A blue moon will grace the night sky tonight (Aug. 31), giving skywatchers their last chance to observe this celestial phenomenon for nearly three years.
The moon will wax to its full phase at 9:58 p.m. today, bringing August’s full moon count to two (the first one occurred Aug. 1). Two full moons won’t rise in a single month again until July 2015.
But don’t expect tonight’s full moon to actually appear blue, unless you’re peering through a thick haze of volcanic ash or forest fire smoke. “Blue moon” is not a reference to the satellite’s observed color.
The term has long been used to describe rare or absurd happenings. And farmers once employed it to denote the third full moon in a season — spring, summer, autumn or winter — that has four full moons instead of the usual three. [Photos: The Blue Moon and Full Moons of 2012]
‘The next time you walk outside on a clear night and see the moon smiling down at you, think of Neil Armstrong and give him a wink.’
– Armstrong family statement
This somewhat obscure and complicated definition, in fact, is found in the 1937 edition of the “Maine Farmers’ Almanac.” But in 1946, a writer for “Sky and Telescope” magazine misinterpreted it, declaring a blue moon to be the second full moon in a month with two of them.
Widespread adoption of the new (and incorrect) definition apparently began in 1980, after the popular radio program “StarDate” used it during a show.
Blue moons occur because lunar months are not synched up perfectly with our calendar months. It takes the moon 29.5 days to orbit Earth, during which time we see the satellite go through all of its phases. But every calendar month (except February) has 30 or 31 days, so two full moons occasionally get squeezed into a single month.
Though the phrase “once in a blue moon” suggests the phenomenon is exceedingly rare, that’s not quite the case. On average, blue moons come around once every 2.7 years, making them more common than the Summer Olympics, or a presidential election in the United States.
Some years even boast two blue moons. This last happened in 1999, and it will occur again in 2018.
Tonight’s blue moon also happens to fall on the day of late astronaut Neil Armstrong’s memorial service. Armstrong, who on July 20, 1969 became the first person to set foot on the moon, died Aug. 25 following complications from heart surgery.
So stargazers may want to keep Armstrong’s “one small step” in mind as they gaze up tonight.
“For those who may ask what they can do to honor Neil, we have a simple request,” Armstrong’s family wrote in a statement shortly after his death. “Honor his example of service, accomplishment and modesty, and the next time you walk outside on a clear night and see the moon smiling down at you, think of Neil Armstrong and give him a wink.”
A blue moon can refer to either the third full moon in a season with four full moons, or the second full moon in a month. Most years have twelve full moons that occur approximately monthly. In addition to those twelve full lunar cycles, each solar calendar year contains roughly eleven days more than 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 about when to insert such an intercalary or 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 folklore. Different traditions and conventions place the extra “blue” full moon at different times in the year. In the Hindu calendar, this extra month is called ‘Adhik(extra) masa (month)’. It is also known as purushottam maas, so as to give it a devotional name.
- 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.
- Folklore gave each moon a name according to its time of year. A moon that came too early had no folk name, and was called a blue moon, retaining 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.
A “blue moon” is also used colloquially to mean “a rare event”, reflected in the phrase “once in a blue moon”
Early English and Christian
The earliest recorded English usage of the term “blue moon” was in a 1524 pamphlet violently attacking the English clergy, entitled “Rede Me and Be Not Wrothe” (“Read me and be not angry”; or possibly “Counsel Me and Be Not Angry”): “If they say the moon is belewe / We must believe that it is true” [If they say the moon is blue, we must believe that it is true].
Another interpretation uses another Middle English meaning of belewe, which (besides “blue”) can mean “betray”. By the 18th century, before the Gregorian calendar reform, the medieval computus was out of sync with the actual seasons and the moon, and occasionally spring would have begun and a full moon passed a month before the computus put the first spring moon. Thus, the clergy needed to tell the people whether the full moon was the Easter moon or a false one, which they may have called a “betrayer moon” (belewe moon) after which people would have had to continue fasting for another month in accordance with the season of Lent.
Modern interpretation of the term relates to absurdities and impossibilities; the phrase “once in a blue moon” refers to an event that will take place only at incredibly rare occasions.
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, and 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 Mount St. Helens in 1980 and Mount Pinatubo in 1991.
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 micrometre 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 United States were affected by the following day, and two days later, observers in Britain reported an indigo sun in smoke-dimmed skies, followed by an equally blue moon that evening.
The key to a blue moon is having lots of particles slightly wider than the wavelength of red light (0.7 micrometre) — 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 micrometre, 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.
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). Full moon 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. 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 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.
Blue moons between 2009 and 2016
The following blue moons occur between 2009 and 2016. These dates use UTC as the timezone; exact dates vary with different timezones.
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
It seems that The Farmers Almanac, even though it describes the Sky & Telescope ‘invention’ of the new definition, is now using the new definition of blue moon on its calendar, therefore indicating that the blue moon is August 31, 2012 instead of August 21, 2013.
Unlike the astronomical seasonal definition, these dates are dependent on the Gregorian calendar and time zones.
Two full moons in one month:
- 2009: December 2, December 31 (partial lunar eclipse visible in some parts of the world), only in time zones west of UTC+05.
- 2010: January 1 (partial lunar eclipse), January 30, only in time zones east of UTC+04:30.
- 2010: March 1, March 30, only in time zones east of UTC+07.
- 2012: August 2, August 31, only in time zones west of UTC+08.
- 2015: July 2, July 31