This article is about the astronomical phenomenon. For other uses, see Blue Moon (disambiguation).
Blue moon of the December 2009 lunar eclipse

Generally, a full moon occurs only once a month. However, once every two or three years, two full moons will occur in a month. This occurrence is called a "blue moon". This usage results from a misinterpretation of the traditional definition of blue moon in the March 1946 issue of Sky and Telescope.[1] Due to the rarity of a blue moon, the term "blue moon" is used colloquially to mean a rare event, as in the phrase "once in a blue moon".[2]

The apparent colour of the moon, at any time of the year, can be affected by atmospheric circumstances. Volcanic eruptions and exceptionally large fires can leave particles in the atmosphere which give the sky, and thus the moon, a tinge of blue (or other colours).

## Origin[edit]

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.

The term blue moon originates in deviations of the lunar calendar computed by the church, from the real moon.[3] Different traditions place the extra "blue" full moon at different times:

### Different definitions[edit]

- 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 the 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.

## Early English and Christian usage[edit]

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

The earliest recorded English usage of the term blue moon was in a 1524 pamphlet violently attacking the English clergy [3] , entitled "Rede Me and Be Not Wrothe" ("Read me and be not angry"): "If they say the moon is belewe [blue] / We must believe that it is true."

Another interpretation uses a different definition of the Middle English word belewe, which, in addition to "blue", can mean "betray".[4] 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 [5] .[6] 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.

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 .[7]

## Maine Farmers' Almanac blue moons[edit]

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 English called the first moon of summer Hay Moon, the second is called Corn Moon, and the last is called Harvest Moon. 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 .[8] 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[edit]

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]

## Visibly blue moon[edit]

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 ,[9] 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 Chich?n volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991 .[10]

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.[10]

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.[11]

## Blue moons between 2009 and 2021[edit]

The following blue moons occur between 2009 and 2021. These dates use UTC as the timezone; exact dates vary with different timezones.

### Seasonal[edit]

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

### Calendar[edit]

Blue moon of August 31, 2012, viewed from Guayaquil, Ecuador

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"): [12]

- 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

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

## Popular culture[edit]

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 No?l of the same name, Blue Moon refers to two full moons occurring within the same month and the same astrological sign.[13]
- 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 classic popular song. It was written by Richard Rodgers and Lorenz Hart in 1934,

## See also[edit]

| Moon portal

- Black moon
- Full moon names
- Wet moon

## References[edit]

- Oxford English Dictionary

1. ^ a b 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."
2. ^ Smith, Bridie (December 28, 2009). "Once in a Blue Moon ...". The Age. Retrieved September 1, 2012.
3. ^ a b Koelbing, Arthur, Ph.D. (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. Check date values in: `|year=` (help) Online edition: Published January 2000 by Bartleby.com.
4. ^ "What Is a Blue Moon?". Farmers' Almanac. Almanac Publishing Co. August 24, 2009. Retrieved September 1, 2012.
5. ^ Frequently asked questions about the date of Easter. Oikoumene.org (2007-01-31). Retrieved on August 14, 2012.
6. ^ Calendars and their History. Astro.nmsu.edu. Retrieved on August 14, 2012
7. ^ Hiscock, Philip (August 30, 2012). "Folklore of the "Blue Moon"". International Planetarium Society.
8. ^ Clarke, Kevin (1999). "on blue moons". InconstantMoon.com.
9. ^ 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
10. ^ a b Blue Moon. science.nasa.gov (July 7, 2004).
11. ^ Bowling, S. A. (1988-02-22). Blue moons and lavender suns. Alaska Science Forum, Article #861
12. ^ Giesen, Jurgen. "Blue Moon". Physik und Astromonie. Retrieved January 17, 2009.
13. ^ No?l, Alyson (2009). Blue Moon. New York: St. Martin's Griffin. pp. Ch. 37. ISBN 0-312-53276-8.

## External links[edit]

- 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