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Tonight’s the Blue Moon. According to modern folklore, a blue moon is the second full in a month. Blue moons are uncommon, occuring about once every 2 1/2 years. Bob King | Forum Communication Photo Illustration

Misinterpreting the moon: Blue moon definition came from a mistake

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By Bob King,

Forum Communications

Tonight is the blue moon — which isn’t blue, of course, unless you happen to be near a volcano.


It was actually a mistake by an amateur astronomer in Sky & Telescope magazine in 1946 that led to our current definition of “blue moon” as the unusual second full moon in a month.

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