A blue moon is when the moon appears to be a shade of blue (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions; e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere.
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.
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.
- 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”.
- In modern use, when 13 full moons occur in a year, usually 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.
- According to Google Calculator, “once in a blue moon” is equal to 1.16699016 × 10−8 hertz. The hertz is a unit of frequency (one per second), and thus if the mean length of time between blue moons (2.7145 years according to Google) is metricated and converted to a frequency (by calculating the multiplicative inverse), it can be expressed in terms of hertz.