What is a Blue Moon?
A "blue moon" is the second full moon in a month, which happens every two to three years. The use of the term "blue moon," meaning infrequently reoccurring, dates to 1821.
Source:
Post-Crescent, 7-30-1985; Oxford English Dictionary
Notes:
See the Sky and Telescope article What's a Blue Moon? or the Blue Moon web site for more information.
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