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Blue Moon

Music by Richard Rodgers (1934), performed by Harpo Marx in At the Circus (1939).

(Words by Lorenz Hart)

Do you know the expression, "once in a blue moon?"  Meaning "rarely," it refers to the fact that blue moons occur very infrequently.  If something happens only once, or you do something only once during the interval between two blue moons, that's pretty infrequent.

So what is a blue moon, then?  Well, most of you might say a blue moon is the second full moon in a calendar month.  That would probably be enough for many, but here @educhora where we strive to be at our academic best, there's a more correct answer.  A blue moon refers, for reasons unknown to us, to the third full moon in a season with four full moons.  If you're into knowing the details, James Pruett, an American amateur astronomer, misunderstood the 1937 Maine Farmers' Almanac that described this system, and put the now-popular definition in a 1946 edition of Sky & Telescope.  Either way, a blue moon occurs every two to three years, so now you know exactly how infrequent "once in a blue moon" is.  After the full moon of this week, the third full moon of this summer, the next blue moon will be in May of 2016.  Hey, we don't just talk about culture or language here, you know.