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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?

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Today, Pruett's misinterpreted "two full moons in a month rule" is recognized worldwide. Indeed, Sky & Telescope turned a literary lemon into lemonade, proclaiming later that – however unintentional – it changed pop culture and the English language in unexpected ways.

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Meanwhile, the original Maine Farmers' Almanac rule had been all but forgotten.

Playing by the (old) rules

Now, let's come back to this Sunday's full moon.

Under the old Almanac rule, this would technically be a blue moon. In the autumn season of 2010, there are four full moons:

  • Sept. 23
  • Oct. 22
  • Nov. 21
  • Dec. 21

"But wait," you might say. "Dec. 21 is the first day of winter."

And you would be correct, but only if you live north of the equator in the Northern Hemisphere. South of the equator it's the first day of summer.

In 2010, the solstice comes at 6:38 p.m. EST (2338 UT).

But the moon turns full at 3:13 a.m. EST (0813 UT). That's 15 hours and 25 minutes before the solstice occurs. So the Dec. 21 full moon occurs during the waning hours of fall and qualifies as the fourth full moon of the season.

This means that under the original Maine Almanac rule – the one promoted by Lafleur and later misinterpreted by Pruett – the third full moon of the 2010 fall season on Nov. 21 would be a blue moon.

Choose your blue moon

So what Blue Moon definition tickles your fancy? Is it the second full moon in a calendar month, or (as is the case on Sunday) the third full moon in a season with four?

Maybe it's both. The final decision is solely up to you.

Sunday's full moon will look no different than any other full moon. But the moon can change color in certain conditions.

After forest fires or volcanic eruptions, the moon can appear to take on a bluish or even lavender hue. Soot and ash particles, deposited high in the Earth's atmosphere, can sometimes make the moon appear bluish.

In the aftermath of the massive eruption of Mount Pinatubo in the Philippines in June 1991, there were reports of blue moons (and even blue suns) worldwide.

We could even call the next full moon (on Dec. 21) a "red moon," but for a different reason: On that day there will be a total eclipse of the moon and, for a short while, the moon will actually glow with a ruddy reddish hue.

More on that special event in the days to come here at SPACE.com, so stay tuned!

Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y.

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