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A diagram which demonstrates why the moon goes through phases.
Click on image for full size

Phases of the Moon

The diagram shows the Moon in different positions along its orbit around the Earth. The Sun is off in the distance, lighting the Earth-Moon system. At any position, half of the Moon is illuminated by the Sun (the light side of the Moon) and half is not (the dark side). Also, half of the Moon is visible to the Earth (the near side of the Moon) and half is not (the far side). As the Moon moves around the Earth, we can see different fractions of the illuminated half of the Moon.

When the Moon is between the Earth and the Sun (1), the near side of the Moon is the dark side. The Moon cannot be seen. We call this New Moon, the beginning of a new cycle of lunar phases. When the Earth is between the Sun and the Moon (5), the near side is the light side. We call this Full Moon, even though we only see half the Moon.

Halfway in between these times (3 & 7), only half of the near side of the moon is illuminated by the Sun. So we can only see one quarter of the Moon. We call these phases First and Third Quarters.

All the phases of the Moon have special names which indicate how much of the illuminated Moon can be seen from Earth, and whether this part is going to grow or shrink.

Last modified October 19, 2005 by Randy Russell.

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