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libration
libration A periodic wobble of a celestial body (see diagram). The most familiar librations are those of the Moon as seen from Earth. In libration in longitude, the Moon appears to swing slightly from side to side (east–west) by up to 8° 08′ in each direction. This occurs because the Moon's speed along its elliptical orbit varies with its distance from Earth, while its axial rotation remains constant. In libration in latitude, the Moon appears to nod from north to south by up to 6° 53′ in each direction. This is because the Moon's axis of rotation is not perpendicular to its orbital plane. As a result of these two librations, we can see up to 59% of the Moon's surface. A third libration, diurnal libration, occurs because we view the Moon from different sides of the Earth at moonrise and moonset, so we can again see slightly round the eastern and western limbs, although by only about 1°. Libration occurs when two motions are locked in resonance (a synchronous orbit). For this reason, the planet Mercury also exhibits a libration, as do some planetary satellites.
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"libration." A Dictionary of Astronomy. 1997. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. "libration." A Dictionary of Astronomy. 1997. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O80-libration.html "libration." A Dictionary of Astronomy. 1997. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O80-libration.html |
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libration
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.
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AILSA ALLABY and MICHAEL ALLABY. "libration." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "libration." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O13-libration.html AILSA ALLABY and MICHAEL ALLABY. "libration." A Dictionary of Earth Sciences. 1999. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-libration.html |
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libration
libration oscillation, balancing. XVII. — L. lībrātiō, -ōn—, f. lībrāre balance, f. lībra pound weight, scales; see -ATION.
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T. F. HOAD. "libration." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. T. F. HOAD. "libration." The Concise Oxford Dictionary of English Etymology. 1996. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O27-libration.html T. F. HOAD. "libration." The Concise Oxford Dictionary of English Etymology. 1996. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O27-libration.html |
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