According to folklore, the second full Moon in a month, such as the one coming up on New Year’s Eve, is a “blue moon.” Trouble is, nobody had ever heard of this definition before 1980.

By Terence Dickinson

As New Year’s Eve approaches, you will hear on newscasts and read in newspapers and on the Internet that the full Moon occurring on the evening of December 31 is a blue moon, as in the expression “once in a blue moon.” The reports will go on to define a blue moon as the second of two full Moons in the same month.

By this definition, a blue moon occurs once every three years or so. However, this is pure astro-baloney. It’s based on neither astronomical fact nor historical tradition. It’s nothing more than a modern urban legend that emerged in the 1980s as a result of a series of misinterpretations of facts and, believe it or not, the answer to a question in the board game Trivial Pursuit!

Yet there’s not a shred of scientific or historical fact in the definition. The Moon doesn’t turn blue, nor is it larger or smaller than usual. In fact, there is nothing special about it compared with any other full Moon. The legend claims that this is the origin of the phrase “once in a blue moon.” But it isn’t.

Yes, it is relatively rare to have two full Moons in a single month, but the reason is simple: The average time between full Moons—one orbit of the Moon around Earth—is 29.5 days. Since the average length of a month is slightly longer, two full Moons occasionally squeeze in. But how did this become the definition of a blue moon?

According to Memorial University folklore historian Philip Hiscock, in St. John’s, Newfoundland, the term blue moon, meaning once in a lifetime or a long while, has been around for more than 400 years. However, Hiscock says its urban-legend meaning (the second of two full Moons in a month) has become widespread only since the late 1980s. Both folklore historians like Hiscock and astronomy writers like me wondered where the “second full Moon in a month” definition came from so recently and how it spread so fast.

Before the 1980s, if you had asked an astronomer what is meant by a blue moon, he or she would have explained that very rarely, certain types of dust and smoke from volcanoes or forest fires can absorb red light, thereby tinting the Moon blue—the Sun too. The phenomenon is so rare, few people have ever seen it.

The most prevalent sightings of blue moons and blue Suns were in 1950 and were the result of smoke blown across Canada and all the way to Europe from massive forest fires in Alberta. A similar effect was caused by emissions from the 1980 Mount Saint Helens volcanic eruption in Washington State. The rarity of these true blue moons matches the long-standing historical definition of a blue moon: once in a very long time.

The bogus “two full Moons in a month” definition was eventually traced to a factual error published inSky & Telescope magazine in the 1940s, an error that went unnoticed until it was collected by a scriptwriter for “Star Date,” an internationally syndicated radio program. The writer was sifting through old astronomy magazines, took the error as fact and used it on the show in the early 1980s. From there, the author of a children’s book published in 1985 used the radio-show “fact” in the book.

If that were the extent of the exposure, the error almost certainly would have remained in obscurity. The pivotal event was that the children’s book just happened to be in the library used by a researcher compiling information for the immensely popular 1980s board game Trivial Pursuit, Genus II edition. This gave the bogus blue-moon explanation enormous circulation and legitimacy—all to the mystification of astronomers, who had never heard of it.

Sky & Telescope admitted to its “blue moon blooper” in its May 1999 issue, but by then, more than a decade had passed since the Trivial Pursuit appearance, and it was too late to undo the mess.
As a cross-check to ensure that the erroneous “two full Moons in a month” idea was not, in fact, century-old folklore, every edition of The Old Farmer’s Almanac has been thoroughly examined, and nowhere in any of them is there a reference to the blue moon as the second full Moon in a month.

Now that we know this definition has neither historical nor scientific validity, can anything be done to reverse it? Regrettably, it’s now way too late. The mistaken concept of a blue moon as the second full Moon in a month now appears to be part of the English language. It is given as the first meaning for “blue moon” in the American Heritage Dictionary (Houghton Mifflin Co., 4th edition, 2000) and as the second entry in my favourite dictionary, Merriam-Webster’s Collegiate (Merriam-Webster, Inc., 11th edition, 2005). The final blow came in June 2007, the last month with a double full Moon, when the U.S. Naval Observatory, the official time and calendar authority for civil, military and scientific application, included the “two full Moons in a month” definition of a blue moon in a news release.

I cringe every time I hear the blue moon referred to as an astronomical event. It isn’t. There’s nothing astronomical about it. It’s mentioned here and elsewhere in this issue of SkyNews to explain what’s really going on.

And I’m not giving up. Every time a so-called blue moon comes up on the calendar, I’ll be fighting for truth, justice and the astronomically correct way!

Comments on this article or anything else in SkyNews magazine or on our website (www.skynews.ca) can be directed to editor Terence Dickinson at skynewseditor@reztel.net.