The angular separation between a planet and the Sun as seen from Earth, determining when and where the planet is visible in the sky. Maximum elongation occurs when inner planets appear farthest from the Sun.
From Latin 'elongatio' meaning 'a lengthening' or 'extension,' from 'e' (out) and 'longus' (long). The astronomical usage developed in the 17th century as astronomers began precise measurements of planetary positions relative to the Sun.
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