62 SATURN AND HIS RINGS. SECT. IX. 



tion occasions a very great compression in his form. 

 His equatorial axis exceeds his polar axis by 6000 miles, 

 whereas the difference in the axes of the earth is only 

 about twenty-six and a half. It is an evident conse- 

 quence of Kepler's law of the squares of the periodic 

 times of the planets being as the cubes of the major 

 axes of their orbits, that the heavenly bodies move 

 slower the farther they are from the sun. In compa- 

 ring the periods of the revolutions of Jupiter and Saturn 

 with the times of their rotation, it appears that a year 

 of Jupiter contains nearly ten thousand of his days, and 

 that of Saturn about thirty thousand Saturnian days. 



The appearance of Saturn is unparalleled in the sys- 

 tem of the world. He is a spheroid nearly 1000 times 

 larger than the earth, surrounded by a ring even brighter 

 than himself, which always remains suspended in the 

 plane of his equator ; and, viewed with a very good 

 telescope, it is found to consist of two concentric rings, 

 divided by a dark band. The mean distance of the 

 interior part of this double ring from the surface of the 

 planet is about 22,240 miles ; it is no less than 33,360 

 miles broad, but, by the estimation of Sir John Herschel, 

 its thickness does not much exceed 300 miles, so that it 

 appears like a plane. By the laws of mechanics, it is 

 impossible that^this body can retain its position by the 

 adhesion of its v particles alone. It must necessarily 

 revolve with a velocity that will generate a centrifugal 

 force sufficient to balance the attraction of Saturn! Ob- 

 servation confirms the truth of these principles, showing 

 that the rings rotate from west to east about the planet 

 in ten hours and a half, which is nearly the time a satel- 

 lite would take to revolve about Saturn at the same dis- 

 tance. Their plane is inclined to the ecliptic, at an 

 angle of 28 10' 44"-5 ; in consequence of this obliquity 

 of position, they always appear elliptical to us, but with 

 an eccentricity so variable as even to be occasionally like 

 a straight line drawn across the planet. In the begin- 

 ning of October, 1832, the plane of the rings passed 

 through the center of the earth ; in that position they 

 are only visible with very superior instruments, and 

 appear like a fine line across the disc of Saturn. About 

 the middle of December, in the same year, the rings 



