Phase angle

Phase angle is the elongation or angle between an orbiting body and the Sun as viewed from a particular perspective such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. Inferior planets such as Venus generally have a low phase angle as seen from Earth, so they are often viewed as a crescent. Superior planets such as Mars and Jupiter usually have a high phase angle, so little of the shadowed side is visible.

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