Blue Moon: The term “blue moon” refers to an extra moon in a season: if a season has four full moons (rather than the more common three), then the third of the four moons is known as a blue moon.
A mistaken definition, that the second full moon in a calendar month is known as a blue moon, became common during the second half of the twentieth century due to a misinterpretation of the Maine Farmers’ Almanac in the March 1946 Sky & telescope magazine; this was corrected in 1999.
Since there are 12.37 full moons in a year, a “blue moon” must occur on the average every 2.7 years.
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