THE SATELLITES OF JUPITER. 



109 



The Satellites of Jupiter. 



Even so early as the brilliant epoch of Galileo, the cor reel 

 opinion was formed that the suhordhiate planetary system 

 of Jupiter might present, in many relations of space and time, 

 a picture in miniature of the Solar System. This view, rap- 

 idly diffused at that time, as well as the discovery, shortly 

 afterward, of the phases of Venus (February, 1610), contrib- 

 uted greatly to the general introduction of the Copernican 

 system. The quadruple group of satellites of Jupiter is the 

 only one of the exterior principal planets which has not been 

 increased by any new discovery, during a period of nearly 

 two centuries and a half, since the epoch of their first dis- 

 covery by Simon Marius on the 29th of December, 1609. 



The following table contains the 'periods of sidereal revo- 

 lution of the satelhtes of Jupiter, their mean distances ex- 

 pressed in diameters of the primary, their diameters in geo- 

 graphical miles, and their Tnasses as parts of the mass of 

 Jupiter. 



If yo4 7'-8y 9 expresses the mass of Jupiter with his satel- 

 lites, then his mass without the satellites is tot/oTq"' ^^^1 

 about 6-oVo" smaller. 



The comparisons of the 7rjig7iitndes, distances, and ec- 

 centricities with other satellite systems has already been 

 given [Cosmos, vol. iv., p. 105-127). The luminous in- 

 tensity of Jupiter's satellites is various, and not in propor- 

 tion to their volume, since, as a general rule, the third and 

 i\\e first, whose relation of magnitude is as 8 : 5, appear the 

 brightest. The smallest and densest of all — the second — is 

 generally brighter than the la.rgor fourth, which is ordinarily 

 called the least luminous. Accidental (temporary) fluctua- 

 tions in the luminous intensity have, as already remarked, 

 been ascribed sometimes to changes of the surface, sometimes 

 to obscurations in the atmosphere of the satellites.* They 

 all appear, moreover, to reflect a more intense light than the 

 primary. When the Earth is situated between Jupiter and 

 the Sun, and the satellites, therefore, moving from east to 

 * Sir John Herschel, Outlines, $ 540. 



Vol . IV._H 



