(CNN) -- 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.
"The phrase 'blue moon' has been around a long time, well over 400 years, but during that time its meaning has shifted," he said. "I have counted six different meanings which have been carried by the term, and at least four of them are still current today. That makes discussion of the term a little complicated."
Full moons used to have 12 names, one for each month, such as "harvest moon," NASA said. The term "blue moon" referred to the 13th full moon in a year.
A blue moon on December 31 is rare.
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