Blue moon

Blue moon
31 December 2009 Blue Moon with partial lunar eclipse

A blue moon can refer to the third full moon in a season with four full moons.[1] Most years have twelve full moons that occur approximately monthly. In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. Lunisolar calendars have rules about when to insert such an intercalary or embolismic ("leap") month, and what name it is given; e.g. in the Hebrew calendar the month Adar is duplicated. The term "blue moon" comes from folklore. Different traditions and conventions place the extra "blue" full moon at different times in the year.

  • In calculating the dates for Lent and Easter, the Clergy identify the Lent Moon. It is thought that historically when the moon's timing was too early, they named an earlier moon as a "betrayer moon" (belewe moon), thus the Lent moon came at its expected time.[2]
  • Folklore gave each moon a name according to its time of year. A moon that came too early had no folk name, and was called a blue moon, retaining the correct seasonal timings for future moons.
  • The Farmers' Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.

A "blue moon" is also used colloquially to mean "a rare event", reflected in the phrase "once in a blue moon".[3]


Early English and Christian

The earliest recorded English usage of the term "blue moon" was in a 1528 pamphlet violently attacking the English clergy,[4] entitled "Rede Me and Be Not Wrothe" ("Read me and be not angry"; or possibly "Counsel Me and Be Not Angry" [5]): "If they say the moon is belewe / We must believe that it is true" [If they say the moon is blue, we must believe that it is true].

Another interpretation uses another Middle English meaning of belewe, which (besides "blue") can mean "betray".[2] By the 16th century, before the Gregorian calendar reform, the medieval computus was out of sync with the actual seasons and the moon, and occasionally spring would have begun and a full moon passed a month before the computus put the first spring moon.[6][7] Thus, the clergy needed to tell the people whether the full moon was the Easter moon or a false one, which they may have called a "betrayer moon" (belewe moon) after which people would have had to continue fasting for another month in accordance with the season of Lent.[8]

Modern interpretation of the term relates to absurdities and impossibilities; the phrase "once in a blue moon" refers to an event that will take place only at incredibly rare occasions.[9]

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,[10] and after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanos have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico, and there are reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.[11]

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 U.S. were affected by the following day, and two days later, observers in England reported an indigo sun in smoke-dimmed skies, followed by an equally blue moon that evening.[11]

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.[12]

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 are given to each moon in a season: For example, the first moon of summer is called the early summer moon, the second is called the midsummer moon, and the last is called the late summer moon. When a season has four moons the third is called the blue moon so that the last can continue to be called the late moon.

The division of the year into quarters starts with the nominal vernal equinox on or around March 21.[13] 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.

Some[weasel words] naming conventions[citation needed] keep the moon's seasonal name for its entire cycle, from its appearance as a new moon through the full moon to the next new moon. In this convention a blue moon starts with a new moon and continues until the next new moon starts the late season moon.

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.[1]

Blue moons between 2009 and 2016

The following blue moons occur between 2009 and 2016. These dates use UTC as the timezone; exact dates vary with different timezones.


Using the Farmers' Almanac definition of blue moon (meaning the third full moon in a season of four full moons), blue moons occur

  • November 21, 2010
  • August 21, 2013
  • May 21, 2016


Note that, unlike the astronomical seasonal definition, these dates are dependent on the Gregorian calendar and time zones.

Two full moons in one month:[14]

  • 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
  • 2015: July 2, July 31

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

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 article was also mentioned.

See also


  1. ^ a b Sinnott, Roger W., Donald W. Olson, and Richard Tresch Fienberg (May 1999). "What's a Blue Moon?". Sky & Telescope. Retrieved 2008-02-09. "The trendy definition of "blue Moon" as the second full Moon in a month is a mistake." 
  2. ^ a b "What is a "Blue Moon"?". Farmers' Almanac. 
  3. ^ Smith, Bridie (28 December 2009). Once in a Blue Moon. The Age. Retrieved on 3 March 2010.
  4. ^ Koelbing, Arthur, Ph.D. (1907–21). "Barclay and Skelton: German influence on English literature". The Cambridge History of English and American Literature, Volume III. 
  5. ^ from the old English "rede" [vb.] to advise, to warn, or "rede"[n.] a warning, an injunction[citation needed])
  6. ^
  7. ^
  8. ^
  9. ^ Hiscock, Philip (June 19, 2006). "Folklore of the "Blue Moon"". International Planetarium Society. 
  10. ^ 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
  11. ^ a b NASA
  12. ^ Bowling, S. A. (1988). Blue moons and lavender suns. Alaska Science Forum, Article #861
  13. ^ Clarke, Kevin (1999). "on blue moons". 
  14. ^ Giesen, Jurgen. "Blue Moon". Physik und Astromonie. Retrieved 2009-01-17. 

External links

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  • Blue Moon — bezeichnet: Blue Moon (Astronomie), ein kalendarisch zusätzlicher Vollmond im Monat Blue Moon (Lied), Lied von Lorenz Hart (Text) und Richard Rodgers Blue Moon (Hörfunksendung), Hörfunksendung des RBB Senders Fritz Blue Moon (Kentucky), Ort in… …   Deutsch Wikipedia

  • blue moon — lue moon The second full moon occurring in the same month; derived from the expression {once in a blue moon}; as, we had a blue moon on January 31, 1999, and another in March. [PJC] {once in a blue moon} very rarely; from the observation that… …   The Collaborative International Dictionary of English

  • Blue Moon — (рус. Голубая Луна)  английское название астрономического явления голубой луны, имеющее, кроме того, следующие значения: Содержание 1 В музыке 2 На экране 3 В литератур …   Википедия

  • blue moon — Moon Moon (m[=oo]n), n. [OE. mone, AS. m[=o]na; akin to D. maan, OS. & OHG. m[=a]no, G. mond, Icel. m[=a]ni, Dan. maane, Sw. m[*a]ne, Goth. m[=e]na, Lith. men[*u], L. mensis month, Gr. mh nh moon, mh n month, Skr. m[=a]s moon, month; prob. from a …   The Collaborative International Dictionary of English

  • blue moon — n. [< a rare atmospheric condition in which layers of smoke, dust, etc., as from forest fires, volcanoes, etc., cause the surface of the moon to appear to be blue] a very long time once in a blue moon very seldom; almost never …   English World dictionary

  • blue moon — 1821 as a specific term in the sense very rarely, perhaps suggesting something that, in fact, never happens (Cf. at the Greek calends); suggested earliest in this couplet from 1528: Yf they say the mone is blewe, We must beleve that it is true.… …   Etymology dictionary

  • Blue Moon — Cette page d’homonymie répertorie les différents sujets et articles partageant un même nom. Sommaire 1 Cinéma et télévision 2 Musique 3 …   Wikipédia en Français

  • blue moon — /blu ˈmun/ (say blooh moohn) noun 1. the second full moon in a calendar month, such second full moons occurring slightly less than once a year. –phrase 2. once in a blue moon, seldom; very rarely. {? from the US Maine Farmers Almanac (1932) which …  

  • blue moon — noun a long time something that happens once in blue moon almost never happens • Usage Domain: ↑colloquialism • Hypernyms: ↑long time, ↑age, ↑years * * * noun once in a blue moon …   Useful english dictionary

  • blue moon — noun Date: 1821 1. a very long period of time usually used in the phrase once in a blue moon < such people happen along only once in a blue moon Saturday Review > 2. a second full moon in a calendar month …   New Collegiate Dictionary

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