"I remember the last time this happened," says professor Philip Hiscock of the Dept. of Folklore at the Memorial University of Newfoundland. "December 1990 ended with a Blue Moon, and many New Year's Eve parties were themed by the event. It was a lot of fun."
Don't expect the Moon to actually turn blue, though. "The 'Blue Moon' is a creature of folklore," he explains. "It's the second full Moon in a calendar month."
Right: The full moon of Dec. 2, 2009, over Turin, Italy. Photographer Stefano De Rosa notes that the blue colors are cast by Christmas lights surrounding the pictured church.
Most months have only one full Moon. The 29.5-day cadence of the lunar cycle matches up almost perfectly with the 28- to 31-day length of calendar months. Indeed, the word "month" comes from "Moon."
Occasionally, however, the one-to-one correspondence breaks down when two full Moons squeeze into a single month. December 2009 is such a month. The first full Moon appeared on December 2nd; the second, a "Blue Moon," will come on December 31st.