Libration

Libration (from Lat. libra, 'a balance,' meaning an oscillating motion), a term denoting certain movements of the moon, chiefly apparent, which have an important effect on the apparent position of the lunar formations. A short study of these reveals puzzling changes in their place from night to night. Those near the edge of the disc disappear and reappear in a seemingly irregular way, while central formations approach or leave the centre in harmony with this motion. These appearances are due to an apparent motion of the moon by which its globe seems to turn slightly round to each side alternately, so that we see a little further round her globe on all sides in turn than we would do if she kept absolutely the same face towards us. This motion, as it refers to the north and south edges of the moon's disc, is called libration in latitude; as it refers to the east and west edges, it is called libration in longitude. The libration in latitude arises from the inclination of both the lunar equator and orbit to the ecliptic. From the relation between these two factors their effects always reinforce each other, so that when the moon rises above the ecliptic in her orbit she also inclines her under side to us, and when below the ecliptic, her upper side. The libration in longitude arises from the unequal speed of the moon in her orbit (see MOON) combined with her sensibly uniform rotation. She is thus sometimes before or behind her mean place, and we can see a little round her west or east edge respectively. An observer at the north or south pole of the earth will also from his position see a little round the north or south edge of the moon's disc, and for intermediate positions the effect has intermediate values. In the same way an observer in the tropics will see further round the west or east edges of the moon, as he is carried from west to east by the earth's rotation. These effects are known as the diurnal or parallactic libration. The maximum libration in longitude is nearly 6° 50'. That in latitude equals 7° 53'. The diurnal libration may rise to 1° 2'. These numbers refer to the apparent displacement of lunar markings in lunar latitude and longitude.