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## parallax

parallax (pâr´əlăks), any alteration in the relative apparent positions of objects produced by a shift in the position of the observer. In astronomy the term is used for several techniques for determining distance. Trigonometric parallax is the apparent displacement of a nearby star against the background of more distant stars resulting from the motion of the earth in its orbit around the sun. Formally, the parallax of a star is the angle at the star that is subtended by the mean distance between the earth and the sun. A shift in the angular position of a star will be greatest when observed at intervals of six months; this makes the parallax equal to the value of one half of the semiannual displacement of the star. If a star's parallax can be measured, it then determines the distance to the star. A unit of stellar measurement is the parsec; it is the distance at which a star would have a parallax of one second of arc and is equivalent to 206,265 times the distance from the earth to the sun, or about 3.3 light-years. A star's distance d in parsecs is the reciprocal of its parallax p (or d = 1/p). The first stellar parallax was measured in 1838 by Friedrich Bessel for the star 61 Cygni. Its parallax of 0.3 places it at a distance of 3.3 parsecs or about 11 light-years. The technique of stellar parallax is useful for stars within 100 parsecs. Spectroscopic parallax is the most widely used technique for determining the distances of stars that are too distant for their stellar parallaxes to be measured. From the analysis of a star's spectrum, its position on the Hertzsprung-Russell diagram is determined. This diagram correlates the spectral class of the star with its absolute magnitude. By comparing the absolute magnitude to its apparent brightness, the star's distance is calculated. Dynamical parallax is a method for determining the distance to a visual binary star. The angular diameter of the orbit of the stars around each other and their apparent brightness are observed. By applying Kepler's laws and the mass-luminosity relation, the distance of the binary star can be determined. Geocentric parallax is a technique similar to stellar parallax, which uses the diameter of the earth rather than the diameter of its orbit as a baseline. Because this baseline is relatively small, the technique is useful only for close celestial objects such as the moon or the asteroids.

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"parallax." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. 10 Dec. 2017 <http://www.encyclopedia.com>.

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## parallax

par·al·lax / ˈparəˌlaks/ • n. the effect whereby the position or direction of an object appears to differ when viewed from different positions, e.g., through the viewfinder and the lens of a camera. ∎  the angular amount of this in a particular case, esp. that of a star viewed from different points in the earth's orbit. DERIVATIVES: par·al·lac·tic / ˌparəˈlaktik/ adj.

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"parallax." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 10 Dec. 2017 <http://www.encyclopedia.com>.

"parallax." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (December 10, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/parallax-0

"parallax." The Oxford Pocket Dictionary of Current English. . Retrieved December 10, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/parallax-0

## parallax

parallax Angular distance by which a celestial object appears to be displaced with respect to more distant objects, when viewed from opposite ends of a baseline. The parallax of a star (annual parallax) is the angle subtended at the star by the mean radius of the Earth's orbit (one astronomical unit); the smaller the angle, the more distant the star. See also parsec

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"parallax." World Encyclopedia. . Encyclopedia.com. 10 Dec. 2017 <http://www.encyclopedia.com>.

"parallax." World Encyclopedia. . Encyclopedia.com. (December 10, 2017). http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/parallax

"parallax." World Encyclopedia. . Retrieved December 10, 2017 from Encyclopedia.com: http://www.encyclopedia.com/environment/encyclopedias-almanacs-transcripts-and-maps/parallax

## parallax

parallax XVII. — F. parallaxe — Gr. parállaxis change, alternation, mutual inclination of two lines meeting in an angle, f. parallássein, -allakt- alter, alternate, f. PARA-1 + allássein exchange, f. állos other.
So parallactic XVII. — Gr.

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"parallax." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. 10 Dec. 2017 <http://www.encyclopedia.com>.

"parallax." The Concise Oxford Dictionary of English Etymology. . Encyclopedia.com. (December 10, 2017). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/parallax-1

"parallax." The Concise Oxford Dictionary of English Etymology. . Retrieved December 10, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/parallax-1

## parallax

parallax The apparent change in position of an object in relation to another when the viewpoint is changed. See also STEREOPTIC VISION.

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"parallax." A Dictionary of Earth Sciences. . Encyclopedia.com. 10 Dec. 2017 <http://www.encyclopedia.com>.

"parallax." A Dictionary of Earth Sciences. . Encyclopedia.com. (December 10, 2017). http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/parallax

"parallax." A Dictionary of Earth Sciences. . Retrieved December 10, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/parallax

## parallax

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