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A Blue Moon This Way Comes

August 28, 2012

“Once in a blue moon” is an expression you  may have heard before. It is now taken to mean “rarely”, deriving from an 1821 writing, Real Life in London by Pierce Egan, where he writes:

“How’s Harry and Ben? – haven’t seen you this blue moon.”

A celestial “blue moon” is now popularly known as the second full moon in a calendar month, and it in fact doesn’t happen that rarely; about once every 2.5 years. This Friday (August 31st) will be one such occurrence. So be sure to look up! The next one won’t be until July 2015.

Full Moon

Full Moon. (Credit: Lick Observatory)

Will the Moon look blue? Not likely. A blue-colored moon is actually quite rare, and is the result of unusual sky conditions where particles of dust or smoke can make the Moon appear blue. The color we will most likely see will be gorgeous pearly gray (as shown above).

The now-popularized celestial definition of a “blue moon” stems from a 1946 article titled, “Once in a Blue Moon” by James Hugh Pruett in the magazine Sky and Telescope, in which he reportedly misinterpreted the description in the 1937 Maine Farmer’s Almanac. The definition of the blue moon as the second full moon in a month was subsequently featured in children’s publications and the game Trivial Pursuit, thus catching on in popular culture.

The average lunar cycle is once every 29.5 days, and so a blue moon can in fact occur in all months except February, which has only 28 days. The year 1999 showcased two blue moons (January and March), and the next year of double blue moons will be 2018; this year, use the phrase “twice in a blue moon” with conviction, as this event happens about once every 19 years!

That’s impossible! The phrase, “once in a blue moon” used to imply something impossible, rather than rare. This earlier definition dates back to medieval England; in 1528, William Barlow, the Bishop of Chichester, wrote in the Treatyse of the Buryall of the Masse:

Yf they saye the mone is belewe,
We must beleve that it is true.”

In 1529, John Frith wrote in his essay,  A pistle to the christen reader:

“They wold make men beleue… that ye mone is made of grene chese.”

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One Comment leave one →
  1. August 29, 2012 3:00 pm

    Reblogged this on NC Museum of Natural Sciences Blogs and commented:

    by Rachel L. Smith

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