PORTION OF THE COSMOS. THE PLANETS. 



379 



Satellites of Jupiter. 



At the brilliant epoch of Galileo, the just view was already 

 propounded,, that the subordinate system of Jupiter pre- 

 sented, in regard to many relations of time and space, an 

 image or picture, on a smaller scale, of the entire solar or 

 planetary system. This view, which then spread rapidly, 

 together with the discovery of the phases of Yenus which 

 followed soon afterwards (February 1610), contributed much 

 to promote the more general reception of the Copernican 

 system. The four satellites of Jupiter are the only group 

 of satellites belonging to the outer planets which has not 

 received any augmentation since the epoch of its first disco- 

 very ( 618 ) (by Simon Harms, on the 29th of December, 

 1609), a period of nearly two centuries and a half. 



The following Table contains, according to Hansen, the 

 sidereal periods of revolution of Jupiter's satellites, their 

 mean distances expressed in semi-diameters of the central 

 planet, and the mass of each in parts of the mass of Jupiter : 



If 



104 7'TTT 



expresses the mass of Jupiter, together with 



