SECT. IX. LIBRATIOXS OF THE MOON. 69 



greatest axis, and consequently the same hemisphere, towards us, 

 which makes her rotation participate in the secular variations of 

 her mean motion of revolution. Even if the angular velocities 

 of rotation and revolution had not been nicely balanced in the 

 beginning of the moon's motion, the attraction of the earth 

 would have recalled the greatest axis to the direction of the line 

 joining the centres of the moon and earth ; so that it would have 

 vibrated on each side of that line in the same manner as a 

 pendulum oscillates on each side of the vertical from the influence 

 of gravitation. No such libration is perceptible ; and, as the 

 smallest disturbance would make it evident, it is clear that, if 

 the moon has ever been touched by a comet, the mass of the 

 latter must have been extremely small. If it had been only 

 the hundred thousandth part of that of the earth, it would have 

 rendered the libration sensible. According to analysis, a similar 

 libration exists in the motions of Jupiter's satellites, which still 

 remains insensible to observation, and yet the comet of 1770 

 passed twice through the midst of them. 



The moon, it is true, is liable to librations depending upon the 

 position of the spectator. At her rising, part of the western edge 

 of her disc is visible, which is invisible at her setting, and the 

 contrary takes place with regard to her eastern edge. There are 

 also librations arising from the relative positions of the earth and 

 moon in their respective orbits ; but, as they are only optical 

 appearances, one hemisphere will be eternally concealed from the 

 earth. For the same reason the earth, which must be so splendid 

 an object to one lunar hemisphere, will be for ever veiled from 

 the other. On account of these circumstances, the remoter 

 hemisphere of the moon has its day a fortnight long, and a night 

 of the same duration, not even enlightened by a moon, while the 

 favoured side is illuminated by the reflection of the earth during 

 its long night. A planet exhibiting a surface thirteen times 

 larger than that of the moon, with all the varieties of clouds, 

 land, and water, coming successively into view, must be a 

 splendid object to a lunar traveller in a journey to his antipodes. 

 The great height of the lunar mountains probably has a consi- 

 derable influence on the phenomena of her motion, the more so 

 as her compression is small, and her mass considerable. In the 

 curve passing through the poles, and that diameter of the moon 

 which always points to the earth, nature has furnished a perma- 



