Sky Watch: Week of Dec. 27
Thursday brings the second full Moon in December. Some almanacs assert that when two full Moons occur in a calendar month, the second full moon is called the blue Moon. The phrase “once in a blue Moon” implies a very seldom event. Yet two full Moons in the same month are not as rare as one might think; this occurs, on average, about every 32 months.
Joe Rao, New York Times