<P> A blue moon is an additional full moon that appears in a subdivision of a year: either the third of four full moons in a season, or a second full moon in a month of the common calendar . </P> <P> The phrase has nothing to do with the actual color of the moon, although a literal "blue moon" (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions: e.g., if volcanic eruptions or fires leave particles in the atmosphere of just the right size to preferentially scatter red light . </P> <P> The term has traditionally referred to an "extra" moon, where a year which normally has 12 moons has 13 instead . The "blue moon" reference is applied to the third moon in a season with four moons, thus correcting the timing of the last month of a season that would have otherwise been expected too early . This happens every two to three years (seven times in the Metonic cycle of 19 years). The March 1946 issue of Sky & Telescope misinterpreted the traditional definition, which led to the modern colloquial misunderstanding that a blue moon is a second full moon in a single solar calendar month with no seasonal link . </P> <P> Owing to the rarity of a blue moon, the term "blue moon" is used colloquially to mean a rare event, as in the phrase "once in a blue moon". </P>

Where did the blue moon get its name