The elongation is the angular separation between the Sun and a planet or other Solar System body as observed from the Earth. Elongations are measured in degrees eastward or westward of the Sun. The greatest eastward or westward elongation is the maximum angle between the Sun and a Solar System body. Specific names are given to three main values of the elongation:
|Conjunction||0o||The Solar System body lies on the same side of the Earth as the Sun|
|Quadrature||90o||The Sun, Earth and Solar System body lie at the corners of a right-angled triangle|
|Opposition||180o||The Solar System body lies on the opposite side of the Earth as the Sun|
When Mercury or Venus (the inferior planets) reach their greatest eastern elongation, they are visible as evening objects that set after the Sun. At greatest western elongation, they are visible as morning objects that rise before the Sun.