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((short description|Angle between the light incident on, and reflected from, an astronomical object))
((More citations needed|date=October 2023))
[[File:Phase Angle 3.svg|thumb|Phase angle diagram]]

In [[observational astronomy]], '''phase angle''' is the [[angle]] between the light [[Ray (optics)|incident]] onto an observed [[Astronomical object|object]] and the light [[reflected]] from the object. In the context of astronomical observations, this is usually the angle [[Sun]]-object-observer.

For terrestrial observations, "Sun–object–Earth" is often nearly the same thing as "Sun–object–observer", since the difference depends on the [[parallax]], which in the case of observations of the [[Moon]] can be as much as 1°, or two full Moon diameters. With the development of [[Spaceflight|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 [[etymology]] of the term is related to the notion of [[planetary phase]]s, since the [[brightness]] of an object and its appearance as a "phase" is the function of the phase angle.

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&ndash;180° range. The [[superior planet]]s cover shorter ranges. For example, for [[Mars]] the maximum phase angle is about 45°. For Jupiter, the maximum is 11.1° and for Saturn  6°.<ref>((cite web |last1=Talcott |first1=Richard |title=Do the Outer Planets have Phases |url=https://www.astronomy.com/observing/do-the-outer-planets-have-phases/ |website=astronomy.com |access-date=17 October 2023))</ref>

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]].

==See also==
*[[Illumination angle]]
*[[Incidence angle (optics)]]

==References==
((reflist))

==External links==
*[https://web.archive.org/web/20140819084901/http://www.oxforddictionaries.com/definition/english/phase-angle?q=phase+angle Oxford dictionary definition]

((Portal bar|Astronomy|Stars|Spaceflight|Outer space|Solar System))

((DEFAULTSORT:Phase Angle (Astronomy)))
[[Category:Angle]]
[[Category:Observational astronomy]]
[[Category:Radiometry]]
[[Category:Scattering, absorption and radiative transfer (optics)]]

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