Blue moon
A blue moon refers either to the presence of a second full moon in a calendar month, to the third full moon in a season containing four, or to a moon that appears blue due to atmospheric effects.
The calendrical meaning of "blue moon" is unconnected to the other meanings. It is often referred to as "traditional", but since no occurrences are known prior to 1937 it is better described as an invented tradition or "modern American folklore". The practice of designating the second full moon in a month as "blue" originated with amateur astronomer James Hugh Pruett in 1946. It does not come from Native American lunar tradition, as is sometimes supposed.
The moonnot necessarily fullcan sometimes appear blue due to atmospheric emissions from large forest fires or volcanoes, though the phenomenon is rare and unpredictable. A calendrical blue moon is predictable and relatively common, happening 7 times in every 19 years. Calendrical blue moons occur because the time between successive full moons is shorter than the average calendar month. They are of no astronomical or historical significance, and are not a product of actual lunisolar timekeeping or intercalation.
Phrase origin
A 1528 satire, Rede Me and Be Nott Wrothe, contained the lines, "Yf they saye the mone is belewe / We must beleve that it is true." The intended sense was of an absurd belief, like the moon being made of cheese. There is nothing to connect it with the later metaphorical or calendrical meanings of "blue moon". However, a confusion of belewe with belǽwan ) led to a false etymology for the calendrical term that remains widely circulated, despite its originator having acknowledged it as groundless.Percy Bysshe Shelley's poem "Alastor" mentioned an erupting volcano and a "blue moon / Low in the west." It was written at a time when the eruption of Mount Tambora was causing global climate effects, and not long before the first recorded instances of "blue moon" as a metaphor.
The OED cites Pierce Egan's Real Life in London as the earliest known occurrence of "blue moon" in the metaphorical sense of a long time. An 1823 revision of Francis Grose's Classical Dictionary of the Vulgar Tongue, edited by Egan, included the definition: "Blue moon. In allusion to a long time before such a circumstance happens. 'O yes, in a blue moon'." An earlier version of the same dictionary had not included the phrase, so it was likely coined some time in the 1810s. "Once in a blue moon" is recorded from 1833.
The use of blue moon to mean a specific calendrical event dates from 1937, when the Maine Farmers' Almanac used the term in a slightly different sense from the one now in common use. According to the OED, "Earlier occurrences of the sense given in the Maine Farmers' Almanac have not been traced, either in editions of the Almanac prior to 1937, or elsewhere; the source of this application of the term is unclear." The conjecture of editorial invention is further supported by the spurious explanation the almanac gave:
The Moon usually comes full twelve times in a year, three times in each
season... However, occasionally the moon comes full thirteen times in a year.
This was considered a very unfortunate circumstance, especially by the
monks who had charge of the calendar. It became necessary for them
to make a calendar of thirteen months, and it upset the regular arrangement
of church festivals. For this reason thirteen came to be considered an
unlucky number. Also, this extra moon had a way of coming in each of
the seasons so that it could not be given a name appropriate to the time
of year like the other moons. It was usually called the Blue Moon... In olden times the almanac
makers had much difficulty calculating the occurrence of the Blue Moon
and this uncertainty gave rise to the expression "Once in a Blue Moon".
There is no evidence that an extra moon in a month, season or year was considered unlucky, or that it led to 13 being considered unlucky, or that the extra moon was called "blue", or that it led to the phrase "once in a blue moon". There is good reason to suspect that the 1937 article was a hoax, a practical joke, or simply misinformed. It is however true that the date of the Christian festival of Easter depended on an accurate computation of full moon dates, and important work was done by the monks Dionysius Exiguus and Bede, explained by the latter in The Reckoning of Time, written c725 CE. According to Bede, "Whenever it was a common year, gave three lunar months to each season. When an embolismic year occurred they assigned the extra month to summer, so that three months together bore the name Litha; hence they called year Thrilithi. It had four summer months, with the usual three for the other seasons." The name Litha is now applied by some Neo-Pagans to midsummer.
The 1937 Maine Farmers' Almanac article was misinterpreted by James Hugh Pruett in a 1946 Sky and Telescope article, leading to the calendrical definition of "blue moon" that is now commonly used, i.e. the second full moon in a calendar month. "A blue moon in the original Maine Farmers' Almanac sense can only occur in the months of February, May, August, and November. In the latter sense, one can occur in any month except February." This later sense gained currency from its use in a United States radio programme, StarDate on January 31, 1980 and in a question in the Trivial Pursuit game in 1986.
Several songs have been titled "Blue Moon", seen as a "symbol of sadness and loneliness."
Visually blue moon
The moon can appear blue under certain atmospheric conditions – for instance, if volcanic eruptions or large-scale fires release particles into the atmosphere of just the right size to preferentially scatter red light. According to the Encyclopaedia Britannica, scattering is the cause of "that epitome of rare occurrences, the blue Moon."A Royal Society report on the 1883 Krakatoa eruption gave a detailed account of "blue, green, and other coloured appearances of the sun and moon" seen in many places for months afterwards.. The report mentioned that in February 1884 an observer in central America saw the crescent moon as "a magnificent emerald-green" while its earthlit part was "pale green". Venus, bright stars and a comet were also green. The report authors suspected that green moons were a contrast effect, since in those cases the surrounding sky was seen as red.
People saw blue moons in 1983 after the eruption of the El Chichón volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991.
The moon looked blue after forest fires in Sweden and Canada in 1950 and 1951, 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 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.
Ice particles might have a similar effect. The Antarctic diary of Robert Falcon Scott for July 11, 1911 mentioned "the air thick with snow, and the moon a vague blue".
The key to a blue moon is having many particles slightly wider than the wavelength of red light —and no other sizes present. 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 micrometer, 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.
Calendrical blue moon
Blue moon as a calendrical term originated with the 1937 Maine Farmers' Almanac, a provincial U.S. magazine that is not to be confused with the Farmers' Almanac, Old Farmer's Almanac, or other American almanacs. There is no evidence of "blue moon" having been used as a specific calendrical term before 1937, and it was possibly invented by the magazine's editor, Henry Porter Trefethen. As a term for the second full moon in a calendar month it began to be widely known in the U.S. in the mid-1980s and became internationally known in the late 1990s when calendrical matters were of special interest given the approaching millennium. It created a misapprehension that the calendrical meaning of "blue moon" had preceded the metaphorical one, and inspired various folk etymologies, e.g. the "betrayer" speculation mentioned earlier, or that it came from a printing convention in calendars or a saying in Czech. A 1997 Taiwanese movie, Blue Moon, had the log line "There is usually only one full moon every month, but occasionally there are two – and that second full moon is called the Blue Moon. It is said that when a person sees a blue moon and makes a wish, he will be granted a second chance in things."In 1999 folklorist Philip Hiscock presented a timeline for the calendrical term. First, the August page of the 1937 Maine Farmers' Almanac ran a sidebar claiming that the term was used "in olden times" for an extra full moon in a season, and gave some examples. Six years later, Laurence J. Lafleur quoted the almanac in the U.S. magazine Sky & Telescope in answer to a reader's question about the meaning of "blue moon". Then James Hugh Pruett quoted it again in Sky & Telescope, saying "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". In 1980 the term was used in a U.S. radio program, Star Date, and in 1985 it appeared in a U.S. children's book, The Kids' World Almanac of Records and Facts In 1986 it was included as a question in Trivial Pursuit, and in 1988 a forthcoming blue moon received widespread press coverage.
In 1999 U.S. astronomer Donald W. Olson researched the original articles and published the results in a Sky & Telescope article co-authored with Richard T Fienberg and Roger W. Sinnott. From the examples given by Trefethen in the 1937 Maine Farmers' Almanac they deduced a "rule" he must effectively have used. "Seasonal Moon names are assigned near the spring equinox in accordance with the ecclesiastical rules for determining the dates of Easter and Lent. The beginnings of summer, fall, and winter are determined by the dynamical mean Sun. When a season contains four full Moons, the third is called a Blue Moon." They termed this the "Maine rule" for blue moons, as distinct from Pruett's 1946 definition that was seen to have been a misinterpretation.
In popular astronomy the Maine rule is sometimes called the "seasonal", "true" or "traditional" rule. Blue moons by Pruett's definition are sometimes called "calendar blue moons". The "seasonal" blue moon rule is itself ambiguous since it depends which definition of season is used. The Maine rule used seasons of equal length with the ecclesiastical equinox. An alternative is to use the astronomical seasons, which are of unequal length.
There is also reference in modern popular astrology to "zodiacal blue moons".