SACRAMENTO, CA - Friday night will bring the second full moon of the month. This is popularly referred to as a blue moon, but this definition has only recently come about.
It was perpetuated by a 1946 article in Sky & Telescope magazine. The traditional definition comes from the Maine Farmer's Almanac. It states that the blue moon is the third full moon in a season of four full moons. So why two definitions?
The more recent definition was a mistake. An amateur astronomer, James Hugh Pruett, mistakenly wrote an erroneous definition some 60 years ago in Sky & Telescope magazine. The author referred to the second full moon of the month as a "blue Moon." The idea caught on and spread, so much so that most people use this definition over the traditional one relating to the Christian calendar.