A blue moon during the December 2009 lunar eclipse

"Once in a blue moon" is a traditional phrase of unknown origin meaning "very rarely indeed".[1] Webster's Dictionary (1949) defined a blue moon as meaning, "A phenomenon never, or very rarely, seen; hence, a rarely recurring occasion."[2] However, since the mid-20th century the term blue moon has commonly been applied to the occurrence of a second full moon within a single calendar month, something that happens every two or three years, thus obscuring the original meaning of the term.

The use of blue moon to mean a specific calendrical event dates from 1937, when the Maine Farmers' Almanac gave a definition slightly different from the one now in common use, and was followed by a 1946 article in Sky and Telescope which led to the current usage, popular since the 1980s.[3][4]

The phrase in modern usage has nothing to do with the actual color of the Moon, although a visually blue Moon (the Moon appearing with a bluish tinge) may occur under certain atmospheric conditions—for instance, if volcanic eruptions or fires release particles in the atmosphere of just the right size to preferentially scatter red light.[5] This is a genuinely uncommon occurrence, more in accord with the original meaning of the phrase "once in a blue moon".


Main article: Intercalary month

The term has traditionally, in the Maine Farmer's Almanac[citation needed], referred to an "extra" full moon, where a year which usually 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,[6] 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).[7] The author of a March 1946 article in Sky & Telescope attempted to decipher the traditional practice of the editors of the Maine Farmers' Almanac by examining old issues of the almanac. Without enough almanacs to see the correct pattern, he conjectured the wrong rule for 'blue moons', which led to the modern colloquial misunderstanding that a blue moon is a second full moon in a single solar calendar month, with no link to the order it occurs in a season. The phrase "once in a blue moon" is also used idiomatically, which means an event happens rarely, like the blue moon itself. An example in a sentence: "My sister lives in another state, so I only see her once in a blue moon."[8][9][10]

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[11]) in a year, and normally there is one full moon each month, with the date of the full moon falling back by nearly one day every calendar 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 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.


Blue moon of August 31, 2012, viewed from Slobozia, Romania.

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 not wrothe, for I say no thynge but trothe. The relevant passage reads:[14]

O churche men are wyly foxes [...] Yf they say the mone is blewe / We must beleve that it is true / Admittynge their interpretacion.[15]

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.

"Betrayer moon" speculation

In 2007, Joe Rao, Skywatching Columnist at Space.com, stated that many years previously he had speculated in Natural History magazine that the "Blue Moon Rule" might derive from belewe, which he described as an Old English word meaning 'to betray', because the extra full moon "betrays the usual perception of one full moon per month."[16] He then added that his speculation had been "innovative", but "completely wrong".[16] In 2009, a less detailed version of this speculation was mentioned in Farmers' Almanac.[17]

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. The seasons used were those of the mean tropical year, equal in length, as opposed to the astronomical seasons which vary in length because the earth's speed in its orbit round the sun is not uniform.

To compare, in 1983 the equal seasons began at 1.48 AM on 23 March, 9.15 AM on 22 June, 4.42 PM on 21 September and 12.10 AM on 22 December, while the astronomical seasons began at 4.39 AM on 21 March, 11.09 PM on 21 June, 2.42 PM on 23 September and 10.30 AM on 22 December (all times GMT). When a season has four full moons the third is called the "blue moon" so that the last can continue to be called with the proper name for that season.[6]

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[6] and in a question in the Trivial Pursuit game in 1986.[10]

Whole blue moon

Image of a pale, visibly blue moon on the night of 25 December 2012.

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,[18] and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years.[19] 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.[20] 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".[21]

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.[20][22]

The key to a blue moon is having many 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.[23]

Blue moons between 2009 and 2037

The following blue moons occur between 2009 and 2037. These dates use UTC as the time zone; exact dates vary with different time zones. Our last up to date blue moon was seen on August 22, 2021, since blue moons happen every two-three years, we can expect to see one in our near future.


Using the Maine Farmers' Almanac definition of blue moon (meaning the third full moon in a season of four full moons, but referenced to astronomical rather than equal seasons), blue moons have occurred on:


Blue moon of July 31, 2015, viewed from Port Coquitlam, Canada

Unlike the astronomical seasonal definition, these dates are dependent on the Gregorian calendar and time zones. These blue moons cannot occur in February.

Two full moons in one month (the second of which is a "blue moon"):[25] A blue moon can occur in January and the following March if there is no full moon at all in February, as is the case in the years 1999, 2018, and 2037.

The next time New Year's Eve falls on a Blue Moon (as occurred on December 31, 2009 in time zones west of UTC+05) is after one Metonic cycle, in 2028 in time zones west of UTC+08. At that time there will be a total lunar eclipse.

See also


  1. ^ Brewers Dictionary Of Phrase And Fable, Revised Edition (1922), p119. "Blue moon. Once in a blue moon. Very rarely indeed."
  2. ^ Webster's New International Dictionary, Second Edition (1949), p296.
  3. ^ Sinnott, Roger W.; Olson, Donald W.; Fienberg, Richard Tresch (May 1999). "What's a Blue Moon?". Sky & Telescope. Retrieved March 1, 2024.
  4. ^ Hiscock, Philip (August 30, 2012). "Folklore of the 'Blue Moon'". International Planetarium Society. Retrieved March 1, 2024.
  5. ^ Gibbs, Philip (May 1997). "Why is the sky blue?". math.ucr.edu. Retrieved November 4, 2015. ... may cause the moon to have a blue tinge since the red light has been scattered out.
  6. ^ a b c d Sinnott, Roger W.; Olson, Donald W.; Fienberg, Richard Tresch (May 1999). "What's a Blue Moon?". Sky & Telescope. Archived from the original on March 22, 2008. Retrieved September 1, 2012. The trendy definition of 'blue Moon' as the second full Moon in a month is a mistake.
  7. ^ Plait, Phil There is also a myth that it came from a calendar printer who printed the phases of the moon in white. When it came to pass that there were two full moons in the same month, they colored the second one blue. "Today's full moon is the 13th and last of 2012".((cite news)): CS1 maint: multiple names: authors list (link)
  8. ^ "Once in a blue moon". Theidioms.com. The Idioms Dictionary. Retrieved June 22, 2022.
  9. ^ Smith, Bridie (December 28, 2009). "Once in a Blue Moon ..." The Age. Australia. Retrieved September 1, 2012.
  10. ^ a b Hiscock, Philip (August 30, 2012). "Folklore of the 'Blue Moon'". International Planetarium Society.
  11. ^ "Month | Define Month at Dictionary.com". Dictionary.reference.com. Retrieved August 25, 2013.
  12. ^ Espanek, Fred. "Eclipses and the moon's orbit". NASA. Retrieved March 10, 2016.
  13. ^ Harper, David (2018). "The Double Blue Moon of 2018". Obliquity. Retrieved January 22, 2022.
  14. ^ 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. (eds.). 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.
  15. ^ ed. Arber 1871 p. 114
  16. ^ a b Joe Rao, Space.com Skywatching Columnist (May 25, 2007). "The Truth Behind This Month's Blue Moon". Space.com. Retrieved July 28, 2018.
  17. ^ "What Is a Blue Moon?". Farmers' Almanac. Almanac Publishing Co. August 24, 2009. Archived from the original on September 7, 2012. Retrieved September 1, 2012.
  18. ^ 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
  19. ^ Philip Hiscock (August 24, 2012). "Once in a Blue Moon". Sky & Telescope. Retrieved May 27, 2019.
  20. ^ a b Blue Moon Archived March 23, 2010, at the Wayback Machine. science.nasa.gov (July 7, 2004).
  21. ^ Scott, Captain R. F. (July 2019). "Scott's Last Expedition". The Journals of Captain R. F. Scott. 12. Retrieved June 22, 2022.
  22. ^ Scott Lingley (July 28, 2015). "The year the sun turned blue". Phys.org. Retrieved May 27, 2019.
  23. ^ Bowling, S. A. (1988-02-22). Blue moons and lavender suns Archived March 21, 2012, at the Wayback Machine. Alaska Science Forum, Article #861
  24. ^ "Sunday's Full Moon is "Blue" by a Different Definition". August 19, 2021.
  25. ^ Giesen, Jurgen. "Blue Moon". Physik und Astromonie. Retrieved January 17, 2009.