observed, that those eclipses of 

 the first satellite which happen when Jupiter is near conjunction 



