Crossword Dictionary

What's The Definition Of Parallax?

\Par"al*lax\, n. [Gr. ? alternation, the mutual inclination of two lines forming an angle, fr. ? to change a little, go aside, deviate; ? beside, beyond + ? to change: cf. F. parallaxe. Cf. {Parallel}.] 1. The apparent displacement, or difference of position, of an object, as seen from two different stations, or points of view. 2. (Astron.) The apparent difference in position of a body (as the sun, or a star) as seen from some point on the earth's surface, and as seen from some other conventional point, as the earth's center or the sun. {Annual parallax}, the greatest value of the heliocentric parallax, or the greatest annual apparent change of place of a body as seen from the earth and sun; as, the annual parallax of a fixed star. {Binocular parallax}, the apparent difference in position of an object as seen separately by one eye, and then by the other, the head remaining unmoved. {Diurnal}, or {Geocentric}, {parallax}, the parallax of a body with reference to the earth's center. This is the kind of parallax that is generally understood when the term is used without qualification. {Heliocentric parallax}, the parallax of a body with reference to the sun, or the angle subtended at the body by lines drawn from it to the earth and sun; as, the heliocentric parallax of a planet. {Horizontal parallax}, the geocentric parallx of a heavenly body when in the horizon, or the angle subtended at the body by the earth's radius. {Optical parallax}, the apparent displacement in position undergone by an object when viewed by either eye singly. --Brande & C. {Parallax of the cross wires} (of an optical instrument), their apparent displacement when the eye changes its position, caused by their not being exactly in the focus of the object glass. {Stellar parallax}, the annual parallax of a fixed star.