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1 The proportion of the visible disk of the Moon or a planet that is illuminated as seen from Earth.

2 A measure of position along a cyclically varying quantity, such as a wave or a periodic vibration. Phase is measured as an angle, where one complete cycle is equivalent to a phase of 360° (or 2π radians), or occasionally as a number between 0.0 and 1.0. Two or more waves of the same frequency are said to be in phase when their maxima and minima occur at the same instants. Otherwise, they are said to be out of phase, or to have a phase difference. If their phases differ by exactly 180° they are said to be in antiphase.

Subjects: Astronomy and Astrophysics.

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