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.