Aug. 30, 2012  -- Once in a blue moon, we get a night like tonight. 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, tonight's full moon has been agreed upon, somewhere, as a blue moon, and if you go out after dark, we hope you will enjoy its light.

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 "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. That's become the most popular modern use of the term. But as the folklore scholar Philip Hiscock discovered at Canada's Memorial University of Newfoundland, there's no easily-found reference to the two-moons-in-a-month definition from before then.

"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 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.

On Friday the moon is fullest at 9:58 a.m. EDT, so you can enjoy it on Thursday or Friday night from the U.S.

There is one rarity to go with Friday's full moon. NASA announced Thursday that Neil Armstrong's private memorial would take place Friday in Cincinnati; a public ceremony, yet to be scheduled, is being planned for Washington in the next two weeks. 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."