The phases of the moon are changes in the moon’s appearance during the month, which are caused by the moon’s turning different portions of its illuminated hemisphere toward Earth. When the moon is between Earth and the sun, its daylight side is turned away from Earth, so it is not seen. This is called the new moon. As the moon continues its revolution around Earth, more and more of its surface becomes visible. This is called the waxing crescent phase. About a week after the new moon, half the moon is visible—the first quarter phase. During the next week, more than half of the moon is seen; this is called the waxing gibbous phase. Finally, about two weeks after the new moon, the moon and sun are on opposite sides of Earth.

The side of the moon facing the sun is also facing Earth, and all the moon’s illuminated side is seen as a full moon. In the next two weeks the moon goes through the same phases, but in reverse from a waning gibbous to third or last quarter to waning crescent phase. Gradually, less and less of the moon is visible until a new moon occurs again.