What Is a Moon Phase?

The phases of the Moon are changes in the appearance of the Moon relative to the position of the Earth and the Sun. As the Moon travels through its monthly orbit, varying amounts of the side facing the Earth are illuminated by the light of the Sun. As the ratio of illuminated surface to shadow changes, it creates different lunar phases.

When the Moon is on the opposite side of Earth from the Sun, its entire face is illuminated by the light of the Sun.This creates a full moon. As it begins to move through its orbit, more of its face falls into shadow, causing the Moon to wane. It passes through the gibbous phase, in which about three-quarters of its surface facing Earth is illuminated. When the Moon is 90 degrees away from the Earth-Sun axis, it appears to be half illuminated and half shadowed. As it continues to move towards the sun, the visible illumination shrinks to a crescent. Finally, when the Moon is between the Sun and the Earth, its face lies completely in shadow, producing a new moon.

The transition from full to new moon takes approximately two weeks, and then the Moon spends two weeks waxing through the crescent, half, and gibbous phases until it reaches another full moon.