Blue moon
Encyclopedia
A blue moon can refer to the third full moon
Full moon
Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

 in a season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

 with four full moons. 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 lunation
Lunation
Lunation is the mean time for one lunar phase cycle .  It is on average 29.530589 days, or 29 days 12 hours 44 minutes and 3 seconds...

s. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle
Metonic cycle
In astronomy and calendar studies, the Metonic cycle or Enneadecaeteris is a period of very close to 19 years which is remarkable for being very nearly a common multiple of the solar year and the synodic month...

), there is an extra full moon. Lunisolar calendar
Lunisolar calendar
A lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year. If the solar year is defined as a tropical year then a lunisolar calendar will give an indication of the season; if it is taken as a sidereal year then the calendar will...

s have rules about when to insert such an intercalary
Intercalation
Intercalation is the insertion of a leap day, week or month into some calendar years to make the calendar follow the seasons or moon phases. Lunisolar calendars may require intercalations of both days and months.- Solar calendars :...

 or embolismic ("leap") month, and what name it is given; e.g. in the Hebrew calendar
Hebrew calendar
The Hebrew calendar , or Jewish calendar, is a lunisolar calendar used today predominantly for Jewish religious observances. It determines the dates for Jewish holidays and the appropriate public reading of Torah portions, yahrzeits , and daily Psalm reading, among many ceremonial uses...

 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
    Lent
    In the Christian tradition, Lent is the period of the liturgical year from Ash Wednesday to Easter. The traditional purpose of Lent is the preparation of the believer – through prayer, repentance, almsgiving and self-denial – for the annual commemoration during Holy Week of the Death and...

     and Easter
    Easter
    Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

    , 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 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".

Early English and Christian

The earliest recorded English usage of the term "blue moon" was in a 1528 pamphlet violently attacking the English clergy, entitled "Rede Me and Be Not Wrothe" ("Read me and be not angry"; or possibly "Counsel Me and Be Not Angry" ): "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
Middle English
Middle English is the stage in the history of the English language during the High and Late Middle Ages, or roughly during the four centuries between the late 11th and the late 15th century....

 meaning of belewe, which (besides "blue") can mean "betray". By the 16th century, before the Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

 reform, the medieval computus
Computus
Computus is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age....

 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. Thus, the clergy needed to tell the people whether the full moon was the Easter
Easter
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

 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
Lent
In the Christian tradition, Lent is the period of the liturgical year from Ash Wednesday to Easter. The traditional purpose of Lent is the preparation of the believer – through prayer, repentance, almsgiving and self-denial – for the annual commemoration during Holy Week of the Death and...

.

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.

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, and after the eruption of Krakatoa
Krakatoa
Krakatoa is a volcanic island made of a'a lava in the Sunda Strait between the islands of Java and Sumatra in Indonesia. The name is used for the island group, the main island , and the volcano as a whole. The island exploded in 1883, killing approximately 40,000 people, although some estimates...

 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
El Chichón
El Chichón, also known as El Chichonal is an active volcano in Francisco León Municipality in northwestern Chiapas, Mexico. Its only recorded eruptive activity was on March 29, April 3 and April 4, 1982 , when it produced a one km-wide caldera that then filled with an acidic crater lake...

 volcano in Mexico, and there are reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo
Mount Pinatubo
Mount Pinatubo is an active stratovolcano located on the island of Luzon, near the tripoint of the Philippine provinces of Zambales, Tarlac, and Pampanga. It is located in the Tri-Cabusilan Mountain range separating the west coast of Luzon from the central plains, and is west of the dormant and...

 in 1991.

On September 23, 1950, several muskeg
Muskeg
Muskeg is an acidic soil type common in Arctic and boreal areas, although it is found in other northern climates as well. Muskeg is approximately synonymous with bogland but muskeg is the standard term in Western Canada and Alaska, while 'bog' is common elsewhere. The term is of Cree origin, maskek...

 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
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...

 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.

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.

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
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

: 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
Equinox
An equinox occurs twice a year, when the tilt of the Earth's axis is inclined neither away from nor towards the Sun, the center of the Sun being in the same plane as the Earth's equator...

 on or around March 21. This is close to the astronomical season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

 but follows the Christian computus
Computus
Computus is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age....

 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
New moon
In astronomical terminology, the new moon is the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth...

 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
StarDate (radio)
StarDate is a science radio program of the University of Texas at Austin McDonald Observatory.The show began in 1976 as a astronomy telephone message service established by science journalist Deborah Byrd. This evolved at Austin radio station KLBJ-FM into the daily radio program Have You Seen the...

 on January 31, 1980.

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.

Seasonal

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

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:
  • 2009: December 2, December 31 (partial lunar eclipse
    December 2009 lunar eclipse
    A partial lunar eclipse was visible on New Year's Eve, December 31, 2009. It was the last and largest of four minor lunar eclipses in 2009. This lunar eclipse is also notable, because it occurred during a blue moon...

     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
New Year's Eve
New Year's Eve is observed annually on December 31, the final day of any given year in the Gregorian calendar. In modern societies, New Year's Eve is often celebrated at social gatherings, during which participants dance, eat, consume alcoholic beverages, and watch or light fireworks to mark the...

 falls on a Blue Moon (as occurred on 2009 December 31) is after one Metonic cycle
Metonic cycle
In astronomy and calendar studies, the Metonic cycle or Enneadecaeteris is a period of very close to 19 years which is remarkable for being very nearly a common multiple of the solar year and the synodic month...

, in 2028. At that time there will be a total lunar eclipse
December 2028 lunar eclipse
A total lunar eclipse will take place on December 31, 2028.-Saros series:This is the 19th of 26 total lunar eclipses in series 125. The previous occurrence was on December 21, 2010 and the next will occur on January 12, 2047....

.

Popular culture

Blue moons have been referenced in popular culture, such as:
  • In the 2011 movie The Smurfs
    The Smurfs (film)
    The Smurfs is a 2011 American 3D family film based on The Smurfs comic book series created by Peyo and the 1980s animated TV series it spawned. It was directed by Raja Gosnell and stars Neil Patrick Harris, Hank Azaria, Jayma Mays, and Sofía Vergara. It is the first CGI/live-action hybrid film to...

    . 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

  • Black moon
    Black moon
    In astronomy, the term black moon is neither well-known nor frequently used. As a consequence it has no accepted definition, but seems to have occasionally been applied to at least four different situations:-Other names:...

  • Wet moon
    Wet moon
    A wet moon is a lunar phase when the "horns" of the crescent moon point up at an angle, away from the horizon...

  • Full moon names

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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