Blue Moon

A blue moon is usually used to describe a second full moon in a single solar calendar month, which happens every two to three years (seven times in the Metonic cycle of 19 years). The usage of the term blue moon to describe the second full moon in one calendar month results from a misinterpretation of the traditional definition of that term in the March 1946 issue of Sky & Telescope. Beforehand, the term was used to describe the third full moon in a season with four.