Earth and the Moon

The Moon

How do the lunar phases work?

The new moon occurs when the Moon is between Earth and the Sun. All of the sunlight striking the Moon bounces away from Earth, so we do not see any of the Moon at all. Over the next two weeks or so, the phase of the Moon changes from new to waxing crescent, then first quarter, then waxing gibbous, until Earth is between the Sun and the Moon. At that point, all of the sunlight striking the Moon bounces toward Earth, so we see the entire disk of the Moon. This phase is called the full moon. Then, over the next two weeks after the Moon is full, the phase changes to waning gibbous, then third quarter (also called last quarter), then waning crescent, until the phase of the Moon is new again.


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