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.

(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.

• The new moon is the recognized beginning of the cycle. From Earth only the shaded side can be seen.
• After four days the waxing crescent moon is much in evidence. The moon is moving around the Earth in a counter-clockwise direction. Thus, it is the right side of the moon which can be seen in quarter.
• At seven days into the cycle (about a week) the half moon can be seen. This is said to be the first quarter (not a quarter moon, but a quarter of the cycle).
• At the ten day mark we see the waxing gibbous moon. Pronounced "jibbus", this moon is kind of the reverse of the crescent moon, only a sliver is left unseen.
• At two week into the cycle the moon is full. From the Earth only the sunny side of the moon can be seen.
• At day 18 the waning gibbous moon shows itself. The counter-clockwise (from above) orbit of the moon continues. Now Earthlings will see less and less of the moon each day until the new moon.
• Twenty-one days into the cycle the half moon appears in the sky. It is the last quarter of the cycle. Three weeks have passed.
• The 25th day marks the waning crescent moon. The cycle completes, and begins again as the moon slips back into the new moon.

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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