elongation

Elongation.The positions of the Sun and Earth in relation to inferior and superior planets at greatest elongation, conjunction, opposition and quadrature. 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 18° and 28° according to circumstances; the equivalent range for Venus is 45°-47°. Any elongation is possible for the planets further from the Sun than the Earth. An elongation of 90° is called quadrature, of 0° conjunction and of 180° opposition.