greatest elongation
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The maximum separation between Mercury or Venus and the Sun in the sky. Mercury and Venus peak as "evening stars" at their greatest eastern elongations, and as "morning stars" during their greatest western elongation. Geometrically, the greatest elongation of Venus occurs when a line tangent to the orbit of Venus intersects Venus and the Earth, and the greatest elongation of Mercury occurs when a line tangent to the orbit of Mercury intersects Mercury and the Earth.