A blue moon is an extra full moon that appears in a subdivision of a year, either the third of four full moons in a season or, recently, a second full moon in a month of the common calendar. Metaphorically, a "blue moon" is a rare event, as in the expression "once in a blue moon".
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., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere.
The term blue moon is currently used commonly to describe 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). This usage of the term "blue moon" to describe the second full moon in one calendar month results from a misinterpretation of the traditional definition of that term in the March 1946 issue of Sky & Telescope. Before then, the term was used to describe the third full moon in a season with four.
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, 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 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. 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.
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 that 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 is not 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 priests require laymen to believe in statements even if they are patently false. 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. In the Antarctic diary of Robert Falcon Scott for July 11, 1911 his entry says, "...the air thick with snow, and the moon a vague blue."  On that date the moon phase would have looked full.
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 micrometer)—and no other sizes present. It 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 micrometer, 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 20, 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
Blue moons have been referenced in popular culture, such as:
- In the novels of Simon R. Green blue moon plays a significant magical role, and also figures in titles of two his novels: Blue Moon Rising and Beyond the Blue Moon, both of the Hawk & Fisher fantasy series.
- In the 1998 paranormal romance fiction novel by Laurell K. Hamilton of the same name, Blue Moon refers to two full moons occurring within the same month.
- 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.
- "Blue Moon" is a popular music standard, written by Richard Rodgers and Lorenz Hart in 1934.
- "New Blue Moon", released in 1990, was a song written and recorded by the Traveling Wilburys.
- "The Moon Is Blue", 1953 movie starring William Holden; nominated for 3 Oscars.
- A Blue Moon appears as a significant element in the 2011 film The Smurfs. The Wikipedia page on Blue Moon is viewed in the movie.
- The theme song for "The Soprano's", "Woke up this Morning" by Alabama3 references born under a blue moon in its lyrics.
- 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.
- "Month | Define Month at Dictionary.com". Dictionary.reference.com. Retrieved 2013-08-25.
- "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.
- Hamilton, Laurell (1998). Blue Moon. New York City: Ace Books. pp. Ch. 46. ISBN 0-441-00574-8.
- Noël, Alyson (2009). Blue Moon. New York: St. Martin's Griffin. pp. Ch. 37. ISBN 0-312-53276-8.
- "The Moon is Blue 1953". YouTube. Retrieved 2013-08-25.
- 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" at infoplease.com
- "Blue moon" at World Wide Words
- Blue moon calculator by David Harper
- "On Blue Moons" by Kevin Clarke
- "It's a big night for a 'blue moon'" at nbcnews.com
- "What does blue moon really mean?" blog entry on lunar calendars and computing
- "Blue moon definition based on a mistake by Tony Rice
- "Blue Moon" (list of blue moons this century) slide show by Tofique Fatehi