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parallax|ˈpærəlæks|
Also 6–7 paralax, 7 parallaxe; also 7 in Gr. form parallaxis.
[a. F. parallaxe (1557 in Hatz.-Darm.), ad. Gr. παράλλαξις change, alteration, alternation, mutual inclination of two lines meeting in an angle, f. παραλλάσσειν to alter, alternate; in mod.L. parallaxis.]
1. a. (Astron.) Apparent displacement, or difference in the apparent position, of an object, caused by actual change (or difference) of position of the point of observation; spec. the angular amount of such displacement or difference of position, being the angle contained between the two straight lines drawn to the object from the two different points of view, and constituting a measure of the distance of the object. Also transf. (quot. 1881).
In Astronomy there are two kinds of parallax, viz. diurnal and annual, the former when a celestial object is observed from opposite points on the earth's surface, the latter when observed from opposite points of the earth's orbit. As the mean or proper position of the body is that which it would have if viewed in the one case from the earth's centre (or a point in a line with it), in the other case from the centre of its orbit, the parallax is actually calculated and stated from these central points, and called geocentric and heliocentric respectively, the base lines of these being the earth's radius and the radius of its orbit. horizontal parallax: the diurnal parallax of a heavenly body seen on the horizon.
1612Selden Illustr. Drayton's Poly-olb. xiv. 235 Those learned Mathematicians, by omitting of Paralax and refractions, deceiued themselues and posterity. 1663Boyle Exp. Nat. Philos. i. ii. 33 Which they not irrationally prove by the Parallaxis (or Circular difference betwixt the place of a Star, suppos'd to be taken by two Observations, the one made at the Centre, and the other on the surface of the Earth). 1665Phil. Trans. I. 106 He hath deduced the Horizontal Parallax of this very Comet. 1696Whiston Th. Earth i. (1722) 32 The fix'd Stars..till very lately..were thought subject to no parallax at all. a1711Ken Hymns Evang. Poet. Wks. 1721 I. 44, I saw it moving in a Sphear so high, Scarce any Parallax I cou'd descry. 1812Woodhouse Astron. xii. 98 The parallax of Mars was found to be about 23 seconds. 1867–77G. F. Chambers Astron. i. i. 2 The problem..when solved [gives] the amount of the Sun's equatorial horizontal parallax. 1881Tait in Nature XXV. 91/1 In these thermometers..no provision is made for avoiding parallax or personal equation.
b. fig.
1594J. Davis Seaman's Secr. (1607) 19 To amend the parallax of false shadow of your sight. 1599Daniel Musophilus 606 Vndeceiued with the Paralax Of a mistaking eye of passion. 1682Sir T. Browne Chr. Mor. ii. §3 Many things are known, as some are seen, that is by Parallaxis, or at some distance from their true and proper beings. 1870Max Müller Sc. Relig. (1873) 43 Unless we make allowance for this mental parallax [between material and spiritual]. 1892Nation 7 Apr. 262/3 The sort of parallax which exhibits the light of Whitman's fame at so different an angle in his own country and in England.
†2. In general sense of Gr. παράλλαξις: Change, alteration. Obs. rare—1.
1677Gale Crt. Gentiles II. iv. 258 The Sun although it is not so variable as the Moon, yet..it has its παραλλαγας or παραλαξεις, Paralaxes and Changes: it appears otherwise at rising, otherwise at noon, otherwise at setting.
3. Special comb.: parallax error, error in reading an instrument caused by parallax when the scale and the indicator are not precisely coincident.
1901C. J. Leaper Graduated Exercises Elem. Pract. Physics p. iii/1 (Index), Parallax error. 1906Bower & Satterly Pract. Physics v. 83 The reading, especially of the burette, is very liable to parallax error. 1967Electronics 6 Mar. 117/2 With analog instruments..operator and parallax errors, meter movement wear and aging often reduce their nominal accuracies. |