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Inobservational astronomy,phase angle is theangle between the lightincident onto an observedobject and the lightreflected from the object. In the context of astronomical observations, this is usually the angleSun-object-observer.
For terrestrial observations, "Sun–object–Earth" is often nearly the same thing as "Sun–object–observer", since the difference depends on theparallax, which in the case of observations of theMoon can be as much as 1°, or two full Moon diameters. With the development ofspace travel, as well as in hypothetical observations from other points in space, the notion of phase angle became independent of Sun and Earth.
Theetymology of the term is related to the notion ofplanetary phases, since thebrightness 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 arecollinear (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 asinferior conjunction. Values less than 90° representbackscattering; values greater than 90° representforward scattering.
For some objects, such as theMoon (seelunar phases),Venus andMercury the phase angle (as seen from the Earth) covers the full 0–180° range. Thesuperior planets cover shorter ranges. For example, forMars the maximum phase angle is about 45°. For Jupiter, the maximum is 11.1° and for Saturn 6°.[1]
Thebrightness of an object is a function of the phase angle, which is generally smooth, except for the so-calledopposition spike near 0°, which does not affectgas giants or bodies with pronouncedatmospheres, and when the object becomes fainter as the angle approaches 180°. This relationship is referred to as thephase curve.
