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What are the phases of the moon?

A phase of the moon is defined by the shape of the illuminated portion of the moon, as it appears to observers on Earth. The moon's appearance changes throughout the month, as it travels on its orbit around the Earth. The relative positions of the sun, Earth, and moon determine which portion of the moon's sunlit surface can be seen.

When the moon is between the Earth and the sun, its daylight side is turned away from the Earth, so the moon can't be seen at all. This is called the "new moon." As the moon continues its revolution around the Earth, a slice of its surface becomes visible. This is called the "waxing crescent phase. About a week after the new moon, half the moon is visible. T his is the "first quarter" phase.

During the following week, more than half the surface of the moon can be seen; this is called the "waxing gibbous" phase. Finally, about two weeks after the new moon, the...

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