Blue MoonBlue Moon

It happens only once in a blue moon -- and scientists say a blue moon is exactly what we'll see in the skies this New Year's Eve.

Don't expect an azure glow over our lunar satellite, however. The term "blue moon" simply refers to the second full moon in a calendar month, something that hasn't happened on a New Year's Eve for nearly 20 years, NASA says.

"December 1990 ended with a blue moon, and many New Year's Eve parties were themed by the event," said Professor Philip Hiscock of the department of folklore at Memorial University of Newfoundland, in Canada. "It was a lot of fun."

Most months have just one full moon, because the 29.5-day cycle of the moon matches up pretty well with the length of calendar months. Occasionally, there will be two full moons in a month, something that happens about every 2½ years, NASA says.

But a blue moon on December 31 is rare.

Elvis Presley crooned about it when he sang the old Rodgers and Hart song "Blue Moon," in which he stood alone without a dream in his heart or a love of his own.

He struck a more hopeful tone in another tune, singing about his love returning to his arms "When My Blue Moon Turns to Gold Again." He also covered Bill Monroe's bluegrass classic, "Blue Moon of Kentucky." [CNN]

A selection of blue moon videos & songs below....

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Blue Moon in Grease

blue moon- sha na na- flip wilson

The Marcels - Blue Moon

Bill Monroe - Blue Moon Of Kentucky

Blue Moon-Elvis Presley

Ella Fitzgerald - Blue Moon