The changing fraction of the disk of an astronomical object that is illuminated, as seen from some particular location. The monthly phases of the Moon are a familiar example (see illustration). When the Sun is approximately on the far side of the Moon as seen from Earth (conjunction), the dark side of the Moon faces the Earth and there is a new moon. The phase waxes, beginning with crescent phases, as an increasing fraction of the illuminated face of the Moon is seen. At quadrature, when half the visible face of the Moon is illuminated, the phase is called the first-quarter moon, since the Moon is now one-quarter of the way through its cycle of phases. The waxing moon continues through its gibbous phases until it is in opposition; the entire visible face of the Moon is illuminated, the full moon. During the full moon, the Moon and the Sun are on opposite sides of the Earth, a configuration known as a syzygy. Then the Moon wanes, going through waning gibbous, third-quarter, and waning crescent phases until it is new again. The cycle of moon phases takes approximately 29.53 days and explains the origin of the word month. See also Moon.
From Earth, different fractions of the illuminated half of the Moon are seen at different times as the Moon goes through a 29.53-day cycle of phases.
Galileo discovered the phases of the planet Venus when he observed the sky with his telescope in 1610. Giovanni Zupus discovered the phases of the planet Mercury in 1639. Because of the angle at which the outer planets are seen from Earth, and because of their great distance, they do not appear to go through phases as seen from Earth.
