Phase angle

For other uses of this term, see phase angle (wave).

Phase angle is the angle between the Sun and Earth seen from the center of a planet. It is related to the phase of a planet or portion of the illuminated disk as seen from Earth. For an inner planet such as Mercury and Venus, the phase angle acquires any value, where 0 in the upper conjunction, 180 in the lower conjunction. For an outer planet the phase angle is bounded by sin Fmax = 1 / r where r is the distance from the outer planet to the Sun in UA. It acquires this maximum value in the squares and it counts 0 in the conjunction and in the opposition.

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