26 THEORY OF JUPITER'S SATELLITES. SECT. IV. 



SECTION IV. 



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

 Satellites Position of theif Orbits Singular Laws among- the Motions 

 of the first three Satellites Eclipses of the Satellites Velocity of Light 

 Aberration Ethereal Medium Satellites of Saturn and Uranus. 



THE changes which take place in the planetary sys- 

 tem are exhibited on a smaller scale by Jupiter and his 

 satellites ; and, as the period requisite for the develop- 

 ment of the inequalities of these moons only extends to 

 a few centuries, it may be regarded as an epitome of 

 that grand cycle which will not be accomplished by the 

 planets in myriads of ages. The revolutions 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 elliptical orbits : but, as the masses 

 of the satellites are nearly 100,000 times less than that 

 of Jupiter ; and as the compression of Jupiter's sphe- 

 roid 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 al- 

 ways maintain. The third and fourth satellites, being 

 farther removed from its influence, revolve in orbits 

 with a very small eccentricity. And although the first 

 two sensibly move in circles, their orbits acquire a 

 small ellipticity, from the disturbances they experience 

 (N. 86). 



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

 an exterior body is the same as if its mass were united 

 in one particle in its center of gravity, and therefore 

 inversely as the square of the distance. In a spheroid, 

 however, there is an additional force arising from the 

 bulging mass at its equator, which, not following the 

 exact law of gravity, acts as a disturbing force. One 



