SECT, iv.] THEORY OF JUPITER's SATELLITES. 31 



SECTION IV. 



Theory of Jupiter's Satellites Effects of the Figure of Jupiter upon his Satel- 

 litesPosition of their Orbits Singular Laws among the Motions of the 

 first Three Satellites Eclipses of the Satellites Velocity of Light Aber- 

 ration Ethereal Medium Satellites of Saturn and Uranus. 



THE changes which take place in the planetary system are 

 exhibited on a smaller scale by Jupiter and his satellites ; and, 

 as the period requisite for the development of the inequalities 

 of these moons only extends to a few centuries, it may be re- 

 garded as an epitome of that grand cycle which will not be 

 accomplished by the planets in myriads of ages. The revo- 

 lutions of the satellites about Jupiter are precisely similar to 

 those of the planets about the sun : it is true they are disturbed 

 by the sun, but his distance is so great, that their motions are 

 nearly the same as if they were not under his influence. The 

 satellites, like the planets, were probably projected in ellip- 

 tical orbits : but, as the masses of the satellites are nearly 

 100,000 times less than that of Jupiter ; and as the compres- 

 sion of Jupiter's spheroid is so great, in consequence of his 

 rapid rotation, that his equatorial diameter exceeds his polar 

 diameter by no less than 6000 miles ; the immense quantity of 

 prominent matter at his equator must soon have given the 

 circular form observed in the orbits of the first and second 

 satellites, which its superior attraction will always maintain. 

 The third and fourth satellites, being farther removed from 

 its influence, revolve in orbits with a very small excentricity. 

 And, although the first two sensibly move in circles, their or- 

 bits acquire a small ellipticity, from the disturbances they 

 experience (N. 86). 

 It has been stated, that the attraction of a sphere on an 



