[ad. L. lībrātiōn-em, n. of action f. lībrā-re to LIBRATE. Cf. F. libration.]
1. a. The action of librating; motion like that of the beam of a balance oscillating upon its pivot; swaying to and fro. b. The state of being balanced or in equipoise; equipoise, balance.
1603. Sir C. Heydon, Jud. Astrol., xviii. 381. This Thebit … perceiuing the quantitie of the tropike yeare to varie, first inuented the libration of the 8. sphere.
1625. N. Carpenter, Geog. Del., I. iv. 73. Some others … imagine the Center … of the Earth to be moued vp and down by a certaine motion of Libration. Ibid., II. vi. 85. This libration or motion of the Water cannot bee caused by the winde or Aire.
1653. Jer. Taylor, Serm. Gold. Grove, Winter, v. 60. The poor bird was beaten back … descending more at every breath of the tempest then it could recover by the libration and frequent weighing of his wings.
1684. T. Burnet, Th. Earth, II. 51. This must needs make it lose its former poise and libration.
1694. W. Holder, Harmony (1731), 29. The Librations of the Pendulum.
1728–46. Thomson, Spring, 742. Their pinions still, In loose libration stretched.
1791. E. Darwin, Bot. Gard., II. 26. So turns the needle to the pole it loves, With fine librations quivering, as it moves.
1853. Kane, Grinnell Exp., xlviii. (1856), 446. Others [viz. icebergs] a congeries of rubbish, and illustrating every possible condition of libration.
1874. H. R. Reynolds, John Bapt., V. iii. 341. A dazzling brightness above the Splendour of the Sun was drawing nearer with gentle librations of its wings.
c. transf. and fig.
1650. Anthroposophia Theomagica, 92. Such chiming and clinching of words, Antithetall Librations, and Symphonicall rappings.
1659. H. More, Immort. Soul, II. x. 218. The Libration or Reciprocation of the Spirits in the Tensility of the Muscles.
1659. J. Harrington, Lawgiving, Wks. (1700), 431. Such a libration or poize of Orders.
1659. O. Walker, Oratory, 97. The short [period] is adverse to Metaphors &c. the long to exact correspondence and libration of its parts.
1670. Dryden, 2nd Pt. Conq. Granada, III. i. Wks. 1808, IV. 151. The bounds of thy libration here are set.
1840. Blackw. Mag., XLVII. 719. The tremulous libration of the equipoise.
1882. J. H. Blunt. Ref. Ch. Eng., II. 480. Oxford has its regular periods of theological libration.
2. Astron. A real or apparent motion of an oscillating kind. Libration of the moon: an apparent irregularity of the moon’s motion which makes it appear to oscillate in such a manner that the parts near the edge of the disk are alternately visible and invisible. (There are three kinds, called libration in latitude, libration in longitude, and diurnal or parallactic libration.)
1669. J. Flamstead, in Phil. Trans., IV. 1109. If the Libration of the Moon be known, the protraction of the Star’s way in this Appearance will be facile. Ibid. (1670), V. 2061. Doubtless, as there is a certain Libration in the Moon, so ’tis not absurd to me, to hold a kind of Libration in the Earth, from the Annual and Diurnal motion of the same.
1678. Norris, Coll. Misc. (1699), 181. We are nonplus’d at a thousand Phenomenas in Nature, which if they were not done, we should have thought them absolutely impossible, as for instance the central Libration of the Earth.
1690. Leybourn, Curs. Math., 754. Now this Libration of the Eccentrick they commonly call the Deviation.
1728. trans. Newton’s Treat. Syst. World, 61. The Moon’s libration in longitude.
1804. Herschel, in Phil. Trans., XCIV. 374. Some small annual variation, or libration of position, which might lead to a discovery of the parallax of the fixed stars.
1812–6. J. Smith, Panorama Sci. & Art, I. 547. Her libration in latitude, is when either of her poles appears to dip a little towards the earth.
1831. Brewster, Newton (1855), I. vi. 128. Galileo had discovered and explained the diurnal libration, arising from the spectator not viewing the moon from the centre of the earth.
1834. Mrs. Somerville, Connex. Phys. Sci., ix. (1849), 78. The moon … is liable to librations depending upon the position of the spectator.
1867–77. G. F. Chambers, Astron., I. vii. 79. When the North Pole [of the Moon] leans towards the earth we see somewhat more of the region surrounding it;… this is known as libration in latitude.
1874. Farrar, Christ, 51. There is one hemisphere of the lunar surface on which in its entirety, no human eye has ever gazed, while at the same time the moon’s librations enable us to conjecture of its general character.
† 3. Weighing (lit. and fig.). Obs.
1657. W. Morice, Coena quasi Κοινὴ, xiii. 185. We … have made libration, what weight the judgment and practice of the ancient Church doth bear.
1667. Waterhouse, Fire Lond., 48. Prudent libration of what weight they will and will not beare.
1770. Emerson (title), Calculation, libration, and mensuration; or the arts of reckoning, weighing, and measuring.
Hence Librational a., pertaining to (the moon’s) libration.
1880. Proctor, Rough Ways made Smooth, 110. Photographs of the moon should be taken in every aspect and in every stage of her librational swayings.
