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Definition of PARALLAX
: the apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object; especially: the angular difference in direction of a celestial body as measured from two points on the earth's orbit
: the apparent displacement or the difference in apparent direction of an object as seen from two different points not on a straight line with the object <motion parallax governs the way objects appear to move when the eyes or head are shifted—Edmund Andrews>
Difference in the direction of a celestial object as seen by observers from two widely separated points, a measurement used to find a body's distance. The two positions of the observers and that of the object form a triangle; its apex angle (at the object) is twice the parallax, which becomes smaller with increasing distance. Observations for calculating the Sun's parallax can be made simultaneously from two different places on Earth's surface; that value reaches a maximum of 8.794 seconds of arc for observers at points separated by Earth's diameter. Observing the difference in an object's position as seen from Earth at points six months apart in its orbit (stellar, or annual, parallax) allows measurements of distances (e.g., of stars) too large to be made from two places on Earth's surface. The nearest star system, Alpha Centauri, has a stellar parallax of 0.76 second of arc. Highly precise parallaxes, and thus the positions, of more than 100,000 stars in the Sun's vicinity were determined from data collected by the European Space Agency's Hipparcos satellite (launched 1989).