Blue moon

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This article is about the astronomical phenomenon. For other uses, see Blue Moon (disambiguation).

A blue moon is a full moon that is not timed to the regular monthly pattern. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles each calendar year contains an excess of roughly eleven days. The extra days accumulate, so that every two or three years (on average about every 2.7154 years) there is an extra full moon. The extra moon is called a "blue moon." Different definitions place the "extra" moon at different times.

  • 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.
  • Folklore gave each moon a name according to its time of year. A moon which came too early had no folk name - and was called a blue moon - bringing the correct seasonal timings for future moons
  • The Farmer's 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.
  • Recent popular usage defined a blue moon as the second full moon in a month, stemming from an interpretation error made in 1946 that was discovered in 1999.[1]

The term "blue moon" is commonly used metaphorically to describe the rarity of an event, as in the saying "once in a blue moon."

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[edit] Early English and Christian usage

The earliest recorded English usage of the term "blue moon" was in 1528 in a pamphlet violently attacking the English clergy,[2] entitled Rede Me and Be Not Wrothe [Read me and be not angry]: "Yf they say the mone is belewe / We must believe that it is true" [If they say the moon is blue, we must believe that it is true].

Some interpret this "blue moon" as relating to absurdities and impossibilities,[3] and a similar moon-related adage was first recorded in the following year: "They would make men beleue ... that þe Moone is made of grene chese" [They would make men believe ... that the moon is made of green cheese].

An alternative interpretation uses the other old English meaning of "belewe" (which can mean "blue" or "betrayer").[4] The church was responsible for the calendar and used the complex computus to calculate the important date of Easter, which is based on the full moon. Lent falls before Easter starting at the beginning of the Lent moon cycle (late winter moon). The next moon is the egg moon (early spring moon), and Easter usually falls on the first Sunday after the full egg moon. Every one to three years the Lent and egg moons would come too early, so the clergy would have to tell people whether the moon was the Lent moon or a false one, which they may have called a "betrayer moon".

[edit] 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, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. The particles in the atmosphere have to be about one micrometer in diameter; under these circumstances, long-wavelength light, which appears red to a viewer, is scattered out of the line of sight and short-wavelength light, which appears blue to a viewer, is selectively transmitted into a viewer's eyes.[5]

[edit] Folklore

Historically, moons were given folk names, twelve each year, to help people to prepare for different times of the year and the related weather and crop needs. Names varied with locality and culture, often with descriptive names such as harvest moon, growing moon, snow moon, and egg moon. Most years have 12 moons (given 12 names) - but in the years with thirteen full moons the monthly "seasons" would be expected to come too early - hens would not recommence laying their eggs when expected after winter as it was still too cold - so the early moon was named a "blue moon", which then re-aligned the rest of the year's moons and 'seasons'.

The origin of the term "blue moon" is steeped in folklore, and its meaning has changed and acquired new nuances over time. Some folktales say that when there is a full blue moon, the moon had a face and talked to those in its light.

[edit] Farmer's Almanac blue moons

In the 19th and early 20th centuries, the Maine Farmer's 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). 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.[6] This is close to the astronomical season but follows the Christian computus used for calculations of Easter, which places each equinox evenly between the summer and winter solstices to calculate seasons rather than using the actual equinox.

Some naming conventions 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.

To calculate the moon names for the seasons using the appearance of the new moon:

  1. Locate the new moons that are nearest to the solstices and equinoxes. These are the early season moons. Mark the new moons as follows: nearest December 21 - the early winter moon; nearest March 20 - the early spring moon; nearest June 20 - the early summer moon; nearest September 22 - the early fall moon. Note: This makes the full moon of that season about two weeks later, always after the 20th or 21st of the month.
  2. Locate the new moons following the early season moons marked above. Mark them as the midseason moons. For example, the new moon that follows the early winter moon is marked as the midwinter moon.
  3. Locate the new moons before the early season moons marked in step 1. Mark them as the late season moons of the previous season. For example, the new moon that precedes the early winter moon is the late fall moon.
  4. Locate all new moons that have not been marked either early, mid, or late moons. These are the blue moons.

Seasons are reversed in the southern hemisphere. Adjust the above instructions for your location.

[edit] Sky and Telescope calendrical misinterpretation

The March 1946 Sky and Telescope article "Once in a Blue Moon" by James Hugh Pruett misinterpreted the 1937 Maine Farmer's 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]

[edit] Blue moons between 2009 and 2015

The following blue moons will occur between 2009 and 2015. These dates use UTC as the timezone; months will vary with different timezones.

[edit] Seasonal

Using the Farmer's 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

[edit] Calendar

Two full moons in one month:[7]

  • 2009: December 2, December 31
  • 2012: August 2, August 31
  • 2015: July 2, July 31

[edit] See also

[edit] References

  1. ^ a b Sinnott, Roger W., Donald W. Olson, and Richard Tresch Fienberg (May 1999). "What's a Blue Moon?". Sky & Telescope. http://skyandtelescope.com/observing/objects/moon/article_127_1.asp. Retrieved on 2008-02-09. "The trendy definition of "blue Moon" as the second full Moon in a month is a mistake." 
  2. ^ Koelbing, Arthur, Ph.D. (1907–21). "Barclay and Skelton: German influence on English literature". The Cambridge History of English and American Literature, Volume III. Bartleby.com. http://www.bartleby.com/213/0414.html. 
  3. ^ Hiscock, Philip (June 19, 2006). "Folklore of the "Blue Moon"". International Planetarium Society. http://www.ips-planetarium.org/planetarian/articles/folkloreBlueMoon.html. 
  4. ^ "What is a "Blue Moon"?". Farmers' Almanac. http://www.farmersalmanac.com/what-is-a-blue-moon. 
  5. ^ Bowling, S. A. (1988). Blue moons and lavender suns. Alaska Science Forum, Article #861 http://www.gi.alaska.edu/ScienceForum/ASF8/861.html
  6. ^ Clarke, Kevin (1999). "on blue moons". InconstantMoon.com. http://www.inconstantmoon.com/cyc_blue.htm. 
  7. ^ Giesen, Jurgen. "Blue Moon". Physik und Astromonie. http://jgiesen.de/moon/BlueMoon/. Retrieved on 2009-01-17. 

[edit] External links

Retrieved from "http://en.wikipedia.org/wiki/Blue_moon"
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