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
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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