Once in a New Year's Eve Blue Moon

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As you ring in the New Year, take your glass of champagne and toast the blue moon in the sky above. It's a once in a generation thing, a special astronomical happening that reminds us that while life is highly unpredictable, the heavens are not.

Tonight's New Year's Eve blue moon is the first since 1990. Another won't roll around for another 19 years.  So the time to celebrate is now. I'll walk outside tonight as the clock strikes twelve, bundled up and looking for a blue moon shining over the snowy mountain peaks of Aspen, CO. The blue moon is expected to be visible to New Year's Eve celebrants across the U.S., Canada, Europe, South America and Africa, according to the AP's Alicia Chang. And in New York City, the full moon will be competing with the glittery ball dropping in Times Square (maybe the cameras can capture both).

 Of course, the name has nothing to do with its color (although a moon can sometimes appear bluish from the smoke of a forest fire or the ash of a volcano). The old expression, "once in a blue moon," has more to do with something that is rare, special, uncommon, even absurd--but not impossible.

The most popular current definition of a blue moon is the second full moon in a calendar month. A full moon occurred on December 2 and tonight's will be the second, a phenomenon that occurs every 2.5 years (the next will be in August, 2012).  But the New Year's Eve blue moon is more exceptional. The next won't occur until 2028. There's no way of knowing where I'll be then, but it is reassuring that, just like clockwork, the blue moon will be there.

There's another way of counting blue moons that dates back to medieval times. Since 1819, the Maine Farmers Almanac has listed the dates for blue moons based on a seasonal counting system:  in a season which has four full moons instead of the usual three, the third full moon is the special one, the "blue moon." By that method, tonight's full moon is the first of the winter season but doesn't actually qualify as "blue." The twice-in-a-month definition of blue moon is a more modern interpretation that is credited to a 1946 article in Sky & Telescope magazine (some say the writer just made a mistake, but I'm sticking with his version).

The 'blue moon' expression itself dates back to old England. A 1528 work by William Barlow, the Bishop of Chichester, the Treatyse of the Buryall of the Masse, included a reference to a blue moon:

    "Yf they saye the mone is belewe, We must beleve that it is true."

I believe in blue moons. But not that the moon is made of green cheese.

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Cristine Russell is a senior fellow at Harvard Kennedy School of Government and the president of the Council for the Advancement of Science Writing.

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