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Is this a true blue moon?

Sunday night's full moon qualifies as a 'blue moon,' but it's not the second full moon in November. So, why is it a blue moon?

A full moon rises behind the skyline of midtown Manhattan in New York, Sunday Nov, 21,

Gary Hershorn/Reuters

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The full moon of November arrives Sunday night and will bring with it a cosmic addition: It will also be a so-called "blue moon."

"But wait a minute," you might ask. "Isn't a 'blue moon' defined as the second full moon that occurs during a calendar month? Tonight's full moon falls on Nov. 21 and it will be the only full moon in November 2010. So how can it be a 'blue' moon?"

Indeed, November's full moon is blue moon – but only if we follow a rule that's now somewhat obscure.

In fact, the current "two- full moons in one month" rule has superseded an older rule that would allow us to call Sunday's moon "blue." To be clear, the moon does not actually appear a blue color during a blue moon, it has to do with lunar mechanics.

Confused yet?

Well, as the late Paul Harvey used to say — here now, is the rest of the story:

IN PICTURES: Full Moons

The blue moon rule

Back in the July 1943 issue of Sky & Telescope magazine, in a question and answer column written by Lawrence J. Lafleur, there was a reference made to the term "blue moon." [Gallery - Full Moon Fever]

Lafleur cited the unusual term from a copy of the 1937 edition of the now-defunct Maine Farmers' Almanac (NOT to be confused with The Farmers' Almanac of Lewiston, Maine, which is still in business).

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