Los Angeles 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.
A full moon occurred on Dec. 2 and will appear again on Thursday, New Year’s Eve.
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.