Once in a Blue Moon

Posted by Rich Talcott
on Thursday, May 31, 2007

Head outside this evening, and you can't help but notice a Full Moon rising in the southeast. Nothing unusual about that — Full Moons occur, on average, every 29.5306 days. But if you look at your calendar for May, you'll see the previous Full Moon fell May 2, making tonight's the second Full Moon of the month.

Can everyone say "Blue Moon"?

Modern folklore holds that the second Full Moon in a given calendar month should be called a Blue Moon. This tradition began in the not-so-distant past: 1946, to be precise. Surprisingly, this modern definition resulted from a misinterpretation of the previous, and more arcane, rule for defining a Blue Moon.

The term "Blue Moon" has appeared in literature since at least the 16th century. And the phrase, "Once in a Blue Moon," has been around seemingly forever, as well. This phrase has little to do with the Moon, however, and typically refers to any rare event. Seeing the Moon turn the color blue certainly falls into that category — but it's not unheard of. Smoke and dust in our atmosphere can turn the Moon bluish. Both forest fires and volcanic eruptions have been known to color the Moon blue, most notably after the massive eruptions of Krakatoa in Indonesia in 1833 and Mount Pinatubo in the Philippines in 1991.

The Blue Moon apparently got tied to an extra Full Moon in the pages of the Maine Farmers' Almanac. As I mentioned earlier, Full Moons recur every 29.5306 days. It doesn't take a genius to figure out that, occasionally, two Full Moons will occur in a given month, or 13 Full Moons in a given year.

The Maine Farmers' Almanac used a slightly different formula. Because folklore traditionally ties the name of a Full Moon to the season (think of the Harvest Moon near the autumnal equinox, and the Moon before Yule and the Moon after Yule around Christmas), a Moon with no name would occur if four Full Moons happened in a given season. And it was always the third of those Full Moons that would be nameless. The Maine Farmers' Almanac filled that void with the term "Blue Moon."

So where does our current Blue Moon definition come from? For that, we can thank our friends at Sky & Telescope. Amateur astronomer James Hugh Pruett wrote an article "Once in a Blue Moon," in the March 1946 issue. In it, he erroneously interpreted the Maine Farmers' Almanac's Blue Moon rule to be the second Full Moon in a calendar month. (Unfortunately, he didn't have a copy of the almanac at hand.) Sky & Tel subsequently adopted Pruett's new definition. But the Blue Moon rule really took off when Deborah Byrd, a former Astronomy columnist, used this definition on her popular radio show, StarDate, on the January 31, 1980, broadcast.

So there you have it. Tonight's Blue Moon will shine on, blissfully unaware of how we humans market its image. Or should I say, how we in the Americas market it. For, you see, Full Moon arrives precisely at 9:04 p.m. EDT (6:04 P.M. PDT) tonight. That translates to the morning of June 1 in Europe, Africa, and Asia. So if you live in the Eastern Hemisphere, there's no Blue Moon for you tonight. You'll have to wait until June 30 to see this colorless Moon with a colorful history.

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