There's something special about tonight's full moon. If you look up at the night sky, you'll see a rare blue moon. No, it won't actually appear blue, nor is it the second full moon of the month. So what makes it a blue moon?

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Tonight's moon is what's now called a "seasonal blue moon." It's the third full moon in a season that has four, rather than the usual three, according to That's actually the original definition of the term, which is thought to come from an Old English word meaning betrayer, because a third full moon usually means the end of a season. Seasonal full moons are rare. Miss this one, and you won't get another chance until May 21, 2016.

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The other definition of blue moon came about relatively recently. The phrase was first used to describe the second full moon in a calendar month by an amateur astronomer writing for Sky and Telescope magazine in 1946. It was a mistake, but after the phrase was repeated by a syndicated radio program in 1980, the definition stuck. The magazine wrote in 2006 that the modern definition is "like a genie that can't be forced back into its bottle," The Weather Channel reported.

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Of course, there is a third definition, where the moon actually does appear blue in color. To see that, you'd have to live near an erupting volcano. NASA's Tony Phillips said that large particles of ash from an exploding volcano can scatter red light and allow other colors to pass. That would cause white moonbeams to appear blue or sometimes green.

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Weather permitting, tonight's blue moon will be a beautiful sight. It may not look very different from any other full moon, but then this kind of thing happens only once in a blue moon.

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MSN Photos: What a moon!