SECT, ix.] KOTATION OF THE MOON. 77 



be irregular solids, of unequal breadth in different parts of 

 the circumference, so that their centres of gravity do not 

 coincide with the centres of their figures. Professor Struve 

 has also discovered that the centre of the ring is not con- 

 centric with the centre 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 ll"-272, and, on the other side, 

 the interval is 11"'390, consequently there is an excentricity 

 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 centre 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 T {g part of that of the planet. 



The periods of rotation of the moon and the other satel- 

 lites 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 ultimately amount to many 

 circumferences (N. 107), if the rotation of the moon were 

 perfectly uniform and not affected by the same inequalities, it 

 would cease exactly to counterbalance the motion of revolu- 

 tion ; and the moon, in the course of ages, would successively 

 and gradually discover every point of her surface to the 

 earth. But theory proves that this never can happen ; for 

 the rotation of the moon, though it does not partake of the 

 periodic inequalities of her revolution, is affected by the same 

 secular variations, so that her motions of rotation and revolu- 

 tion round the earth will always balance each other, and 

 remain equal. This circumstance arises from the form of the 

 lunar spheroid, which has three principal axes of different 

 lengths at right angles to each other. 



The moon is flattened at her poles from her centrifugal 



