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 in the year. 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. 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. 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. The Maine Farmers' Almanac called the third full moon in a season that had four the "blue moon". 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. Using the common 1946 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, and in 2018 in almost every time zone) both January and March each have two full moons, so that the second one in each month is called a "blue moon", and the month of February, with only 28 or 29 days, has no full moon. Under this misinterpretation, certain tropical years with 13 full moons, which would normally be said to have one blue moon, can be said to have two blue moons, making blue moons appear to be more frequent. Etymology The earliest recorded English usage of the term blue moon is found in an anti-clerical pamphlet (attacking the Roman clergy, and Cardinal