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octant|ˈɒktənt|
[ad. late L. octans, -tant-em a half quadrant (Vitruv.), f. octo eight: cf. quadrans quadrant. So F. octant, in sense 3 (1683 in Hatz.-Darm.).]
1. The eighth part of a circle; i.e. either (a) an arc of a circle, forming one eighth of the circumference, or (b) one eighth of the area of a circle, contained within two radii at an angle of 45°.
1750Phil. Trans. XLVII. 69 Thro' the whole octant OA, it is continually decreasing. 1875T. R. Robinson ibid. CLXV. 411 The irregularity of the wind..varies in each octant.
b. Each of the eight parts into which a solid figure or body (e.g. a sphere), or the space around a central point, is divided by three planes (usually mutually at right angles) intersecting at the central point.
1790Wildbore in Phil. Trans. LXXX. 497 Disposed in the eight octants of a regular parallelopipedon. 1875Bennett & Dyer tr. Sachs' Bot. 288 The globule [= antheridium of Nitella] now consists of four lower and four upper octants of a sphere... Each octant now breaks up..into an outer and an inner cell. 1895Story-Maskelyne Crystallogr. ii. §15 These planes, YZ, ZX, XY, divide the space round the origin into eight hollow quoins or octants.
2. Astron. That point in the apparent course of a planet at which it is 45° distant from another planet, from the sun, or from some particular point; spec. each of the four points at which the moon is 45° from conjunction with or opposition to the sun, or midway between the syzygies and quadratures.
1690Leybourn Curs. Math. 773 About the Octants from the Aphelion and Perihelion. 1706Phillips, Octant or Octile (in Astrol.), when a Planet is in such an Aspect or Position with respect to another, that their Places are only distant an eighth part of a Circle, or 45 Degrees. c1716Machin in Rigaud Corr. Sci. Men (1841) I. 275 It will be of great use, if I could have a few places of the moon when in the octants or near. 1787Bonnycastle Astron. xxi. 363 In her third octant..she again appears gibbous. 1834Hist. Astron. ix. 45/1 (U.K.S.) A third [inequality of the moon], called the variation,..is greatest in the octants.
3. An instrument in the form of a graduated eighth of a circle, used for making angular measurements, esp. in astronomy and navigation. (In Fr., mentioned 1683 in Le Cordier, Instruments des Pilotes.)
1731Hadley in Phil. Trans. XXXVII. 150 The Instrument consists of an Octant ABC, having on its Limb BC an Arch of 45 Degrees, divided into 90 Parts or half Degrees. 1774M. Mackenzie Maritime Surv. 2 The principal Instruments used in surveying; such as the Theodolite, the astronomical Quadrant, and Hadley's Octant, or Sextant. 1825J. Nicholson Operat. Mechanic 316 Binding himself to divide all sextants and octants by the same engine.
Hence ocˈtantal a., of or pertaining to an octant; vanishing once in each octant of the compass.
1776Horsley in Phil. Trans. LXVI. 363 If from these..we reject the octantal days. 1928L. S. Palmer Wireless Princ. & Pract. xii. 472 Any error of this nature occurring twice in each quadrant is termed an octantal error. 1954Electronic Engin. XXVI. 39/1 Marconi's have developed a unique fixed (non⁓rotating) aerial with low octantal error. |