libration The slow oscillation of a satellite, as seen from the planet around which it revolves. One libration is due to a parallax effect: e.g., because of the rotation of the Earth, more of the eastern limb of the Moon is visible at moonrise, while more of the western limb can be seen at moonset. In the Earth—Moon system, a second libration is that of lunar longitude, with a monthly period, since the Moon's revolution around the Earth sometimes exceeds and sometimes lags behind its axial rotation, and the third lunar libration is of latitude, due to the 6° inclination of the lunar equator to its orbital plane, so that more of the polar regions become visible when the Moon is north or south of the ecliptic.

libration oscillation, balancing. XVII. — L. lībrātiō, -ōn—, f. lībrāre balance, f. lībra pound weight, scales; see -ATION.