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added reference and values for Jupiter and Saturn phase angles |
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The phase angle varies from 0° to 180°. The value of 0° corresponds to the position where the illuminator, the observer, and the object are [[collinear]] (all lying along the same line), with the illuminator and the observer on the same side of the object. The value of 180° is the position where the object is between the illuminator and the observer, known as [[inferior conjunction]]. Values less than 90° represent [[backscatter]]ing; values greater than 90° represent [[forward scatter]]ing.
For some objects, such as the [[Moon]] (see [[lunar phases]]), [[Venus]] and [[Mercury (planet)|Mercury]] the phase angle (as seen from the Earth) covers the full 0–180° range. The [[superior planet]]s cover shorter ranges. For example, for [[Mars]] the maximum phase angle is about 45°.
The [[brightness]] of an object is a function of the phase angle, which is generally smooth, except for the so-called [[opposition spike]] near 0°, which does not affect [[gas giant]]s or bodies with pronounced [[atmosphere]]s, and when the object becomes fainter as the angle approaches 180°. This relationship is referred to as the [[Phase curve (astronomy)|phase curve]].
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