phase

Phase. The changing phases of the Moon during the course of a synodic month. The ratio of the illuminated area of the apparent disc of a celestial body to the area of the entire apparent disc, taken as a circle. The phases of the Moon are the recurring cycle of apparent forms of the Moon (see illustration). New Moon, first quarter, full Moon and last quarter are formally defined as the times at which the excess of the apparent celestial longitude of the Moon over that of the Sun is 0°, 90°, 180° and 270° respectively. The Moon and planets show phases because bodies of this kind emit no light of their own, shining only by reflected sunlight. The hemisphere of a moon or planet facing the Sun is bright, its other hemisphere dark. The phase as seen from the Earth depends on the relative positions of the Earth and Sun with respect to the body, since this determines what proportion of its illuminated half is visible.