Caption: The astronomical phase angle (or phase angle for short) illustrated.

The phase angle is the angle between an incident and reflected light rays.

Usually, the phase angle is the angle between the Earth and Sun subtended by an astronomical object (e.g., planet as in the diagram).

If the elongation of the astronomical object is east/west, the Sun is east/west of the Earth.

Credit/Permission: © User:Planetary photometry, 2010 / CC BY-SA 3.0.
Image link: Wikimedia Commons.
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