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Tonight’s the Blue Moon. According to modern folklore, a blue moon is the second full in a month. Blue moons are uncommon, occuring about once every 2 1/2 years. Bob King | Forum Communication Photo Illustration

Misinterpreting the moon: Blue moon definition came from a mistake

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By Bob King,

Forum Communications

Tonight is the blue moon — which isn’t blue, of course, unless you happen to be near a volcano.

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It was actually a mistake by an amateur astronomer in Sky & Telescope magazine in 1946 that led to our current definition of “blue moon” as the unusual second full moon in a month.

The term “blue moon” goes back hundreds of years, but it had a different meaning then of “impossible” or “absurd.” The term later morphed into a reference for something uncommon or that rarely occurred.

There are normally three full moons in each of the four seasons, for a total of 12 per year. In the early 1930s, the Maine Farmers’ Almanac (unrelated to the Old Farmer’s) named the third full moon in a season that had an extra fourth full moon a blue moon. It’s unclear where the “blue” part came from, but it’s possible it refers to that earlier meaning — an event that rarely happens.

Then, in the March 1946 issue of Sky & Telescope magazine, American amateur astronomer James Hugh Pruett wrote an article titled “Once in a Blue Moon.” He either misread the Maine almanac’s definition or interpreted the meaning of “blue moon” differently, calling it the second full moon in a month. Sky and Telescope later adopted Pruett’s definition.

The blue moon snowballed into popular culture when Deborah Byrd, host of National Public Radio’s Star Date program, used Pruett’s definition during a broadcast on Jan. 31, 1980. Word got around, and now you know the rest of the story.

Fascinating, isn’t it, that the current blue moon definition is based on one person’s (mis)interpretation of an earlier definition? Makes you wonder what other accepted “facts” are based on odd turns of events and errors in interpretation.

I personally like the modern definition. It still catches the gist of the old almanac sense in a way that’s easy to remember. The next blue moon for North America will be in July 2015. Even better, there will be two blue moons in 2018 — one in January and one in March, with no full moon at all in February. The last time that happened was in 1999.

Now, if you’re near a volcano …

Volcanic ash and forest fires can turn the moon blue. The secret? It’s the ash. If all the ash particles are about 1 micron in size (the period at the end of this sentence is 600 microns across), they efficiently scatter away all the warm colors in moonlight, leaving a pale blue orb.

I’ve never seen the phenomenon, but much of the planet saw blue moons for months after the eruption of the Indonesian volcano Krakatoa in 1883. Ditto for Mount St. Helens in 1980 and Mount Pinatubo in 1991. If you live in western U.S. where forest fires have been rife this summer, perhaps you’ve seen one too many blue moons.

Most of us will never get to see a real blue moon, but the calendar version will shine in Pisces tonight. We normally get one full moon a month, but every 2½ years there’s room for another to squeeze in.

That’s because the time between full moons is 29.5 days, while most months are 30 or 31 days. Because the first full moon of August was on the 1st, there’s enough time left in the month to make room for a second one on the 31st. If the moon were always full at the beginning of each 30 or 31-day month, we’d get 11 blue moons a year. Now, wouldn’t that be nice. That doesn’t happen because the moon’s not in sync with the calendar — it marches to its own 29.5-day rhythm.

Full moons have acquired a variety of names handed down from past generations. We get our moon names from the various American Indian tribes as well as the early colonists. Two common monikers for the August full moon are the sturgeon and red moons. The first refers to August being a great time to catch sturgeon and the second to the color of the moon when it rises during the hazy summer months. According to the Old Farmer’s Almanac, the first full moon of August was the sturgeon and the second, the red moon. It’s a fun coincidence that this month’s red moon is also blue.

I’m looking forward to a fine moonlit walk tonight, and I wish you the same.

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BOB KING is photo editor at the News Tribune. His Area Voices blog, Astro Bob, is at astrobob.areavoices.com.

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