There’s A “Blue Moon” Tonight And It Won’t Happen Again For 3 Years

According to NASA, an early definition of a “blue moon,” from the 1940s, was considerably more complicated and involved “factors such as ecclesiastical dates of Easter and Lent, tropical years, and the timing of seasons according to the dynamical mean sun.”

An author named James Hugh Pruett attempted to explain this complex definition (from a Maine Farmer's Almanac) to the readers of the Sky & Telescope magazine in 1946, offering his interpretation that a blue moon was simply the second full moon in a single month. He was wrong, but the definition stuck.

The last time it happened was in September 2012. The next one won't be until January 2018.