Blue Moon: Rare Second Full Moon of Month Visible Tonight

Aug. 31, 2012 — -- 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, and if you go out after dark, we hope you will enjoy its light.

The moon was actually at its fullest at 9:58 a.m. EDT today, 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.

"Blue Moon
You saw me standing alone
Without a dream in my heart
Without a love of my own...."

--"Blue Moon," by Richard Rodgers and Lorenz Hart

Perhaps we are best off with expressions of sadness or loneliness. There are certainly enough songs about it, though there's also Nick Drake's more upbeat "Pink Moon," which refers to the full moon that comes in April.

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.

Today's full moon does coincide with today's private memorial service for Neil Armstrong in Cincinnati. When Armstrong's family announced his death on Saturday, they made a request: "Honor his example of service, accomplishment and modesty, and the next time you walk outside on a clear night and see the moon smiling down at you, think of Neil Armstrong and give him a wink."