Today there will be a blue moon – the second full moon within one calendar month. This doesn’t happen very often – once every few years. The next one will be August 2012, with the full moons falling either side of the London Olympics.
This time, there will also be a partial lunar eclipse.
Those reading from Australia don’t get their full moon until January 1st, which makes January the blue moon month for them.
I read about this first on Wikipedia by accident quite some time ago, and put it in my diary to look out for when it happens next.
This piece on Yahoo News goes on to explain that it’s not really a significant occurrence for real astronomers:
A full moon occurs every 29.5 days, and most years have 12. On average, an extra full moon in a month — a blue moon — occurs every 2.5 years. The last time there was a lunar double take was in May 2007. New Year’s Eve blue moons are rarer, occurring every 19 years. The last time was in 1990; the next one won’t come again until 2028.
Blue moons have no astronomical significance, said Greg Laughlin, an astronomer at the University of California, Santa Cruz.
“`Blue moon’ is just a name in the same sense as a `hunter’s moon’ or a `harvest moon,'” Laughlin said in an e-mail.