This article is about the astronomical phenomenon. For other uses, see Blue Moon (disambiguation).
The term blue moon is commonly used metaphorically to describe a rare event, as in the saying "once in a blue moon". Metaphor (from the Greek: μεταφορά - metaphora, meaning "transfer" is language that directly compares seemingly unrelated subjects
A blue moon is actually a name given to an irregularly timed full moon. Full moon is a Lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. Most years have twelve full moons which occur approximately monthly, but each calendar year contains those twelve full lunar cycles plus about eleven days to spare. The extra days accumulate, so that every two or three years there is an extra full moon (this happens every 2. 72 years). Different definitions place the extra moon at different times - the extra moon is called a "blue moon".
The moon has also literally had a visible blue coloring on rare occassions, caused by atmospheric disturbances.
The earliest recorded English usage of the term "Blue moon" was in 1528 in a pamphlet violently attacking the English Clergy, entitled Rede Me and Be Not Wrothe [Read me and be not angry]: "Yf they say the mone is belewe / We must beleve 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, 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") The church was responsible for the calendar and used the complex computus to calculate the most important date of Easter, which is based on the full moon. Computus ( Latin for Computation) is the Calculation of the date of Easter in the Christian calendar. 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. The Clergy were responsible for telling people when it was Lent & Easter - it was critical to celebrate Lent, the trials and resurrection of Christ at the correct time. Every 2 to 3 years the Lent and Egg moons would come too early, the Clergy would have to tell people whether the moon was the Lent moon or a false one - they may have called this a "Betrayer 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. "Sverige" redirects here For other uses see Sweden (disambiguation and Sverige (disambiguation. Country to "Dominion of Canada" or "Canadian Federation" or anything else please read the Talk Page Krakatoa ( Indonesian: Krakatau) also spelled Krakatao or Krakatowa, is a volcanic island in the Sunda Strait
Full moons were given names in folklore, twelve each year, corresponding to times of the year and the related weather and crop needs - with folk names such as Harvest moon, Growing moon, and Snow moon (varying widely with locality & culture - see other full moon names). The harvest moon is the moon at and about the period of fullness that is nearest to the autumnal equinox Full moon is a Lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. A year has slightly more than 12 full moons, so in the years with 13 full moons, one moon would not align with the correct season and was named a blue moon, which then re-aligned the rest of the year's twelve moons (so that corn was planted and harvested at the correct seasonal time, and so on).
The origin of the term blue moon is steeped in folklore, and its meaning has changed and acquired new nuances over time. Some folklore said that when there was a blue moon, the moon had a face and talked to the items in its moonlight.
In the 1800s and early 1900s, the Maine Farmer's Almanac listed Blue Moon dates for farmers. This article is about the Farmers' Almanac; for a similarly titled publication see Old Farmer's Almanac. 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. A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. The second is called the mid summer moon. 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 summer moon.
The division of the year into quarters starts with the nominal Vernal equinox - on or around March 21. An equinox is the event of the Sun passing over the Earth's equator in its annual cycle  This is close to the astronomical seasons but follows the Christian computus used for calculations of Easter (this places each equinox evenly between the Summer & Winter solstices to calculate seasons, rather than using the actual equinox). A season is one of the major divisions of the Year, generally based on yearly periodic changes in Weather. Computus ( Latin for Computation) is the Calculation of the date of Easter in the Christian calendar. Solstices occur twice a year when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes An equinox is the event of the Sun passing over the Earth's equator in its annual cycle
Some naming conventions keep the moon's seasonal name for its entire cycle - from its appearance as a new moon, through the full moon in the middle, to the next new moon. This article is about the lunar phase for other uses see New Moon (disambiguation. 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:
Seasons are reversed in the southern hemisphere. Adjust the above instructions for your location.
This definition was used until 1946, when the dates & meaning provided by the Farmer's Almanac were misinterpreted in popular products (after its editors had died), and was only recovered in 1999.  According to this calculation the year 2008 has one blue moon that occurs in the spring.
For the year 2008, these are the dates of the moons in the northern hemisphere. These dates use the actual solstices and not the artificial solstices that give each season an equal number of days.
From 1946, people started calling a full moon a blue moon if it was the second of two full moons to occur in the same calendar month. This definition of blue moon originated from a mistake in an article in the March 1946 Sky & Telescope magazine, which misinterpreted the dates & meaning provided by the Farmer's Almanac. Sky & Telescope (S&T is an American monthly Magazine covering all aspects of Amateur astronomy, including current events in This article is about the Farmers' Almanac; for a similarly titled publication see Old Farmer's Almanac. It was helped to popularity when Earth & Sky used this definition in the radio series Star Date for some years, and as a result the game Trivial Pursuit used it in a question and answer about blue moon. Since its initial broadcast in 1991 the Earth & Sky radio series has presented daily information about Science and Nature to radio listeners from around Trivial Pursuit is a Board game in which progress is determined by a player's ability to answer general knowledge and Popular culture questions It was recovered only in 1999 when researchers for Sky & Telescope magazine discovered the error. Sky & Telescope (S&T is an American monthly Magazine covering all aspects of Amateur astronomy, including current events in They noticed that the Maine Farmer's Almanac from 1829 to 1937 reported blue moons that did not fit the meaning of the term calendar blue moon. This article is about the Farmers' Almanac; for a similarly titled publication see Old Farmer's Almanac.  Sky & Telescope printed a retraction and correction though by this time the calendar definition was in common use. Sky & Telescope (S&T is an American monthly Magazine covering all aspects of Amateur astronomy, including current events in
Calendar Blue moons occur infrequently, in years with thirteen full moons. There are also some years in which there is no full moon in February at all, since February is slightly shorter than the time from one full moon to the next. This condition, known as a black moon, gives additional 'blue' moons in the preceding and following months (namely January and March). In Astronomy, the term black moon is neither well-known nor frequently used The last time this occurred was in 1999, and the next occurrence will be in 2018, according to UTC.
The previous calendar blue moon (based on UTC) was on June 30, 2007. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, is defeated and killed by troops of the Usurper Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. The first full moon would have occurred on June 1, 2007. Events 193 - Roman Emperor Didius Julianus is Assassinated 987 - Hugh Capet is elected Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. That was May 31, 2007 in the Western Hemisphere, making that full moon the second occurrence in May in the Western Hemisphere (see below); the next calendar blue moon will be December 31, 2009. Events 1279 BC - Rameses II (The Great (19th dynasty becomes pharaoh of Ancient Egypt. Year 2007 ( MMVII) was a Common year starting on Monday of the Gregorian calendar in the 21st century. Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia. This article is about the year For the film see 2009 Lost Memories.
Occasionally whether a moon is called blue depends on the time zone. Any full moon occurs simultaneously everywhere, but at that moment clocks and calendars are not the same. Full moon is a Lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun.
Example, when it is early evening on August 31 in Europe, it is already early morning September 1 in New Zealand. New Zealand is an Island country in the south-western Pacific Ocean comprising two main landmasses (the North Island and the South Island Hence, residents of London seeing a full moon when their clocks and calendar say it is August 31 would call what they see a calendar blue moon. London ( ˈlʌndən is the capital and largest urban area in the United Kingdom. People seeing the same full moon from Auckland would note by their clocks and calendar that it is the early morning of September 1, and they would not term it a blue moon. The Auckland metropolitan area or Greater Auckland, in the North Island of New Zealand, is the largest and most populous urban area in the country But they would probably have a calendar blue moon at the end of September, or perhaps October. 
Because this is confusing, astronomers worldwide and the calendar makers who rely on them typically choose the time zone of the Royal Greenwich Observatory in the United Kingdom, known as Greenwich Mean Time, or the nearly identical UTC time zone. The Royal Observatory Greenwich (formerly the Royal Greenwich Observatory or RGO) was commissioned in 1675 by King Charles II, with the The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom, the UK or Britain,is a Sovereign state located Greenwich Mean Time ( GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London As a practical matter, because the moon seems to the casual viewer to be full for almost three days, the use of a foreign time zone for calendar markings for full moons makes little difference.
The following Blue Moons occur between 2005 and 2015. These dates use UTC as the timezone, months will vary with different timezones.
Using the Farmer's Almanac definition of blue moon (meaning the third full moon in a season of four full moons) blue moons occur:
If the "Calendar Blue Moon" (1946-1999) definition is taken (meaning the second full moon in any given month) then blue moons occur :
Note that the year 2018 (as well as 2037, 1961, 1942, 1999 etc. Events 43 BC - Octavian, later known as Augustus compels the Roman Senate to elect him Consul. Year 2005 ( MMV) was a Common year starting on Saturday (link displays full calendar of the Gregorian calendar. Events 1535 - French explorer Jacques Cartier sets sail on his second voyage to North America with three ships 110 men and 2008 ( MMVIII) is the current year in accordance with the Gregorian calendar, a Leap year that started on Tuesday of the Common Events 164 BC - Judas Maccabaeus, son of Mattathias of the Hasmonean family restores the Temple in Jerusalem. For the film see 2010 The Year We Make Contact. For the book see 2010 Odyssey Two. Events 1192 - Minamoto Yoritomo becomes Seii Tai Shōgun and the De facto ruler of Japan. 2013 ( MMXIII) will be a Common year starting on Tuesday of the Gregorian calendar. Events 350 - Roman usurper Nepotianus, of the Constantinian dynasty, is defeated and killed by troops of the Usurper Events 406 – Vandals, Alans and Suebians cross the Rhine, beginning an invasion of Gallia. Events 1056 - Byzantine Empress Theodora becomes ill dying suddenly a few days later without children to succeed the Throne Events 30 BC - Battle of Alexandria: Mark Antony achieves a minor victory over Octavian 's forces but most of his army subsequently ) will have a black moon (no full moon in the short month of February) - and this results in 2 blue moons in January and March.