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

Second Full Moon of the Month Will Ring in 2010
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.