<P> The term has traditionally referred to an "extra" full moon, where a year which normally has 12 full moons has 13 instead . The "blue moon" reference is applied to the third full moon in a season with four full 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 full 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> <Ul> <Li> In calculating the dates for Lent and Easter, Catholic clergy identified a Lenten moon . Historically, when the moons arrived too early, they called the early moon a "betrayer" (belewe) moon, so the Lenten moon came at its expected time . </Li> <Li> Folklore named each of the 12 full moons in a year according to its time of year . The occasional 13th full moon that came too early for its season was called a "blue moon", so the rest of the moons that year retained their customary seasonal names . </Li> <Li> The Maine Farmers' Almanac called the third full moon in a season that had four the "blue moon". </Li> <Li> The frequency of a blue moon can be calculated as follows . It is the period of time it would take for an extra synodic orbit of the moon to occur in a year . Given that a year is approximately 365.2425 days and a synodic orbit is 29.5309 days, then there are about 12.368 synodic months in a year . For this to add up to another full month would take 1 / 0.368 years . Thus it would take about 2.716 years, or 2 years, 8 months and 18 days for another blue moon to occur . </Li> <Li> Using the common Sky & Telescope misunderstanding, when one calendar month has two full moons; the second one is called a "blue moon". On rare occasions in a calendar year (as happened in 2010 in time zones east of UTC + 07), both January and March each have two full moons, so that the second one in each month is called a "blue moon"; in this case, the month of February, with only 28 or 29 days, has no full moon . Under this misinterpretation a blue moon can be more frequent . </Li> </Ul>

Is there a month with two full moons