Resource: Phases of the Moon

As the Moon orbits Earth every 29.5 days, it goes through a cycle of phases — changes in its apparent shape as viewed from Earth. Because the Moon is spherical and the hemisphere that faces the Sun reflects sunlight, half of the Moon is always illuminated. Depending on the Moon's position relative to Earth, different portions of this illuminated hemisphere are visible to us.

Five designations describe the Moon's apparent shape and sequence of phases: new, crescent, quarter, gibbous, and full. Despite this division into five stages, the phases of the Moon are in fact part of a continuum. As the Moon orbits Earth, the portion of its illuminated hemisphere that is visible from Earth changes slightly every day.

During the new moon phase, the Moon is between the Sun and Earth. Because this placement means that the Moon's illuminated half is facing away from Earth, the Moon is not visible from Earth. As the Moon continues its orbit, the illuminated side gradually becomes visible and is seen as a crescent. Over the next few days, the crescent appears to grow, or wax, showing a larger portion of the illuminated side of the Moon each successive day. When half of its illuminated hemisphere becomes visible from Earth, the Moon appears as a half-disk, also known as the "half moon." This is the first-quarter phase.

The Moon continues to wax through gibbous phase, in which more than half of the illuminated side is visible from Earth. When the Moon reaches the point of its orbit at which it is on the opposite side of Earth from the Sun, the entire lit hemisphere is visible and it appears as a full moon — a complete circular disk. As the Moon proceeds around the rest of its orbit, it wanes from full to gibbous to half to crescent until the process starts over again with a new moon.

Over time, gravitational forces between the Moon and Earth have synchronized the Moon's rotation rate with its orbit, such that the Moon takes 29.5 days both to orbit Earth and to rotate on its axis. As a result, the same side of the Moon always faces Earth, and from Earth's surface people never see the far side.

To learn more about how the Moon stays in orbit, check out Why Doesn't the Moon Fall Down?.

To learn more about the Moon's role in eclipses, check out Solar Eclipses.

To learn more about the origin of the Moon, check out The Origin of the Moon.

• Why does the Moon sometimes appear in the sky as a crescent shape?
• What do we call the process of the Moon's changes as it goes from a new moon to a full moon?
• Draw a diagram showing the relationship between the Moon, Earth and the Sun when there is a full moon.
• Although we can't see a new moon, at what time of day is a 'new' moon actually in the sky?
• Discuss how the Moon appears to wax and wane. Use props such as a flashlight and balls to demonstrate its changing appearance.