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SCIENCE FILE | I Didn't Know That

June 27, 1996

Q: What is a blue moon?

A: A blue moon is the second full moon in a month. A blue moon will occur Sunday, and the next will be on March 31, 1999.

The moon goes through a complete cycle of phases in 29.5 days. If a full moon occurs in the first day or two of a month, then that phase can be repeated at the end of the month. A blue moon can thus occur in any month except February, which has only 28 days. On average, blue moons occur once every 2.7 years. Moons can literally take on a blue hue on infrequent occasions when atmospheric conditions are right, and the expression "once in a blue moon" came to mean a rare occurrence as early as the 1500s. But calling the second full moon of the month a blue moon did not start until the late 1800s.

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