Phase angle (astronomy)

In observational astronomy, phase angle is the angle between the light incident onto an observed object and the light reflected from the object.

With the development of space travel, as well as in hypothetical observations from other points in space, the notion of phase angle became independent of Sun and Earth.

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.

[1] 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 giants or bodies with pronounced atmospheres, and when the object becomes fainter as the angle approaches 180°.