elongation, in astronomy, the angular distance between two points in the sky as measured from a third point. The elongation of a planet is usually measured as the angular distance from the sun to the planet as measured from the earth. When a planet lies on the line drawn from the earth to the sun, its elongation is 0 and is said to be in conjunction. When a planet's elongation is 90, it is in quadrature. When its elongation is 180, it is in opposition. Elongation is measured east (eastern quadrature) or west (western quadrature) from the sun. The superior planets can have elongations between 0 and 180; the elongations of the inferior planets are limited by their proximity to the sun. The greatest elongation of Mercury is 28, and of Venus, 47.
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