Blue Moon

A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year. The extra days accumulate, so that every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times

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 New Year of 2010 was a blue moon… some pictures

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.