S.CT. IX. ROTATION OF THE MOON. 63 



became visible with ordinary instruments, on account of 

 their plane passing through the sun. In the end of 

 April, 1833, the rings vanished a second time, and re- 

 appeared in June of that year. Similar phenomena 

 will occur in 1847, and generally as often as Saturn has 

 the same longitude with either node of his rings. Each 

 side of these rings has alternately fifteen years of sun- 

 shine and fifteen years of darkness. A dark line has 

 been seen in the outer ring, supposed to indicate a sub- 

 division. 



It is a singular result of theory that the rings could 

 not maintain their stability of rotation if they were 

 everywhere of uniform thickness ; for the smallest dis- 

 turbance would destroy the equilibrium, which would 

 become more and more deranged, till at last they would 

 be precipitated on the surface of the planet. The rings 

 of Saturn must, therefore, be irregular solids of unequal 

 breadth in different parts of the circumference, so that 

 their centers of gravity do not coincide with the centers 

 of their figures. Professor Strave has also discovered 

 that the center of the ring is not concentric with the 

 center of Saturn. The interval between the outer edge 

 of the globe of the planet and the outer edge of the ring 

 on one side is 11"'272, and on the other side the inter- 

 val is 11"-390, consequently there is an eccentricity of 

 the globe in the ring of 0"-215. If the rings obeyed 

 different forces they would not remain in the same 

 plane ; ' but the powerful attraction of Saturn always 

 maintains them and his satellites in the plane of his 

 equator. The rings, by their mutual action, and that 

 of the sun and satellites, must oscillate about the center 

 of Saturn, and produce phenomena of light and shadow 

 whose periods extend to many years. According to M. 

 Bessel the mass of Saturn's ring is equal to the yfy part 

 of that of the planet. 



The periods of rotation of the moon and the other 

 satellites are equal to the times of their revolutions ; 

 consequently these bodies always turn the same face to 

 their primaries. However, as the mean motion of the 

 moon is subject to a secular inequality which will ulti- 

 mately amount to many circumferences (N. 107), if the 

 rotation of the moon were perfectly uniform and not 



