Blue moons

Have you ever wondered why we consider “once in a blue moon” to be the epitome of rare occurrence?

This is a good time to ask, because July 2004 is one of those rare months when there are two full moons: one on July 2, and a second on July 31. According to folklore, the second of these full moons is a “blue moon.”

That doesn’t really help us with the origin of the phrase, alas; it turns out this particularly bit of folklore only dates back to the dark ages–the dark ages known as the 1980s.

Philip Hiscock, archivist at the Folklore and Language Archive at Memorial University of Newfoundland, has researched the term “blue moon” extensively. He says the phrase first appeared in print in 1528 as an obvious absurdity, something that can’t be true. After a couple of centuries, it was also used to mean “never,” as in, “I’ll marry you when the moon is blue.”

In the 20th century, songwriters used “blue moon” as a symbol of sadness or loneliness. And there’s even a cocktail called a blue moon.

But Hiscock says he first heard the second full moon within a single calendar month called a blue moon in 1988. That year May had two full moons, prompting a rash of newspaper and radio stories that all referred to “folklore” calling the second full moon of a month a blue moon. As a folklorist, Hiscock started looking for the source of this claim–and couldn’t find it.

But couple of years later, his brother reminded him that the 1986 Genus II edition of Trivial Pursuit included that meaning as the answer to a question. The publisher told Hiscock the source was a 1985 book, The Kids’ World Almanac of Records and Facts. The book’s authors, Hiscock believes, got the notion from Deborah Byrd, host of the National Public Radio program Star Date. In late January 1980 she read on-air an article from a 1946 issue of Sky and Telescope magazine that contained that explanation of blue moon.

In 1999 Sky and Telescope set out to discover where the 1946 author had gotten his information, and determined that he had misunderstood the explanation given in a 1943 article. There really was a usage of “blue moon” tied to the calendar–but it was far more complicated than simply being the second full moon in a month. It seems some old almanacs used a method of naming each full moon that was tied to the ecclesiastical calendar. Here’s how Sky and Telescope described it: “Seasonal Moon names are assigned near the spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent. The beginning of summer, fall and winter are determined by the dynamical mean Sun. When a season contains four full Moons, the third is called a Blue Moon.” (The third, not the fourth, gets that name, because the last moon of the season may already have a name, such as the Moon Before Yule.)

Even though the new explanation is essentially bogus, it already seems so entrenched that in 100 years it probably will be considered the true “traditional” explanation.

Of course, there is an even rarer kind of blue moon, and that’s one that actually looks blue. In 1883, following the eruption of Krakatoa, the moon appeared blue all over the world. (There were also lavender suns and spectacular sunsets.) The explosion lifted enormous amounts of ash into the atmosphere. Some of the ash clouds were filled with particles just under a micron (one millionth of a metre) in diameter–which happens to be the perfect size to scatter red light, while allowing other wavelengths to pass. Presto, blue moons (and sometimes green ones).

Blue moons carried on for years after Krakatoa. Other volcanic eruptions over the years have also produced blue moons, including that of Mt. St. Helens. So have some forest fires, such as the ones that erupted in Alberta in September of 1950, when muskeg fires that had smoldered for years suddenly burst to life. Blue moons were reported all over eastern North America and even in the British Isles.

Although forest fires aren’t rare, blue moons still are; most forest-fires produce smoke particles that are smaller than a micron, which scatter blue light.

Which means, since this is the height of forest fire season, July’s blue moon is more likely to be red.

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