Monday, January 04, 2010


Marietta Moon
A full moon shines in the early morning sky.

Pammy reminded me that this New Year’s Eve just past was unusual: the kind of New Year’s we see once in a blue moon. Literally.

According to current popular usage, the expression “blue moon” refers to the second full moon in a calendar month, a phenomenon that takes place about once every two-and-a-half years. That it happens at all is owing to the fact that the Gregorian calendar year is about eleven days longer than the lunar year of 354 days.

There were full moons on both December 2 and December 31, 2009: the second of these is the blue moon. The last time a blue moon showed up on a New Year’s Eve, it was 1990 - nineteen years ago.

[It’s no coincidence that the Jewish calendar, which is based on the solar year while at the same time using the moon’s phases to determine the months, has a repeating cycle of nineteen years. Of course, given that every Jewish month begins with the new moon, there can be no blue moons in the Jewish calendar.]

The next New Year’s Eve blue moon will be in another nineteen years: December 31, 2028. As far as a blue moon closing out a decade, that’s an even more unusual event, taking place once every 190 years. The last time it happened was in 1819; the next time will be in 2199. Barring some major advances in Medical Science, I suspect we won’t, alas, be around to see it.

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