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THE SAGA OF THE BOGUS BLUE MOON

Is there really such as thing as a blue Moon?

by Terence Dickinson
SkyNews Editor

If your calendar shows the phases of the Moon, check either May or June this year for two full Moons in a single month. Depending on whether the calendar maker used moon phase data for the Eastern Time zone (eastern North America) or for Universal Time (Greenwich Observatory, England), the second full Moon will fall on either May 31 or June 30. Whichever date is selected, that second full Moon in the month is said to be the blue Moon.


By this definition, blue Moons occur once every three years or so. However, this two-full-moons-in-a-month definition is pure astro-baloney. It's based on neither astronomical fact or historical tradition. It's nothing more than an urban legend, that emerged in the 1980s as a result of a series of misinterpretations of facts and--believe it or not--a question in the board game Trivial Pursuit!

Yet, as likely as not, you will hear or read about this upcoming bogus blue Moon on news broadcasts and in newspapers. 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. There is not a thing special about it compared to any other full Moon. The legend claims that's where the phrase "once in a blue Moon" comes from. But it isn't.

According to Memorial University (St. John's, Newfoundland) folklore historian Philip Hiscock, the term "blue Moon"--meaning once in a lifetime, or once in 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 became so widespread so fast. Before the mid-1980s, if you asked astronomers what is meant by a blue Moon, they would explain that very rarely, certain types of dust and smoke from volcanoes or forest fires can absorb red light and tint the Moon blue--the Sun too. The phenomenon is so rare, few people have ever seen it.

The last widespread blue Moons and blue Suns seen in this part of the world were back in 1950, caused by smoke blown all the way to Europe from huge forest fires in Alberta. A similar effect was caused by emissions from the 1980 Mount St. Helens volcanic eruption in Washington state. The rarity of these true blue Moons matches the long-standing historical definition of a blue Moon, meaning once in a very long time.

The bogus two-full-Moons-in-a-month definition was eventually traced to a mistake published in Sky & Telescope magazine in the 1940s. Apparently, the error was not noticed for more than three decades until it was gathered by a researcher for Star Date, an internationally syndicated radio program, who was sifting through old magazines and took the error as fact and used it on the show. From there, the author of a children's book published in 1985 used the definition as a "science fact."

Because the children’s book was not a best-seller, the error probably would have returned to obscurity at that point. But the book just happened to be in the library used by a researcher for the immensely popular board game Trivial Pursuit, Genus II edition. This gave the bogus definition widespread 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 it was more than a decade after the Trivial Pursuit appearance and too late to undo the mess.

As a cross-check to ensure that the erroneous two-full-Moons-in-a-month definition might indeed be century-old folklore, every edition of The Old Farmer's Almanac has been thoroughly examined and nowhere in any edition does it contain any reference to the blue Moon as the second full Moon in a month.

Now that we know the second-full-moon-in-a-month definition has neither historical or scientific validity, there’s the problem of what to do to reverse it. My guess is that it’s too late. The erroneous 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 blue Moon definition in the American Heritage Dictionary (Houghton Mifflin Co., 4th edition, 2000), and as the second definition in my favourite dictionary, Merriam-Webster’s Collegiate (Merriam-Webster, Inc., 11th edition, 2003).

But 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!


CELESTIAL OBSERVING TIPS, May 2007

by Todd Carlson
Assistant Editor

With summer approaching and daylight extending further into the evening, sometimes the late spring sky over Canada is not fully dark until after 11 p.m. But brighter objects, especially the Moon and planets, are visible perfectly well in deep twilight, without waiting for total darkness.

Here are the most prominent naked eye observing highlights for May.

1. After 11 p.m. on the night of May 4/5, the Moon is beside Jupiter and easy to identify. As the largest planet in our solar system, Jupiter shines with a brilliant creamy hue, brighter and steadier than any stars. Later in the month, Jupiter is seen rising low in the southeast during dusk, and higher in the south later on in the night.

2. The planet Venus is the brightest celestial object after the Moon. Look for it standing above the western horizon during May evenings. Enshrouded in clouds, Venus reflects 65 percent of the light it receives from the Sun, resulting in a brilliant diamond-like appearance in our twilight sky.

3. On May 19 and 20, the thin crescent Moon will join Venus on the western horizon at dusk. Look for Earthshine, sunlight reflecting off of the Earth and illuminating the night side the Moon, as the sky darkens. Binoculars will enhance the view.

4. Mercury can be observed during the last two weeks of May below and to the right of Venus about 75 minutes after sunset. Use binoculars to aid in sighting Mercury, which looks like a star (brighter than other stars in the vicinity, but much dimmer than brilliant Venus).