Blue moon

From Wikipedia, the free encyclopedia

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".

A blue moon is actually a name given to an irregularly timed full moon. 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".

  • 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.
  • The Farmer's Almanac defined blue moon as an extra full moon that occurred in a season(which would normally have three full moons). If a season had 4 full moons, the 3rd full moon was named a blue moon.
  • Until 1999, the common definition of blue moon was the 2nd full moon in a calendar month (this was a misinterpretation of the Maine Farmer's Almanac in 1946 that became commonly accepted and was discovered in 1999).

The moon has also literally had a visible blue coloring on rare occassions, caused by atmospheric disturbances.

Contents

[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,[1] 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,[2] 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")[3] 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. 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".

[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.

[edit] Folklore

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). 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.

[edit] Farmer's Almanac blue moons

In the 1800s and early 1900s, 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 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.[4] 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).

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. 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 2 weeks later, always after the 20th or 21st of the month.
  2. Locate the new moons following the ones marked above. Mark them as the mid season moons. For example, the new moon that follows the early winter moon is marked as the mid winter moon. This is most often in January.
  3. Locate the new moons before the ones 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. This is most often in November.
  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.

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.[5] 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.

  1. January 8 - February 5 → Mid Winter moon
  2. February 6 - March 6 → Late Winter moon
  3. March 7 - April 4 → Early Spring moon
  4. April 5 - May 4 → Mid Spring moon
  5. May 5 - June 2 → Blue moon (full Blue Moon on May 19)
  6. June 3 - July 2 → Late Spring moon
  7. July 3 - July 31 → Early Summer moon
  8. August 1 - August 29 → Mid Summer moon
  9. August 30 - September 28 → Late Summer moon
  10. September 29 - October 27 → Early Fall moon
  11. October 28 - November 26 → Mid Fall moon
  12. November 27 - December 26 → Late Fall moon

[edit] Calendar blue moons

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. 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. It was recovered only in 1999 when researchers for Sky & Telescope magazine discovered the error. 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.[5] Sky & Telescope printed a retraction and correction though by this time the calendar definition was in common use.

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). 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. The first full moon would have occurred on June 1, 2007. 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.

[edit] Time zone problems

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.

Example, when it is early evening on August 31 in Europe, it is already early morning September 1 in New Zealand. 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. 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. But they would probably have a calendar blue moon at the end of September, or perhaps October.[6]

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. 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.

[edit] Blue Moons between 2005 and 2015

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 [1]:

Note that the year 2018 (as well as 2037, 1961, 1942, 1999 etc.) 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.


[edit] See also

[edit] References

[edit] Notes

  1. ^ 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.
  2. ^ Hiscock, Philip (19 June 2006). Folklore of the "Blue Moon". International Planetarium Society.
  3. ^ What is a "Blue Moon"?. Farmers' Almanac.
  4. ^ Clarke, Kevin (1999). on blue moons. InconstantMoon.com.
  5. ^ a b Sinnott, Roger W., Donald W. Olson, and Richard Tresch Fienberg (May 1999). What's a Blue Moon?. Sky & Telescope. Retrieved on 2008-02-09. “The trendy definition of "blue Moon" as the second full Moon in a month is a mistake.”
  6. ^ Harper and Stockman (13 October 2007). The Blue Moon of 2007. Obliquity.com.

[edit] External links