Blue moon

This article is about the astronomical phenomenon. For other meanings see Blue Moon (disambiguation).

The term blue moon has at least four related meanings. One is a common metaphorical phrase for a rare event. Full moons are given names in folklore, and two definitions of blue moon are a name for a rare full moon that does not have a folk name. One modern blue moon definition is a result of a misinterpretation of the Maine Farmer's Almanac, where a second full moon occurs in a calendar month. The older definition of blue moon is for an extra full moon that occurs in a quarter of the year, which would normally have three full moons but sometimes has four. Oddly, it is the third full moon in a season that has four which is counted as the "extra" other full moon and named blue moon. According to certain folklore, it is said that when there is a blue moon, the moon has a face and talks to the items in its moonlight.

Earliest use in English

The origin of the term blue moon is steeped in folklore, and its meaning has changed and acquired new and interesting meanings and nuances over time. The earliest known recorded usage was in 1362, in a pamphlet entitled Rede Me and Be Not Wrothe: "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]. This implies the expression had a meaning of something that was absurd, and bears close resemblance to another moon-related adage 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".

Visibly blue moon

The most obvious 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 in 1950 and Canada in 1953 and, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years.

Farmer's Almanac blue moons

The older meaning of blue moon to name an extra full moon, as was used in the Maine Farmer's Almanac, was the third full moon in a quarter of the year when there were four full moons – normally a quarter year has three full moons. The division of the year into quarters for this purpose has the dividing line set between March 21 and March 22. This has to do with the rule for setting the date for the Christian Holy Day of Easter, which depends on the last full moon - as calculated by the computus, a somewhat inaccurate formula - on or before the Equinox on March 21, which is also somewhat inaccurate.

This meaning of blue moon was lost when the editors of the original Farmer's Almanac died. It was recovered only when researchers for Sky & Telescope magazine 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.^[1]

Calendar blue moons

In recent times, people have taken to calling a full moon a blue moon based on the Gregorian calendar. By this use of the term, a blue moon is 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 failed in an attempt to infer the earlier definition used in the original Farmer's Almanac (see above). It was helped to popularity when Deborah Byrd of Earth & Sky used the Sky & Telescope definition in the radio series Star Date for some years. As a result, the game Trivial Pursuit used a question and answer about blue moon. Sky & Telescope discovered the error nearly sixty years later and the magazine printed a retraction and correction.^[2] By the time the correction came the calendar definition had already come into common use.

Calendar blue moons occur infrequently, and the saying once in a blue moon is used to describe a rare event. However, they are inevitable because of the mis-match between the solar and lunar cycles. Each calendar year contains twelve full lunar cycles, plus about eleven days to spare. The extra days accumulate, so that while most years contain twelve full moons to match the twelve months, every two or three years there is a year with thirteen full moons. On average, this happens once every 2.72 years. Additionally, in some years 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 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.

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.^[3]

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 2004 and 2010

The following data is based on the Calendar and Farmers' Almanac definitions.

August 2005 — Third full moon in a season of four full moons
June 2007 — has a second full moon falling on the 30th
May 2008 — Third full moon in a season of four full moons
November 2010 — Third full moon in a season of four full moons

References

^ What's a Blue Moon? Sky and Telegraph website.
^ ibid
^ www.obliquity.com

Oxford English Dictionary

External links

[1] What's a Blue Moon? Sky and Telegraph website.

[2] ^ ibid

[obli-3] www.obliquity.com

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