libration

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A very slow oscillation, real or apparent, of a satellite as viewed from the larger celestial body around which it revolves.

from The Century Dictionary.

• noun The act of librating or balancing, or the state of being balanced; a state of equipoise; balance.
• noun In astronomy, a real or apparent libratory or oscillating motion, like that of a balance before coming to rest.
• noun libration in latitude, in consequenoe of her axis being inclined to the plane of her orbit, so that sometimes one of her poles and sometimes the other declines, as it were, or dips toward the earth
• noun diurnal libration, which is simply a consequence of the lunar parallax. In the last case, an observer at the surface of the earth perceives points near the upper edge of the moon's disk, at the time of her rising, which disappear as her elevation is increased; while new ones on the opposite or lower edge, that were before invisible, come into view as she descends toward the horizon. If the observer were placed at the earth's center he would perceive no diurnal libration.

from the GNU version of the Collaborative International Dictionary of English.

• noun The act or state of librating.
• noun (Astron.) A real or apparent libratory motion, like that of a balance before coming to rest.
• noun any one of those small periodical changes in the position of the moon's surface relatively to the earth, in consequence of which narrow portions at opposite limbs become visible or invisible alternately. It receives different names according to the manner in which it takes place; as: (a) Libration in longitude, that which, depending on the place of the moon in its elliptic orbit, causes small portions near the eastern and western borders alternately to appear and disappear each month. (b) Libration in latitude, that which depends on the varying position of the moon's axis in respect to the spectator, causing the alternate appearance and disappearance of either pole. (c) Diurnal or parallactic libration, that which brings into view on the upper limb, at rising and setting, some parts not in the average visible hemisphere.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The act of librating.
• noun astronomy The apparent wobble or variation in the visible side of the Moon that permanently faces the Earth, allowing observers on Earth to see, over a period of time, slightly more than half of the lunar surface.
• noun by extension A similar rotational or orbital characteristic of some other celestial body.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (astronomy) a real or apparent slow oscillation of a moon or satellite