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What Are the Normal Phases of the Moon?

Over time, the appearance of the moon changes. At all times, at least half of the moon is illuminated by the sun. We can see only the illuminated portion -- the half that is in shadow is not visible to the naked eye. The different phases occur as the angles between the Earth, the sun and the moon change, as the moon orbits the Earth.

Phases
- Their are eight phases of the moon. They are the first quarter, waxing gibbous, full, waning gibbous, third quarter, waning crescent, new and waxing crescent. The main phases are the new moon and the full moon, and the first and third quarters, with other phases in between.
Main Phases
- When the moon is between the Earth and the sun, it's in the new moon phase. The Earth, moon and sun are all in alignment. The portion of the moon that is illuminated is the back side, and we cannot see it. During the full moon phase, the Earth, moon and sun are in alignment again, but the illuminated half of the sun is visible to us. The first and third quarter phases are also called "half-moons." When the moon is at a 90-degree angle to the Earth and the sun, we see half of the moon illuminated - the other half is in shadow and cannot be seen from Earth.
Additional Phases
- During a crescent moon, less than half of the moon is illuminated. During a gibbous moon, more than half of the moon is illuminated. When the moon is in a waxing phase, the illumination is expanding. When the moon is in a waning phase, the illumination is decreasing. Following the new moon, the illuminated portion of the moon is increasing, but less than half is lit, making it a waxing crescent. Following the first quarter, the illumination is increasing and it's lighting more than half the moon, making it a waxing gibbous. Following the full moon, the light is decreasing, creating a waning gibbous. After the third quarter, the light is decreasing until entirely gone, creating a waning crescent.
Orbit
- The moon completes an entire cycle of all the phases in approximately one month. The period is approximately 29 days, not usually an entire calendar month -- and except for Leap Year, more than one month during February. The orbit, or lunation, is the time it takes for the moon to cycle from its starting phase all the way back to its beginning phase. During the month-long moon lunation, the earth is also moving through its yearly orbit around the sun.

Astronomy