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What Are the Cyclical Phases of the Moon?

The phases of the moon change according to the relative positions of the sun, Earth and moon. Although the sun always illuminates half of the moon, the amount of this illumination that is visible from the Earth depends on the moon's angle with regard to the Earth and sun. The moon orbits the Earth once every 27.3 days. However, a full lunar cycle comprises about 29.5 days, because the Earth travels approximately 45 million miles around the sun in the time that it takes the moon to orbit the Earth. Therefore, it takes the moon roughly two extra days to return to its initial position relative to the Earth and sun.

New Moon
- The new moon, or dark moon, occurs when the Earth, moon and sun align. The moon's illuminated side faces away from the Earth and toward the sun, making it invisible from the Earth. During the new moon, the moon and sun rise and set at roughly the same time.
Waxing Crescent
- The waxing crescent phase occurs when the moon's orbit begins to carry it out of its alignment with the Earth and sun. During this time, most of its illuminated side is still facing away from the Earth, and only a thin crescent is visible.
First Quarter
- This phase occurs approximately one week after the new moon. Its name comes from the fact that the moon has completed a quarter of the lunar cycle. During this phase, half of the illuminated side of the moon--or a quarter of the entire moon--is visible from the Earth.
Waxing Gibbous
- The waxing gibbous phase occurs during the second week after the new moon. As the moon waxes, more of its illuminated side becomes visible from the Earth. "Gibbous" means that more than half of the moon's disc is visible from the Earth, but the moon is not yet full.
Full Moon
- The full moon occurs about two weeks after the new moon, when the moon is halfway through its cycle. During this phase, the entire illuminated side of the moon faces the Earth. It is only during the full moon, when the moon and sun are on opposite sides of the Earth, that lunar eclipses can occur.
Waning Gibbous
- During the week following the full moon, the illuminated portion of the moon begins to decrease, or wane. As with the waxing gibbous moon, more than half of the moon's disc is visible during this phase.
Last Quarter
- The last quarter phase occurs about three weeks after the new moon, once again showing half of the moon's illuminated portion, or a quarter of its total surface.
Waning Crescent
- The waning crescent moon occurs during the fourth week of the lunar cycle. Like the waxing crescent moon, it appears as a very thin sliver in the sky. When 29.5 days have passed, the lunar cycle restarts with the new moon.

Astronomy