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.
- 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.