Everything about Blue Moon totally explained
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's 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.
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, entitled
Rede Me and Be Not Wrothe [Readme and be not angry]: "Yf they say the mone is belewe / We must beleve that it's true"
[Ifthey say the moon is blue, we must believe that it's 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. 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".
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 slightly more than 12 full moons, so in the years with 13 full moons, one moon wouldn't 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.
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. 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:
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.
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.
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.
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. 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.
January 8 - February 5 → Mid Winter moon
February 6 - March 6 → Late Winter moon
March 7 - April 4 → Early Spring moon
April 5 - May 4 → Mid Spring moon
May 5 - June 2 → Blue moon (full Blue Moon on May 19)
June 3 - July 2 → Late Spring moon
July 3 - July 31 → Early Summer moon
August 1 - August 29 → Mid Summer moon
August 30 - September 28 → Late Summer moon
September 29 - October 27 → Early Fall moon
October 28 - November 26 → Mid Fall moon
November 27 - December 26 → Late Fall moon
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 didn't fit the meaning of the term calendar blue moon.
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.
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:
August 19, 2005
May 19, 2008
November 21, 2010
August 21, 2013
If the "Calendar Blue Moon" (1946-1999) definition is taken (meaning the second full moon in any given month) then blue moons occur (External Link
):
June 30, 2007
December 31, 2009
August 31, 2012
July 31, 2015
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.
Further Information
Get more info on 'Blue Moon'.
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