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 on December 2. It will appear again on Thursday in time for the New Year's countdown. 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. 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. "`Blue moon' is just a name in the same sense as a `hunter's moon'(a full moon in octorber) 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. If you are in China, in which month can you see the blue moon?
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