Moon phases. Medieval manuscript page showing the changes in the appearance (phases) of the Moon as it orbits around the Earth. Lunar phases occur because the Moon does not produce its own light but reflects that of the Sun. This means that some of the Moon's surface is darkened by its own shadow. The amount of this shadowed area visible depends on the relative positioning of the Sun Moon and Earth. 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. Photographed from the 14th century book Le Propietaire des Choses by Barthelemy de Glandville.