Why the Moon Waxes and Wanes

On a clear night the moon can often be seen in the sky in different attitudes. It seems to cycle through a series of shapes, from full sphere, to a hemisphere, to a crescent, to nothing at all. These strange attitudes can be explained by the fact that the moon does not have its own source of light. What is seen of the moon from Earth is only by the reflected light of the Sun.

New Moon as seen from space
New Moon as seen from Earth Waxing Crescent Moon as seen from Earth Half Moon in First Quarter as seen from Earth Waxing Gibbous Moon as seen from Earth Full Moon as seen from Earth Waning Gibbous Moon as seen from Earth Half Moon in Last Quarter as seen from Earth Waning Crescent Moon as seen from Earth

(Hover over a phase of the moon to see what it looks like from space.)

As the moon travels around the Earth, it shows a different aspect relative to the sun to the terrestrial observer. During a new moon the moon is closer to the sun than the Earth, so people on Earth can only see the shaded side of it. At a full moon, the moon has travelled around the Earth so that the Earth is closer to the sun. Now earthly observers can see the side of the moon that is fully exposed to the sun.

It takes the sun approximately 28 days to make its circuit around the Earth. Thus, the moon seems to wax (or get more full) for 14 days, and appears to wane (or get less full) for the subsequent 14 days. About every three or four days there is a recognized phase.

Because the moon only spins once in every 28 days, only one side of its surface is revealed to the Earth. Meanwhile, this monthly rotation means that one day on the moon takes about a month. Thus, the moon appears to wax and wane because the terrestrial viewpoint of the moon changes relative to the moon's source of light.

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Interesting Fact:

In 1969 Neil Armstrong was the first man to walk on the moon.


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