Astronomical rarity to kick-off 2010.

The two-thousand and ninth year of the Common Era has been somewhat bizarre the whole way through, and this Thursday night it will go out in the same fashion :  a blue moon will hang over New Year’s Eve. Such an event hasn’t coincided with that holiday since 1990.

We know the blue moon is rare and special (and goes great with a slice of orange) but what the heck is it? In modern usage, the term blue moon refers to the second full moon in a single calendar month. Though our Gregorian calendar is based on the solar year, its months almost perfectly coincide with the cycles of the moon. In fact, the word month derives from earlier words for moon.

But every once in a while (in a blue moon?), a year will come along with 13 full moons, and astronomers run out of months to stick them in, so one has to do double duty. This usually occurs about seven times in a given 20 year period.

That’s not the full story, though. This modern definition is actually a misinterpretation of a complex explanation in the 1937 edition of the Maine Farmer’s Almanac. The old formulation printed in that book was based both on astronomical observations and the ecclesiastical date of Easter. A 1946 article by James Hugh Pruett in Sky and Telescope magazine referred to a previous article which in turn had referred to the Almanac. Pruett boiled down the calculation to this simple statement :  “This second [full moon] in a month, so I interpret it, was called Blue Moon.”

A more accurate, though still oversimplified, way of expressing the Almanac’s traditional formula is to call the blue moon the third full moon in a season with four. Why single this one out? Because in the dark ages one of the only approved scientific endeavors was calculating the date of Easter.  This was before Avatar; you had to get your kicks somewhere.

Before the Farmer’s Almanac, the phrase ‘once in a blue moon’ was used to refer to events that never happen. Such events might also occur on a day not ending in ‘y’ or when pigs fly. The earliest known reference is found in this rhyming couplet from 1528 :

Yf they say the mone is blewe,

We must beleve that it is true.

Such a tongue-in-cheek term was likely not intended to be taken seriously. In fact, the moon only appears blue under very rare circumstances not associated with its stage. Photographers have been able to capture a blue-hued moon using reflection from terrestrial light sources, or unique atmospheric conditions, as in the featured NASA photo.

Rest assured, Thursday’s night light will be just as white as any other full moon. Though that’s not to say it won’t be blamed for it’s fair share of wild and ‘unusual’ behavior.