What is the X in X-mas?
The angular distance between two celestial bodies as seen from a third. Elongation is normally conceived as a measure of the angle formed between the Sun and a celestial body, such as a planet or the Moon, with Earth at the vertex. In terms of the celestial sphere, elongation is the distance between the Sun and the body as measured in degrees of celestial longitude. When the body lies on a direct line drawn from Earth to or through the Sun, its elongation is 0° and it is said to be in conjunction. It is said to be in quadrature when it lies at a right angle to a line between the Earth and Sun with an elongation of 90°, and it is in opposition when it lies on the opposite side of Earth from the Sun with an elongation of 180°. Superior planets (those that are farther from the Sun than Earth) have a full range of elongations between 0° and 180°. Inferior planets (those closer to the Sun than Earth) have limited elongations due to their smaller orbits; Venus has a greatest elongation of about 48°, while Mercury's greatest elongation is about 28°. See more at conjunction, opposition.
in astronomy, the angular distance in celestial longitude separating the Moon or a planet from the Sun. The greatest elongation possible for the two inferior planets (those closer than the Earth to the Sun) is about 48 in the case of Venus and about 28 in that of Mercury. Elongation may also refer to the angular distance of any celestial body from another around which it revolves or from a particular point in the sky; e.g., the extreme east or west position of a star with reference to the north celestial pole.