The moon is the only natural satellite of the earth, orbiting it in about 28 days. The orbital position (phase) determines how much of the moon is illuminated by the sun, for earthbound observers. Since the moon has no atmosphere, all debris from space can easily reach the lunar surface, which is the reason for all the craters. Often when the moon is a small crescent, the dark area glows with a faint dark grey light; this is the earthshine, caused by the nearly fully illuminated earth as seen from the moon.

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