“Do you see that full moon in the sky tonight? That happens once in a blue moon.” Scientifically, this happens 7 time in 19 years or about once in 3 years. You won’t see it again until January 2018, the last time was in August 2012. Technically, the full moon occurred at 8:43 a.m. ET today, about 20 minutes after it set for the U.S. East Coast, but it can still be again when the moon rises at at 8:23 p.m. ET.
The primary definition of a Blue Moon, according to the now defunct Maine Farmers’ Almanac, is a 4th occurrence of a moon in a 3-month season rather than the more regularly occurring 3. An alternative and more recent definition that appeared in a 1943 Sky and Telescope Magazine article referring to a second full moon in a month. This second definition has stuck, due to a StarDate radio show reinforcing it in 1980, and the popular board game Trivial Pursuit.
It is also called a Full Sturgeon Moon, Green Corn Moon, Grain Moon or Full Red Moon. And of course, it is also the name of a 1961 #1 hit song by the doo-wop group The Marcels, originally written in 1934 by Rodgers & Hart.
Bill Petro, your friendly neighborhood historian