68 ROTATION OF THE MOOX. SECT. IX. 



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 centres of gravity do not coincide with the centres of their 

 figures. Professor Struve has also discovered that the centre of 

 the rings is not concentric with the centre of Saturn. The 

 interval between the outer edge of the globe of the planet and 

 the outer edge of the rings on one side is 11"'272, and, on the 

 other side, the interval is 11" '390, consequently there is an 

 excentricity of the globe in the rings 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 -^ 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 ultimately amount to many circumferences (N. 108), 

 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 revolution ; 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 revolution 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 force, 

 therefore her polar axis is the least. The other two are in the 

 plane of her equator, but that directed towards the earth is the 

 greatest (N. 142). The attraction of the earth, as if it had 

 drawn out that part of the moon's equator, constantly brings the 



