<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> <P> One lunation (an average lunar cycle) is 29.53 days . There are about 365.24 days in a tropical year . Therefore, about 12.37 lunations (365.24 days divided by 29.53 days) occur in a tropical year . In the widely used Gregorian calendar, there are 12 months (the word month is derived from moon) in a year, and normally there is one full moon each month . Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles . The extra days accumulate, so every two or three years (seven times in the 19 - year Metonic cycle), there is an extra full moon . The extra moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a blue moon . </P>

Once in a blue moon origin of phrase