phases, lunarThe phases exhibited by the Moon (see illustration). A new Moon occurs when the Moon is at conjunction and the nearside is totally unilluminated by the Sun. As the moon moves eastward in its orbit around the Earth, the sunrise terminator crosses the nearside from east to west to produce a crescent Moon. The crescent waxes to first quarter when the Moon, at quadrature, is half illuminated, through a gibbous Moon, and finally to full Moon, when the Moon is at opposition and the nearside is fully illuminated. The sunset terminator then follows to produce a waning gibbous Moon, last quarter, a waning crescent, and the next new Moon. New Moon, first quarter, full Moon, last quarter, and next new Moon occur on average about 7.4 days apart. See also eclipse; synodic month.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006