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What is a 'blue moon' anyway?

August 31 will arrive with a blue moon, and it won't happen again for three years. What exactly is a blue moon? And where did it get its name?

By Joe RaoSpace.com / July 30, 2012

This image of the full moon was taken by Alamelu Sundaramoorthy from Portland, Ore. on July 3, 2012.

Alamelu Sundaramoorthy

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The month of August brings us not one, but two full moons. The first will kick off the month on Wednesday (Aug.1), and will be followed by a second on Aug. 31.

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Some almanacs and calendars assert that when two full moons occur within a calendar month, the second full moon is called a "blue moon."

The full moon that night will likely 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. Smoke from widespread forest fire activity in western Canada created a blue moon across eastern North America in late September 1950. 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. [Infographic: Blue Moons Explained]

Origin of the term

The phrase "once in a blue moon" was first noted in 1824 and refers to occurrences that are uncommon, though not truly rare. Yet, to have two full moons in the same month is not as uncommon as one might think.  In fact, it occurs, on average, about every 2.66 years.  And in the year 1999, it occurred twice in a span of just three months. 

For the longest time no one seemed to have a clue as to where the "blue moon rule" originated.  Many years ago in the pages of Natural History magazine, I speculated that the rule might have evolved out of the fact that the word "belewe" came from the Old English, meaning, "to betray."  "Perhaps," I suggested, "the second full Moon is 'belewe' because it betrays the usual perception of one full moon per month." 

But as innovative as my explanation was, it turned out to be completely wrong.

More mistakes

It was not until that "double blue moon year" of 1999 that the origin of the calendrical term "blue moon" was at long last discovered.  It was during the time frame from 1932 through 1957 that the Maine Farmers' Almanac suggested that if one of the four seasons (winter, spring, summer or fall) contained four full moons instead of the usual three, that the third full moon should be called a blue moon. 

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