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.
The original meaning developed when the Christians needed to name an extra moon in the calendar. There are typically twelve full moons in a year, each one linking to a religious holiday. A year with thirteen moons, threw off the balance thus a need to name the thirteenth moon to keep the calendar on track. That extra moon was deemed the "Blue Moon."
The second, more recent definition, was a mistake published some 60 years ago in Sky & Telescope magazine. The author referred to the second full moon of the month as a "blue Moon." This definition has been more widely used than the one associated with the Christian calendar.
Whichever definition you believe, you won't see the moon turn blue.