Elongation is the apparent angular separation of a celestial body from the Sun, measured at the observer. For a lunar phase, the elongation is the Sun–Earth–Moon angle; in the distant-Sun approximation it is supplementary to the phase angle. A given unsigned elongation can occur on either side of the Sun in the sky.
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In astronomy, **elongation** refers to the angular distance between a celestial body and the Sun as viewed from Earth. It is most commonly used in the context of the planets, particularly inferior planets (those that orbit closer to the Sun than Earth, such as Mercury and Venus). Elongation helps describe the position of these planets in relation to the Sun and Earth.