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How Do the Phases Get Their Names? - Windows to the Universe

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The Phases of the Moon.
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How Do the Phases Get Their Names?

When the Moon appears smaller than a quarter, we call it a crescent. When the Moon appears larger than a quarter, we call it gibbous. When the moon is getting bigger (phases New to Full) it is waxing. When it is getting smaller (phases Full to New) it is waning.

For example, if today the Moon were a waxing crescent, then tomorrow the crescent shape would continue to grow larger, approaching first quarter. After first quarter, the Moon would be a waxing gibbous, and continue growing until it reached full. The Moon would then begin to shrink, becoming first a waning gibbous and eventually reaching third quarter. Following third quarter it becomes a waning crescent, and continues to shrink until it becomes invisible at new Moon.

Just in case you can't remember all of this, there are a few handy ways to recognize whether the Moon is growing or shrinking. A crescent moon which looks like a "C" is shrinking (C for collapsing!). If it looks like a "D", then it is growing. This is true for a gibbous Moon as well, but it's a bit trickier to picture. If the edge of the Moon (the real edge of the Moon, not the edge of night on the Moon) is curved like a "C", the gibbous Moon is shrinking. Another way to think of it is that the Moon always grows or shrinks from the right to the left.

Last modified October 19, 2005 by Randy Russell.

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