A blue moon is usually used to discribe a second full moon in a single solar calendar month, which happens every two to three years (seven times in the Metonic cycle of 19 years). The usage of the term blue moon to describe the second full month in one calendar month results from a misinterpretation of the traditional definition of that term in the March 1946 issue of Sky & Telescope. Previous to that time, the term was used to describe the third full moon in a season with four.
The term has nothing to do with the actual colour of the moon. A literally "blue moon" (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions, e.g. volcanic eruptions and exceptionally large fires can leave particles in the atmosphere.
One lunation (an average lunar cycle) is 29.53 days. There are about 365.25 days in a solar year. Therefore, about 12.37 lunations (365.25 days divided by 29.53 days) occur in a solar 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 (7 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, the Christian 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 that 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 2 full moons; the second one is called a blue moon. On rare occasions in a calendar year (as happened in 2010), both January and March each have 2 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. This calculation is based on the relationship that both "once in a blue moon" and "hertz" describe frequency.
Origin of the term 
The suggestion has been made that the term "blue moon" for "intercalary month" arose by folk etymology, the "blue" replacing the no-longer-understood belewe "to betray". The original meaning would then have been "betrayer moon", referring to a full moon which would "normally" (in non-intercalating years) be the full moon of spring, while in intercalating year, it was "traitorous" in the sense that people would have had to continue fasting for another month in accordance with the season of Lent.
The earliest recorded English usage of the term blue moon is found in an anti-clerical pamphlet (attacking the Roman clergy, and cardinal Thomas Wolsey in particular) by two converted Greenwich friars, William Roy and Jerome Barlow, published in 1528 under the title Rede me and be nott wrothe, for I say no thynge but trothe. The relevant passage reads:
- O churche men are wyly foxes [...] Yf they say the mone is blewe / We must beleve that it is true / Admittynge their interpretacion. (ed. Arber 1871 p. 114)
It isn't clear from the context that this refers to intercalation; the context of the passage is a dialogue between two priest's servants, spoken by the character "Jeffrey" (a brefe dialoge betwene two preste's servauntis, named Watkyn and Ieffraye). The intention may simply be that Jeffrey makes an absurd statement, "the moon is blue" to make the point that priest's will require laymen to believe in statements even if they are patently untrue. But in the above interpretation of "betrayer moon", Jeffrey may also be saying that it is up to the priests' whim to decide whether Lent is over or not, by introducing "blue moons" apparently at random.
Maine Farmers' Almanac blue moons 
In the 19th and early 20th centuries, the Maine Farmers' Almanac listed blue moon dates for farmers. These correspond to the third full moon in a quarter of the year when there were four full moons (normally a quarter year has three full moons). Full moon names were given to each lunation in a season. When a season has four moons the third is called the blue moon so that the last can continue to be called with the proper name for that season.
The division of the year into quarters starts with the nominal vernal equinox on or around March 21. This is close to the astronomical season but follows the Christian computus used for calculations of Easter, which places the equinox at a fixed date in the (Gregorian) calendar.
Sky and Telescope calendar misinterpretation 
The March 1946 Sky and Telescope article "Once in a Blue Moon" by James Hugh Pruett misinterpreted the 1937 Maine Farmers' Almanac. "Seven times in 19 years there were — and still are — 13 full moons in a year. This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon." Widespread adoption of the definition of a "blue moon" as the second full moon in a month followed its use on the popular radio program StarDate on January 31, 1980.
Visibly blue moon 
The most literal meaning of blue moon is when the moon (not necessarily a full moon) appears to a casual observer to be unusually bluish, which is a rare event. The effect can be caused by smoke or dust particles in the atmosphere, as has happened after forest fires in Sweden and Canada in 1950 and 1951, and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanoes have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichón volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991.
On September 23, 1950, several muskeg fires that had been smoldering for several years in Alberta, Canada, suddenly blew up into major—and very smoky—fires. Winds carried the smoke eastward and southward with unusual speed, and the conditions of the fire produced large quantities of oily droplets of just the right size (about 1 micrometre in diameter) to scatter red and yellow light. Wherever the smoke cleared enough so that the sun was visible, it was lavender or blue. Ontario, Canada, and much of the east coast of the United States were affected by the following day, and two days later, observers in Britain reported an indigo sun in smoke-dimmed skies, followed by an equally blue moon that evening.
The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometre)—and no other sizes present. This is rare, but volcanoes sometimes produce such clouds, as do forest fires. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes, with most smaller than 1 micrometre, and they tend to scatter blue light. This kind of cloud makes the moon turn red; thus red moons are far more common than blue moons.
Blue moons between 2009 and 2021 
The following blue moons occur between 2009 and 2021. These dates use UTC as the timezone; exact dates vary with different timezones.
Using the Maine Farmers' Almanac definition of blue moon (meaning the third full moon in a season of four full moons, and where the seasons are marked by equal 3 month intervals between solstices and equinoxes as opposed to calendar quarters), blue moons have occurred or will occur on
- November 21, 2010
- August 21, 2013
- May 21, 2016
- May 18, 2019
- August 22, 2021
Unlike the astronomical seasonal definition, these dates are dependent on the Gregorian calendar and time zones.
Two full moons in one month (the second of which is a "blue moon"): 
- 2009: December 2, December 31 (partial lunar eclipse visible in some parts of the world), only in time zones west of UTC+05.
- 2010: January 1 (partial lunar eclipse), January 30, only in time zones east of UTC+04:30.
- 2010: March 1, March 30, only in time zones east of UTC+07.
- 2012: August 2, August 31, only in time zones west of UTC+10
- 2012: September 1, September 30, only in time zones east of UTC+10:30.
- 2015: July 2, July 31
- 2018: January 2, January 31
- 2018: March 2, March 31
- 2020: October 1, October 31
Popular culture 
Blue moons have been referenced in popular culture, such as:
- In the 2011 movie The Smurfs. In this context, the blue moon was literally a blue-colored moon, a period of time in the Smurfs' medieval world where it becomes possible to cross dimensions via an underground waterfall, which helps set the premise of the film's plot by sending several Smurfs to the real world. This Wikipedia article was also shown on a computer in the movie.
- The Blue Moon Detective Agency, from the television series Moonlighting.
- In the 2009 young adult fiction novel by Alyson Noël of the same name, Blue Moon refers to two full moons occurring within the same month and the same astrological sign.
- The blue moon was referenced in Charmed, where the Charmed Ones were magically transformed into monsters (looked like a werewolf on four legs).
- In 1995 the German Synthpop band De/Vision released a single titled 'Blue Moon'. 'Blue Moon' also appeared on the album 'Unversed In Love.'
- "Blue Moon" is a popular music standard, written by Richard Rodgers and Lorenz Hart in 1934.
- "Once in a Very Blue Moon" was written by the Nashville songwriter Patrick Alger with Eugene Levine. It was recorded by folk/country artist Nanci Griffith in 1984, as well as by Dolly Parton and Mary Black.
See also 
- Plait, Phil. "Today’s Full Moon is the 13th and Last of 2012".
- Sinnott, Roger W.; Olson, Donald W.; Fienberg, Richard Tresch (May 1999). "What's a Blue Moon?". Sky & Telescope. Retrieved September 1, 2012. "The trendy definition of 'blue Moon' as the second full Moon in a month is a mistake."
- Smith, Bridie (December 28, 2009). "Once in a Blue Moon ...". The Age. Retrieved September 1, 2012.
- Hiscock, Philip (August 30, 2012). "Folklore of the "Blue Moon"". International Planetarium Society.
- "What Is a Blue Moon?". Farmers' Almanac. Almanac Publishing Co. August 24, 2009. Retrieved September 1, 2012. Frequently asked questions about the date of Easter. Oikoumene.org (2007-01-31). Retrieved on August 14, 2012.
- Calendars and their History. Astro.nmsu.edu. Retrieved on August 14, 2012
- printed by John Schott at Strasburg in 1528. See also Koelbing, Arthur (1907–21). "Barclay and Skelton: German Influence on English Literature". In A.W. Ward, et al. The Cambridge History of English and American Literature. New York: G.P. Putnam's Sons. vol. III, ch. 4, § 14. ISBN 1-58734-073-9.
- Clarke, Kevin (1999). "on blue moons". InconstantMoon.com.
- Minnaert, M: "De natuurkunde van 't vrije veld" 5th edition Thieme 1974, part I "Licht en kleur in het landschap" par.187 ; ISBN 90-03-90844-3 (out of print); also see ISBN 0-387-97935-2
- Blue Moon. science.nasa.gov (July 7, 2004).
- Bowling, S. A. (1988-02-22). Blue moons and lavender suns. Alaska Science Forum, Article #861
- Giesen, Jurgen. "Blue Moon". Physik und Astromonie. Retrieved January 17, 2009.
- Noël, Alyson (2009). Blue Moon. New York: St. Martin's Griffin. pp. Ch. 37. ISBN 0-312-53276-8.
- Information about the August 2012 Blue Moon
- What is a Blue Moon? by Michael Myers
- Folklore of the Blue Moon by Philip Hiscock
- What's a Blue Moon? by Donald W. Olson, Richard T. Fienberg, and Roger W. Sinnott – Sky & Telescope
- Article explaining that originally a blue moon meant the 3rd full moon in a season of 4 full moons, and how the "2nd in a month" error began
- Once in a Blue Moon – What is a blue moon? by Ann-Marie Imbornoni
- Topical Words – Blue Moon
- Blue Moon: Folklore or fakelore? by Pip Wilson
- A Blue Moon Calculator by David Harper
- On Blue Moons by Kevin Clarke
- Blue Moon by Irineu Gomes Varella (Portuguese)
- 'Blue moon' coming to our skies soon
- Blue Moon – what's the real definition? by David Harper and Lynne Marie Stockman
- blog on lunar calendars and computing
- WRAL blog Tony Rice
- Blue Moon (list of blue moons this century) by Tofique Fatehi