Elongation is the geocentric angle between a planet and the Sun, measured in the plane of the planet, Earth, and Sun. Planetary elongations range from 0° to 180°, east or west of the Sun. Greatest elongation is the instant at which the geocentric angular distance of Mercury or Venus from the Sun is a maximum.

Elongation is also the name of the angle between a planet and one of its satellites as seen from Earth, measured from 0° east or west of the planet, in the plane of the planet, satellite, and Earth.