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The phrase phase angle is used to describe the angle between the path of light from a light source to a celestial body, and the path of this body's reflected light to the observer. An example is the angle between the line to the Moon from the Sun, with the line to the Moon from an (Earth) observer who is viewing the Moon's reflected light. The term is used regarding the Moon in its different phases, and is also useful for solar system planets as a factor regarding their apparent magnitude. It also affects the light curves of transiting extra-solar planets.