elongation

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°.

elongation
1.  The angle between the Sun and a body in the Solar System, as seen from Earth. Elongations are measured from 0° to 180° east or west of the Sun, in the plane passing through the Sun, Earth, and body. When a body is in conjunction with the Sun, its elongation is 0°; at opposition its elongation is 180°; and when at quadrature its elongation is 90° east or west. Greatest elongation is the maximum angular separation of Mercury or Venus from the Sun, either west or east.

2. The angle between a planet and a satellite, as seen from Earth. Elongations are measured from 0° east or west of the planet, in the plane passing through the planet, Earth, and satellite.