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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 the name given to the third full moon in a season which has four, as most seasons only have three full moons. This happens every 2.72 years. Recent popular usage defines a blue moon as the second full moon in a month. This came about from a mistake made by the author of the March 1946 edition of Sky and Telescope, in which he misinterpreted a page of the 1937 Maine Farmers' Almanac, and defined a blue moon as the second full moon in a month.

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

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 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,[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 1 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".

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.

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 either 12 or 13 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 folktales say that when there is a blue moon, the moon had a face and talked to those in its light.

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 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.

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 second-full-Moon-in-a-month definition followed its use on the popular radio program StarDate on January 31, 1980. [5]

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.

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:

  • August 19, 2005
  • May 19, 2008
  • November 21, 2010
  • August 21, 2013

Calendar

Two full moons in one month: [6]

  • 2007, June 1, June 30
  • 2009, December 2, December 31
  • 2012, August 2, August 31
  • 2015, July 2, July 31

Black Moon

The years 2018 and 2037 will have a black moon, meaning there will be no full moon in the short month of February and blue moons in both January and March.

See also

References

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.{{cite web}}: CS1 maint: multiple names: authors list (link)
  2. ^ Hiscock, Philip (June 19, 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. ^ Sinnott, Roger W., Donald W. Olson, and Richard Tresch Fienberg (1999). "What's a Blue Moon?". Sky & Telescope. Retrieved 2008-02-09. The trendy definition of "blue Moon" as the second full Moon in a month is a mistake. {{cite web}}: Italic or bold markup not allowed in: |publisher= (help); Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
  6. ^ Giesen, Jurgen. "Blue Moon". Physik und Astromonie. Retrieved 2009-01-17.
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