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, or the second full moon of a calendar month.[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 on when to insert such an intercalary of 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.

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

Contents

Early English and Christian usage

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]): "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,[6] 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 another Middle English meaning of belewe, which (besides "blue") can mean "betray".[2] The church was responsible for the calendar, and used the complex computus to calculate the important date of Easter based on the full moon. The late winter time before Easter is the Lent period of fasting. Easter falls on the first Sunday after the first full moon of Spring, where the beginning of Spring is fixed at 21 March in the calendar. By the 16th century, before the Gregorian calendar reform, the medieval computus was out of sync with the actual seasons and the real Moon. Occasionally, Spring would have begun and a full moon passed, a month before the computus put the first Spring moon. The clergy would have to tell people whether the full moon was the Easter moon or a false one, which they may have called a "betrayer moon"; after which people would have to continue fasting for another month.

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 the Tyndall effect (in turn, caused by smoke or dust particles in the atmosphere), as has happened after forest fires in Sweden and Canada in 1950 and 1951,[7] 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.[8]

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.[8]

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.[9]

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). 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.[10] 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 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.

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; months vary with different timezones.

Seasonal

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

Calendar

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

Two full moons in one month:[11]

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.

See also

References

  1. ↑ 1.0 1.1 1.2 Sinnott, Roger W., Donald W. Olson, and Richard Tresch Fienberg (May 1999). "What's a Blue Moon?". Sky & Telescope. http://www.skyandtelescope.com/observing/objects/moon/3304131.html?showAll=y&c=y. Retrieved 2008-02-09. "The trendy definition of "blue Moon" as the second full Moon in a month is a mistake." 
  2. ↑ 2.0 2.1 "What is a "Blue Moon"?". Farmers' Almanac. http://www.farmersalmanac.com/what-is-a-blue-moon. 
  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. Bartleby.com. http://www.bartleby.com/213/0414.html. 
  5. ↑ from the old English "rede" [vb.] to advise, to warn, or "rede"[n.] a warning, an injunction )
  6. ↑ Hiscock, Philip (June 19, 2006). "Folklore of the "Blue Moon"". International Planetarium Society. http://www.ips-planetarium.org/planetarian/articles/folkloreBlueMoon.html. 
  7. ↑ 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
  8. ↑ 8.0 8.1 NASA http://science.nasa.gov/headlines/y2004/07jul_bluemoon.htm
  9. ↑ Bowling, S. A. (1988). Blue moons and lavender suns. Alaska Science Forum, Article #861 http://www.gi.alaska.edu/ScienceForum/ASF8/861.html
  10. ↑ Clarke, Kevin (1999). "on blue moons". InconstantMoon.com. http://www.inconstantmoon.com/cyc_blue.htm. 
  11. ↑ Giesen, Jurgen. "Blue Moon". Physik und Astromonie. http://jgiesen.de/moon/BlueMoon/. Retrieved 2009-01-17. 

External links