The astronomical phase angle is the angle at an illuminated body between the directions to its light source and to its observer. For a lunar phase it is the Sun–Moon–observer angle at the Moon, rather than the elongation measured at Earth. A small phase angle gives a nearly full apparent disk in the spherical model.
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In astronomy, the phase angle refers to the angle between the observer, a celestial body (such as a planet or moon), and the source of light illuminating that body (usually the Sun). It is an important concept when discussing the illumination of astronomical objects, particularly those in the solar system, such as planets and their moons. The phase angle can be used to describe the appearance of these objects as viewed from a specific location, typically Earth.