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


from The American Heritage® Dictionary of the English Language, 4th Edition

[Latin lībrātiō, lībrātiōn-, oscillation, from lībrātus, past participle of lībrāre, to balance, from lībra, balance.]

from Wiktionary, Creative Commons Attribution/Share-Alike License

From Latin lībrātiō ("a hurling, swinging"), from lībrō ("poise, cause to swing").


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  • By this means the ship was able to penetrate the so-called libration zone, which was a semispace continuum lying between the 4th and 5th-dimensional universes.

    Blockade, Lepso Brand, Kurt 1976

  • The moon really undergoes considerable libration, recalling the libration of

    Other Worlds Their Nature, Possibilities and Habitability in the Light of the Latest Discoveries 1890

  • This produces the phenomenon that is called libration, the result of which is that, along the border between the day and night hemispheres there is a narrow strip where the sun rises and sets once in each of her years, which are about two hundred and twenty-five of our days in length.

    A Columbus of Space 1890

  • Due to a slight "wobble" in the lunar motion called libration,

    Conservapedia - Recent changes [en] 2009

  • Due to a slight "wobble" in the lunar motion called libration,

    Conservapedia - Recent changes [en] 2009

  • Due to a slight "wobble" in the lunar motion called libration,

    Conservapedia - Recent changes [en] 2009

  • But it would not rise or set: it would be fixed in the sky, and subject only to a minute oscillation to and fro once a month, by reason of the "libration" we have been speaking of.

    Pioneers of Science Oliver Lodge 1895

  • As the moon, except for the slight movement termed its "libration," always turns the same face to us, so that we see in all only about four sevenths of its surface, it has naturally been conjectured that the unseen side, which is probably some miles lower than that turned toward us, might have a different character from that which we behold.

    Outlines of the Earth's History A Popular Study in Physiography Nathaniel Southgate Shaler 1873

  • Now this increased action of the system, during the hot fit, by exhausting the sensorial powers of irritation and association, contributes to induce a renewal of the cold paroxysm; as the accumulation of those sensorial powers in the cold fit produces the increased actions of the hot fit; which two states of the system reciprocally induce each other by a kind of libration, or a plus and minus, of the sensorial powers of irritation and association.

    Zoonomia, Vol. II Or, the Laws of Organic Life Erasmus Darwin 1766

  • Yes, we don't like what we hear about the black libration theology.

    Obama apologizes for 'sweetie' comment 2008


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  • "in astronomy, is an apparent irregularity of the moon's motion, whereby she seems to librate about her axis, sometimes from the east to the west, and now and then from the west to the east; so that the parts in the western limb or margin of the moon sometimes recede from the centre of the disc, and sometimes move towards it, by which means they become alternately visible and invisible to the inhabitants of the earth. This libration is owing to her equable rotation round her own axis, once in a month, in conjunction with her unequal motion in the perimeter of her orbit round the earth."

    Falconer's New Universal Dictionary of the Marine (1816), 222

    October 12, 2008