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°.