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Tonight's the blue moon. According to modern folklore, a blue moon is the second full moon in a month. Blue moons are uncommon, occurring about once every 2½ years. (Photo illustration by Bob King

Astro Bob column: Modern definition of 'blue moon' a case of misinterpretation

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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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