The angular distance between the Sun and a planet or the Moon as viewed from the Earth, i.e. the angle Sun-Earth-Moon/planet (see illustration).
For the inferior planets, Mercury and Venus, elongation is restricted to a limited range. The maximum values, east and west, reached during each orbit are called greatest elongation. The greatest elongation for Mercury lies between 18í and 28í according to circumstances; the equivalent range for Venus is 45í-47í. Any elongation is possible for the planets further from the Sun than the Earth.
An elongation of 90í is called quadrature, of 0í conjunction and of 180í opposition.
