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Astronomy
The Moon
Elvis Presley

What causes a blue moon?

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June 18, 2008 11:28PM

The term "blue moon" refers to the extra moon that falls in a month that has two full moons. Each 2.7 years there is a month that has two moons in it and this rare event (actually the first moon in this month) is referred to as a "blue moon." The phrase, "once in a blue moon" means something is rare.