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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?

By Joe RaoSpace.com / November 21, 2010

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."

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"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In fact, the current "two- full moons in one month" rule has superseded an older rule that would allow us to call Sunday's moon "blue."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).

On the almanac page for August 1937, the calendrical meaning for the term "blue moon" was given.

That explanation said that the moon "... usually comes full twelve times in a year, three times for each season."

Occasionally, however, there will come a year when there are 13 full moons during a year, not the usual 12. The almanac explanation continued:

"This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number."

And with that extra full moon, it also meant that one of the four seasons would contain four full moons instead of the usual three.

"There are seven Blue Moons in a Lunar Cycle of nineteen years," continued the almanac, ending on the comment that, "In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression 'Once in a Blue Moon.'"

An unfortunate oversight

But while LaFleur quoted the almanac's account, he made one very important omission: He never specified the date for this particular blue moon.

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