Moon phases. 18th century French document showing the phases of the moon, from full (pleine) to new (nouvelle) and back again. Lunar phases occur because the Moon reflects the light of the Sun. The proportion of the lit face facing the Earth therefore determines the proportion of the Moon visible. When the Sun and Moon are on opposite sides of the Earth, the Moon appears full. When they are on the same side of Earth, the Moon appears as a crescent. The phases follow a 28-day cycle.