All Things Considered, May 30, 2007 · There is a prevailing myth about what a blue moon is. Thursday, May 31, will bring the second full moon of the month. But that does not constitute a blue moon, as is popularly believed.
Kelly Beatty, editor of Night Sky magazine and executive editor of Sky and Telescope, tells Robert Siegel that a blue moon actually refers to the phenomenon of having four full moons in a season, which ordinarily has three.
Beatty also acknowledged that his magazine had a hand in giving the misconception credence. Sky and Telescope magazine recently put out a press release explaining its role in perpetuating the myth. It read, in part:
Our 1946 writer, amateur astronomer James Hugh Pruett (1886-1955), made an incorrect assumption about how the term had been used in the Maine Farmers' Almanac, where it consistently referred to the third full moon in a three-month season containing four. (By this definition there is no blue moon in May or June 2007, and the next one happens in May 2008.)
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