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Blue Moon: Rare Second Full Moon of Month Visible Friday Night

Stockbyte/Thinkstock(NEW YORK) -- Once in a blue moon, we get a night like this one. If the weather is clear, you will get to see the second full moon of the month -- or perhaps the fourth full moon of a three-month season -- or maybe an early "betrayer moon" (belewe in Old English) -- or any of half a dozen other definitions that have come up over the last 400 years. At any rate, the full moon of Aug. 31 has been agreed upon, somewhere, as a blue moon.

The moon was actually at its fullest at 9:58 a.m. EDT Friday, which means it was below the horizon for most of the Western Hemisphere. If you saw the moon last night, you probably thought of it as full, and when it rises again tonight, it will still be plenty bright.

If it has even a hint of a blue tinge, please let us know immediately. Blue moons have very little to do with the color blue (although the moon can take on a blue cast if there is a lot of volcanic ash in the atmosphere). The phrase "once in a blue moon" has come to mean something that doesn't happen very often, and it's been a part of our folklore since -- well, nobody's quite sure.

Sky & Telescope, a magazine for astronomy enthusiasts, ran an article in March 1946 that defined a blue moon as the second full moon in a month -- but readily admits today that it made a mistake, oversimplifying the four-full-moons-in-a-season definition. The mistake caught on, even though the folklore scholar Philip Hiscock of Canada's Memorial University of Newfoundland said he could find no references to the two-moons-in-a-month definition from before then. (For the record, the moon is full once every 29 1/2 days.)

"The term has been around a long time," said Hiscock. "The earliest uses of that term really meant something like 'never...an impossibility.'"

And even that's not quite the case. August 2012 has had two full moons -- but so did December 2009, and so will July 2015. Months with two full moons -- the reason we're all hearing the term now -- occur, on average, about once every 2.7 years.

Copyright 2012 ABC News Radio

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