Blue Moon
Blue moon
A blue moon is a thirteenth full moon in a year. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. Lunisolar calendars have rules on when to insert such an intercalary of embolismic (”leap”) month, and what name it is given; e.g. in the Hebrew calendar the month Adar is duplicated. The term “blue moon” comes from folk lore. Different traditions and conventions place the extra “blue” full moon at different times in the year.
- In calculating the dates for Lent and Easter, the Clergy identify the Lent Moon. It is thought that historically when the moon’s timing was too early, they named an earlier moon as a “betrayer moon” (belewe moon), thus the Lent moon came at its expected time.[1]
- 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 – bringing the correct seasonal timings for future moons.
- The Farmers’ Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.
- Recent popular usage defined a blue moon as the second full moon in a calendar month, stemming from an interpretation error made in 1946 that was discovered in 1999.[2] For example, December 31, 2009 was a blue moon according to this usage.
A “blue moon” is also used colloquially to mean “a rare event”, especially in the phrase “once in a blue moon”.
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Early English and Christian usage
The earliest recorded English usage of the term “blue moon” was in a 1528 pamphlet violently attacking the English clergy,[3] entitled “Rede Me and Be Not Wrothe” (”Read me and be not angry”; or possibly “Counsel Me and Be Not Angry” [4]): “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,[5] 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 another Middle English meaning of belewe, which (besides “blue”) can mean “betray” [1]. The church was responsible for the calendar and used the complex computus to calculate the important date of Easter, which is based on the full moon. The late winter time before Easter is the Lent period of fasting. Easter falls on the first Sunday after the first full moon of Spring, where the beginning of Spring is fixed at 21 March in the calendar. By the 16th century, before the Gregorian calendar reform, the medieval computus was out of sync with the actual seasons and the real Moon. Occasionally, Spring would have begun and a full moon passed, a month before the computus put the first Spring moon. The clergy would have to tell people whether the full moon was the Easter moon or a false one, which they may have called a “betrayer moon”; after which people would have to continue fasting for another month.[citation needed]
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 1951.[6], and, notably, after the eruption of Krakatoa in 1883, which caused the moon to appear blue for nearly two years. Other less potent volcanos have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichon volcano in Mexico, and there are reports of blue moons caused by Mt. St. Helens in 1980 and Mount Pinatubo in 1991.[7]
On September 23, 1950, several muskeg fires that had been smoldering for several years in Alberta, Canada suddenly blew up into major—and very smoky—fires. Winds carried the smoke eastward and southward with unusual speed, and the conditions of the fire produced large quantities of oily droplets of just the right size (about 1 micron in diameter) to scatter red and yellow light. Wherever the smoke cleared enough so that the sun was visible, it was lavender or blue. Ontario, Canada and much of the east coast of the U.S. were affected by the following day, and two days later, observers in England reported an indigo sun in smoke-dimmed skies, followed by an equally blue moon that evening.[7]
The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micron)–and no other sizes present. This is rare, but volcanoes sometimes produce such clouds, as do forest fires. Ash and dust clouds thrown into the atmosphere by fires and storms usually contain a mixture of particles with a wide range of sizes, with most smaller than 1 micron, and they tend to scatter blue light. This kind of cloud makes the moon turn red; thus red moons are far more common than blue moons.[8]
Farmers’ Almanac blue moons
In the 19th and early 20th centuries, the Maine Farmers’ 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 midsummer moon, and 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 moon.
The division of the year into quarters starts with the nominal vernal equinox on or around March 21.[9] This is close to the astronomical season but follows the Christian computus used for calculations of Easter, which places the equinox at a fixed date in the (Gregorian) calendar.
Some[weasel words] naming conventions[citation needed] keep the moon’s seasonal name for its entire cycle, from its appearance as a new moon through the full moon 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.
Sky and Telescope calendar misinterpretation
The March 1946 Sky and Telescope article “Once in a Blue Moon” by James Hugh Pruett misinterpreted the 1937 Maine Farmers’ 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 definition of a “blue moon” as the second full moon in a month followed its use on the popular radio program StarDate on January 31, 1980.[2]
Blue moons between 2009 and 2016 for the Northern Hemisphere
The following blue moons will occur between 2009 and 2016. These dates use UTC as the timezone; months will vary with different timezones.
Seasonal
Using the Farmers’ Almanac definition of blue moon (meaning the third full moon in a season of four full moons), blue moons occur
- November 21, 2010
- August 21, 2013
- May 21, 2016
Calendar
Two full moons in one month:[10]
- 2009: December 2; December 31 (combined with a partial lunar eclipse visible in some parts of the world)
- 2012: August 2, August 31
- 2015: July 2, July 31
The next time New Year’s Eve will fall on a Blue Moon (as occurred on 2009 December 31) is after one Metonic cycle, in 2028. At that time there will be a total lunar eclipse.
Note that, unlike the astronomical seasonal definition, these dates are dependent on the Gregorian calendar and time zones. The full moon on 2009 December 31 at 19:13 UTC occurred early the next day (2010 January 1 at 0:13 UTC+5) in eastern countries from central Asia east to the International Date Line (IDL). In these locations the calendar blue moon occurred on 2010 January 30 at 11:18 UTC+5 or 20:18 UTC+14.
Furthermore, a second calendar full moon following that in late January will occur on 2010 February 28 at 21:38 UTC+5, but early the next day (2010 March 1 at 0:38 UTC+8) in countries from eastern Asia east to the IDL. Thus in these locations there will be an additional calendar blue moon on 2010 March 30 at 10:25 UTC+8 or 16:25 UTC+14.
The Blue Moons of 2009 and 2010
Our Blue Moon Calculator says that in 2009, there is a Blue Moon in December. You may have heard or read reports which said that the Blue Moon really occurs in January 2010 or March 2010. Who is right?
Well, you could see Blue Moons in December 2009 or in January 2010 or March 2010, but it depends where in the world you live.
Let’s start with the basic astronomical facts. The Full Moons from December 2009 to March 2010 are as follows:
| 2 December 2009 | at | 07:30 GMT |
| 31 December 2009 | at | 19:13 GMT |
| 30 January 2010 | at | 06:17 GMT |
| 28 February 2010 | at | 16:38 GMT |
| 30 March 2010 | at | 02:25 GMT |
These dates and times have been calculated rigorously using the same methods as those employed by the United States Naval Observatory and by Her Majesty’s Nautical Almanac Office. They are given in Greenwich Mean Time (GMT) which is the standard time zone used by astronomers worldwide.
| City | Time zone | Full Moon 1 | Full Moon 2 | Full Moon 3 | Full Moon 4 | Full Moon 5 |
|---|---|---|---|---|---|---|
| Places East of Greenwich | ||||||
| Auckland¹ | GMT + 13 hours | Dec 2 20:30 |
Jan 1 08:13 |
Jan 30 19:17 |
Mar 1 05:38 |
Mar 30 15:25 |
| Sydney¹ | GMT + 11 hours | Dec 2 18:30 |
Jan 1 06:13 |
Jan 30 17:17 |
Mar 1 03:38 |
Mar 30 13:25 |
| Tokyo | GMT + 9 hours | Dec 2 16:30 |
Jan 1 04:13 |
Jan 30 15:17 |
Mar 1 01:38 |
Mar 30 11:25 |
| Beijing Perth |
GMT + 8 hours | Dec 2 15:30 |
Jan 1 03:13 |
Jan 30 14:17 |
Mar 1 00:38 |
Mar 30 10:25 |
| Ho Chi Minh (Saigon) |
GMT + 7 hours | Dec 2 14:30 |
Jan 1 02:13 |
Jan 30 13:17 |
Feb 28 23:38 |
Mar 30 09:25 |
| Mumbai | GMT + 5 hours 30 minutes | Dec 2 13:00 |
Jan 1 00:43 |
Jan 30 11:47 |
Feb 28 22:08 |
Mar 30 07:55 |
| Islamabad | GMT + 5 hours | Dec 2 12:30 |
Jan 1 00:13 |
Jan 30 11:17 |
Feb 28 21:38 |
Mar 30 07:25 |
| Moscow | GMT + 3 hours | Dec 2 10:30 |
Dec 31 22:13 |
Jan 30 09:17 |
Feb 28 19:38 |
Mar 30 05:25 |
| Johannesburg | GMT + 2 hours | Dec 2 09:30 |
Dec 31 21:13 |
Jan 30 08:17 |
Feb 28 18:38 |
Mar 30 06:25 |
| Paris | GMT + 1 hour | Dec 2 08:30 |
Dec 31 20:13 |
Jan 30 07:17 |
Feb 28 17:38 |
Mar 30 05:25 |
| The Greenwich Meridian | ||||||
| London | GMT | Dec 2 07:30 |
Dec 31 19:13 |
Jan 30 06:17 |
Feb 28 16:38 |
Mar 30 02:25 |
| Places West of Greenwich | ||||||
| Rio de Janeiro¹ | GMT - 2 hours | Dec 2 05:30 |
Dec 31 17:13 |
Jan 30 04:17 |
Feb 28 14:38 |
Mar 30 00:25 |
| New York Toronto |
GMT - 5 hours | Dec 2 02:30 |
Dec 31 14:13 |
Jan 30 01:17 |
Feb 28 11:38 |
Mar 29 21:25 |
| Chicago Mexico City |
GMT - 6 hours | Dec 2 01:30 |
Dec 31 13:13 |
Jan 30 00:17 |
Feb 28 10:38 |
Mar 29 20:25 |
| Calgary Tucson |
GMT - 7 hours | Dec 2 00:30 |
Dec 31 12:13 |
Jan 29 23:17 |
Feb 28 09:38 |
Mar 29 19:25 |
| Los Angeles Vancouver |
GMT - 8 hours | Dec 1 23:30 |
Dec 31 11:13 |
Jan 29 22:17 |
Feb 28 08:38 |
Mar 29 18:25 |
| Hawaii | GMT - 10 hours | Dec 1 21:30 |
Dec 31 09:13 |
Jan 29 20:17 |
Feb 28 06:38 |
Mar 29 16:25 |
| ¹ These places observe daylight saving time. | ||||||
As you can see, Europe, Africa, the Americas and Asia west of India have a Blue Moon in December 2009, whilst central Asia has a Blue Moon in January 2010.
Acknowledgements
We thank Australian reader Jim Cahill for alerting us to the fact that the citizens of Western Australia voted not to switch to daylight saving time in 2009, so Perth is in the same time zone as Beijing, not Tokyo. Happily, they still get a double Blue Moon in 2010!
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Last modified on 29 January 2010
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