Once in a Blue Moon on New Year's Eve -- or Not?

Dec. 31, 2009— -- New Year's Eve is never dull. But nights like tonight happen just once in a blue moon.

Not only will a full moon light up the festive evening, but a so-called blue moon will help ring in 2010 for revelers in the Western hemisphere.

So while you're out carousing with friends or family, take a break from your champagne and noisemakers to look up at the night sky.

You may see the "blue moon," but you won't see the moon awash in the color blue.

The term "blue moon" generally refers to the second full moon in a month and occurs just once every 2.7 years. The last blue moon to shine on New Year's Eve was 19 years ago, in 1990.

But while that is how most people define the phrase now, folklorists say it wasn't always the case.

"The term has been around a long time," said Philip Hiscock, a professor of folklore at Canada's Memorial University of Newfoundland. "The earliest uses of that term really meant something like never ... an impossibility."

The first indications of the phrase, he said, date back about 600 years and might have described absurdities, as in, "He'd have you believe the moon was blue."

But later, in the 19th century, as people noticed that on occasion the moon actually did look a little blue in color, say after a volcanic eruption or forest fire, Hiscock said "blue moon" morphed into meaning something rare and fairly random.

"That today is still the major usage of the term," he said. "The moon does actually appear blue from time to time, but it is actually quite rare. It worked well with the phrase."

After Indonesia's Mount Krakatoa erupted in 1883, people looking up at the night sky might have seen a moon that appeared blue as light shone through the volcanic ash. Wildfires, like those that frequently plague the western United States, also could appear to give the moon a blue tint.

The less astronomically-inclined might note that the phrase has also spread to music, commerce and even cocktails.

Crooner Elvis Presley recorded both the melancholy ballad "Blue Moon" and the bluegrass hit "Blue Moon of Kentucky." And come happy hour, your barkeep might offer you a pint of "Blue Moon" beer or a flirty "Blue Moon" cocktail.