SECT. IV. THEORY OF JUPITER'S SATELLITES. 27 



SECTION IV. 



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

 Satellites Position of their 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 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 regarded 

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

 by the planets in myriads of ages. The revolutions of the satel- 

 lites 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 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 excen- 

 tricity. 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 par- 

 ticle in its centre 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. 



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