LOS ANGELES - Once in a blue moon there is one on New Year's Eve.
Revelers ringing in 2010 will be treated to a so-called blue moon. According to popular definition, a blue moon is the second full moon in a month. But don't expect it to be blue - the name has nothing to do with the color of our closest celestial neighbor.
A full moon occurred Dec. 2. It will appear again tomorrow in time for the New Year's countdown.
"If you're in Times Square, you'll see the full moon right above you. It's going to be that brilliant," said Jack Horkheimer, director emeritus of the Miami Space Transit Planetarium and host of a weekly astronomy TV show.
It would be, anyway, if you could actually see it - clouds and perhaps showers are forecast all evening in New York as well as Philadelphia.
Weather, permitting, the New Year's Eve blue moon will be visible in the United States, Canada, Europe, South America, and Africa. For partygoers in Australia and Asia, the full moon does not show up until New Year's Day, making January a blue-moon month for them.
However, the Eastern Hemisphere can celebrate with a partial lunar eclipse on New Year's Eve when part of the moon enters Earth's shadow. The eclipse will not be visible in the Americas.
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 was in 1990; the next will be 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.
The popular definition of blue moon came about after a writer for Sky & Telescope magazine in 1946 misinterpreted the Maine Farmer's Almanac and labeled a blue moon as the second full moon in a month. In fact, the almanac defined a blue moon as the third full moon in a season with four full moons, not the usual three.
Although Sky & Telescope corrected the error decades later, the definition caught on. For purists, however, this New Year's Eve full moon doesn't even qualify as a blue moon. It's just the first full moon of the winter season.