Once in a Blue Moon on New Year's Eve -- or Not? Full moon to shine for New Year's Eve revelers tonight. What makes it blue?

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 Moon Does Appear Blue on Rare Occasions

"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.

Moon Myths Have Existed for Centuries

Those references rely on the romantic and emotional qualities that people have associated with the moon for centuries, Hiscock said.

The phrase lunatic comes from the idea that the human psyche is influenced by the moon and, despite evidence to the contrary, some people persist in linking full moons to bizarre behavior.

Lunar folklore has a special place for parties too, Hiscock said, as some believe that the moon contributes to riotous revelry.

"Blue moon parties are better than other parties -- this is part of modern folklore," Hiscock said, adding that when a blue moon falls on New Year's Eve, superstitions run especially rampant.

Sky & Telescope Magazine Helps Popularize Modern Definition Through Error

In the 19th century, the phrase started appearing in New England almanacs, he said, but with a different definition from the one accepted now. Almanac authors defined "blue moon" as the third full moon in a season with four full moons, not the typical three.

When astronomy magazine Sky & Telescope published a misinterpretation of the Maine Farmer Almanac in 1946, the modern definition was born.

The magazine acknowledged the error in 1999 (with the help of Hiscock and astronomer Donald W. Olson) but, by that time, the new definition had already won out over the old.

In a light-hearted press release issued by the magazine this week, Kelly Beatty, Sky & Telescope's senior contributing editor, said, "In modern usage, the second full moon in a month has come to be called a 'Blue Moon.' But it's not! This colorful term is actually a calendrical goof that worked its way into the pages of Sky & Telescope back in March 1946, and it spread to the world from there."

New 'Blue Moon' Definition Persists

Sticklers might point out that according to the former definition, there is no blue moon to be had this month. The next one won't happen until Nov. 2010.

But those who subscribe to the old meaning are in the minority.

"Usage has just sort of driven it to be the way it is now," said Alan MacRobert, senior editor of Sky & Telescope. Language shifts and evolves and, especially when it comes to the moon, "you can't beat back the tide," he quipped.