A blue moon is full moon that occurs as the second full moon in a given month. Blue moons are not typically blue in color — that happens only, well, once in a blue moon, but there is the possibility for a hint of blue in any full moon (more on this below). The definition of blue moon as the term is used today began when a writer made a mistake. [Blue Moon Lights Up Night Sky This Week]
See photos of the August 2012 blue moon here: The Blue Moon and Full Moon of 2012 (Photos)
The phrase "blue moon" is "a creature of folklore," explains Philip Hiscock, a folklorist at the Memorial University of Newfoundland. "It's the second full moon in a calendar month."
Hiscock helped figure out where the term came from. Long ago, "blue moon" was used to describe absurd things. In 1946, James Hugh Pruett (1886-1955), an amateur astronomer, was writing in Sky & Telescope magazine. Pruett "made an incorrect assumption about how the term had been used in the Maine Farmers' Almanac — which consistently used "blue moon" to mean to the third full moon in a season that contained four of them (rather than the usual three)," Sky & Telescope editors later explained. The error had been repeated in a syndicated radio program in 1980.
Hiscock and Texas astronomer Donald W. Olson helped the magazine sort all this out and admit the mistake back in 1999. The error led to the widely accepted definition of blue moon today: the second full moon in a given month. A blue moon occurs roughly once every 2.7 years.
The next blue moon, according to this folklore, will be July 31, 2015, following a full moon on July 1, 2015.
But can the moon really be blue? Yes, scientists say.
If there's been a recent forest fire or volcanic eruption that pumped significant smoke or ash into the upper atmosphere, it is possible for the moon to take on a bluish hue. Just such an event made the moon turn blue in late September, 1950, when smoke from a forest fire in Canada drifted down to cause a blue moon over eastern North America. The eruption of Mount Pinatubo in June 1991 created blue moons from various perspectives around the planet.
The phrase “once in a blue moon” — meaning something very rare — dates back to 1824.
Now that you know the truth, you might test your lunar knowledge with our moon facts quiz.
— SPACE.com Staff