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Here we go again. Every 2.7 years, on average, there are two full moons in a month. This December is that month. The first full moon occurred on Dec. 2, the second full moon is Monday. We're being told this second one is the blue moon, as in the saying "once in a blue moon." I say bunk.

For centuries this coincidence of calendar and celestial clockwork was ignored by astronomers and everyone else. Suddenly in the last few years, newscasts and weather reports announce that two full moons in a month is the blue moon.

I believe this is a modern myth. I can't find a shred of historical evidence that the blue moon saying, which has been around for generations, refers to two full moons in a month. My research suggests that the two full moons in a month idea must have been the invention of some modern writer and the story has been repeated so often in the last decade it is now taken as fact.

But once in a blue moon must mean something.

I prefer the explanation given by Canadian astronomer Helen Sawyer Hogg, who says that blue moons really do occur, but rarely. They are caused by the dust from major volcanic eruptions and by smoke from forest fires. This type of dust and smoke is often made up of particles slightly larger than a wavelength of light. The particles act like a filter because they scatter red light but do not impede blue light, which has a smaller wavelength.

The result is that only blue light reaches the eye from the moon (or sun). The effect can be quite pronounced when these bodies are near the horizon.

"There seems little doubt that the well-known saying is derived from such circumstances," Dr. Hogg wrote in 1975. "From a study I made of the literature of the past several centuries, it appears that volcanic eruptions and other effects will render a blue moon visible probably once in seven or eight decades" -- roughly once in a lifetime.

The last widely observed blue moons were seen in late September 1950, following a series of huge forest fires in northern Alberta. The region of blue moon visibility stretched from Canada to Florida and east to northern Europe.

Astronomy historian Patrick Moore saw it from England. On Sept. 26, 1950, from East Grinstead, Sussex, he made this entry in his observing log: "The moon shone . . . with a lovely shimmering blueness -- like an electric glimmer -- utterly different from anything I have seen before."

If this is the correct explanation for the blue moon, then few people have ever seen it. The phenomenon's rarity may be the reason why someone decided a substitute was needed, one that occurs more frequently than the real thing and can be explained in a few sentences on the evening news.

But I'm probably backing a lost cause. The media attention being given to this month's fake blue moon equals that of the last two times we had two full moons in a month (1985 and 1988). We're probably stuck with bogus blue moons from now on.

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