Blue moon

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

A blue moon is defined by the Oxford English Dictionary (OED) as (1) A moon (real, depicted, or imagined) that appears blue; (2) (Since c1820) a long or indefinite length of time; a rarely recurring period or event (“once in a blue moon”); (3) (Since 1946) a second full moon in a calendar month, or formerly (from 1937) the third full moon in a season containing four full moons.[1]

The calendrical meaning of "blue moon" (3) is unconnected to the other meanings. It is often referred to as “traditional”[2], but since no occurrences are known prior to 1937 it is better described as an invented tradition or “modern American folklore”.[3] The practice of designating the second full moon in a month as "blue" originated with amateur astronomer James Hugh Pruett in 1946.[4]

The moon - not necessarily full - can sometimes appear blue due to atmospheric emissions from large forest fires or volcanoes, though the phenomenon is rare and unpredictable (hence the saying “once in a blue moon”).[5][6][7] A calendrical blue moon (by Pruett's definition) is predictable and relatively common, happening 7 times in every 19 years (i.e. once every 2 or 3 years).[1]

Phrase origin[edit]

A 1528 satire, Rede Me and Be Nott Wrothe, contained the lines, “Yf they saye the mone is belewe / We must believe that it is true.”[8] 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 (Middle English, “blue”)[9] with belǽwan (Old English “to betray”)[10]) led to a false etymology for the calendrical term that remains widely circulated, despite its originator having acknowledged it as groundless.[11][12][13]

Percy Bysshe Shelley’s poem "Alastor" (1816) [14] mentioned an erupting volcano[14]: 7  and a “blue moon / Low in the west.” [14]: 14  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 (1821) as the earliest known occurrence of “blue moon” in the metaphorical sense of a long time. (“How's Harry and Ben?—haven't seen you this blue moon.”)[15] 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.’”[16] An earlier (1811) version of the same dictionary had not included the phrase, so it was likely coined some time in the 1810s.[17] "Once in a blue moon" is recorded from 1833.[1]

The use of blue moon to mean a specific calendrical event dates from 1937, when the Maine Farmers' Almanac gave a definition slightly different 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 (if it is not a coinage by the editor, H. P. Trefethen) is unclear.”[1] 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".[18]

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, 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, [the Anglo-Saxons] gave three lunar months to each season. When an embolismic year occurred (that is, one of 13 lunar months) they assigned the extra month to summer, so that three months together bore the name ‘‘Litha’’; hence they called [the embolismic] 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.[19]

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 most 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 later sense, one can occur in any month except February."[1] 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.[20][21]

Actual blue moon[edit]

The moon (and sun[22]) can appear blue under certain atmospheric conditions — for instance, if volcanic eruptions or fires release particles in the atmosphere of just the right size to preferentially scatter red light.[23] According to the Encyclopaedia Britannica, Rayleigh scattering is the cause of “that epitome of rare occurrences, the blue Moon (seen when forest fires produce clouds composed of small droplets of organic compounds).” [5]

A Royal Society report on the 1883 Krakatoa eruption[24] gave a detailed account of “blue, green, and other coloured appearances of the sun and moon” seen in many places for months afterwards.[24]: xiii . The report mentioned that in February 1884 an observer in central America saw the crescent moon as “a magnificent emerald-green” while its ashen part was “pale green”. Venus, bright stars and a comet were also green.[24]: 173  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.[25]

The moon looked blue after forest fires in Sweden and Canada in 1950 and 1951,[26] 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.[25][27]

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".[28]

The key to a blue moon is having many particles slightly wider than the wavelength of red light (0.7 micrometer)—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.[29]

Calendrical Use[edit]

Main article: Intercalary month
A calendrical "blue moon" during the December 2009 lunar eclipse

One lunation (an average lunar cycle) is 29.53 days. There are about 365.24 days in a tropical year. Therefore, about 12.37 lunations (365.24 days divided by 29.53 days) occur in a tropical year. In the widely used Gregorian calendar, there are 12 months (the word month is derived from moon[30]) in a year, and normally there is one full moon each month, with the date of the full moon falling back by nearly one day every calendar month. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles. The extra days accumulate, so every two or three years (seven times in the 19 year Metonic cycle), there is an extra full moon in the year. The extra full moon necessarily falls in one of the four seasons (defined as periods between a solstice and equinox), giving that season four full moons instead of the usual three.[31] This was the 1937 sense of "blue moon", and is sometimes referred to as a "seasonal blue moon".[32][33]

The frequency of a blue moon can be calculated as follows: It is the period of time it would take for an extra synodic orbit of the moon to occur in a year. Given that a year is approximately 365.2425 days and a synodic orbit is 29.5309 days,[34] then there are about 12.368 synodic months in a year. For this to add up to another full month would take 1/0.368 years. Thus it would take about 2.716 years, or 2 years, 8 months, and 18 days for another blue moon to occur. Or approximately once in 32.5 months on an average.

There can be two calendrical blue moons within the same calendar year. This happens approximately four times per century.[35]

On rare occasions in a calendar year (as happened in 2010 in time zones east of UTC+07, and in 2018 in almost every time zone) both January and March each have two full moons, and the month of February has no full moon.[36]

The "seasonal blue moons" of the 1937 Maine Farmers' Almanac referred to seasons of the mean tropical year, equal in length, as opposed to the astronomical seasons which vary in length because the earth's speed in its orbit round the sun is not uniform.

To compare, in 1983 the equal seasons began at 1.48 AM on 23 March, 9.15 AM on 22 June, 4.42 PM on 21 September and 12.10 AM on 22 December, while the astronomical seasons began at 4.39 AM on 21 March, 11.09 PM on 21 June, 2.42 PM on 23 September and 10.30 AM on 22 December (all times GMT).

Blue moons between 2009 and 2037[edit]

The following blue moons occur between 2009 and 2037. These dates use UTC as the time zone; exact dates vary with different time zones.

Using the 1937 Maine Farmers' Almanac definition of blue moon (meaning the third full moon in a season of four full moons, but referenced to astronomical rather than equal seasons), blue moons have occurred (or will occur) on:

  • November 21, 2010
  • August 20, 2013
  • May 21, 2016
  • May 18, 2019
  • Aug 22, 2021[37]
  • Aug 19, 2024
  • May 20, 2027
  • Aug 24, 2029
  • Aug 21, 2032
  • May 22, 2035

Blue moons acording to Pruett's 1946 definition (two full moons in one month, the second of which is a "blue moon"):[38] A blue moon can occur in January and the following March if there is no full moon at all in February, as is the case in the years 1999, 2018, and 2037.

  • 2009: December 2 and 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) and 30, only in time zones east of UTC+04:30
  • 2010: March 1 and 30, only in time zones east of and including UTC+07
  • 2012: August 2 and 31, only in time zones west of and including UTC+10
  • 2012: September 1 and 30, only in time zones east of and including UTC+10:30
  • 2015: July 2 and 31
  • 2018: January 2 and 31 (total lunar eclipse visible in some parts of the world), only in time zones west of and including UTC+11
  • 2018: March 2 and 31, only in time zones west of and including UTC+12
  • 2020: October 1 and 31, only in time zones west of and including UTC+10
  • 2020: November 1 and 30 (penumbral lunar eclipse visible in some parts of the world), only in time zones east of and including UTC+9
  • 2023: August 1 and 30
  • 2028: December 2 and 31 (total lunar eclipse visible in some parts of the world), only in time zones west of UTC+8
  • 2029: January 1 (total lunar eclipse) and 30, only in time zones east of UTC+7
  • 2037: January 1 and 31 (total lunar eclipse visible in some parts of the world), plus March 2 and 31

The next time New Year's Eve falls on a Blue Moon (as occurred on December 31, 2009 in time zones west of UTC+05) is after one Metonic cycle, in 2028 in time zones west of UTC+08. At that time there will be a total lunar eclipse.

See also[edit]

References[edit]

  1. ^ a b c d e "blue moon, n." Oxford English Dictionary. Oxford University Press. September 2023. Retrieved May 3, 2024.
  2. ^ Dobrijevic, Daisy (January 13, 2024). "Blue Moon: What is it and when is the next one?". space.com. Retrieved May 4, 2024. A seasonal Blue Moon is the traditional definition of a Blue Moon and refers to the third full moon in a season that has four full moons according to NASA.
  3. ^ "What is a blue moon? Is the moon ever really blue?". Library of Congress. September 19, 2019. Retrieved May 4, 2024. So, 'blue moon' as most of us today know it, is modern American folklore, but with a long interesting history involving calendars and the measuring of the year. Still, no matter what meaning you give it, blue moons are pretty rare, and everyone knows what you mean when you say "once in a blue moon!"
  4. ^ "What is a Blue Moon". Sky and Telescope. July 27, 2006. Retrieved May 7, 2024. Some three years later, in March 1946, an article entitled "Once in a Blue Moon" appeared in Sky & Telescope (page 3). Its author, James Hugh Pruett (1886-1955), was an amateur astronomer living in Eugene, Oregon, and a frequent contributor to Sky & Telescope. Pruett wrote on a variety of topics, especially fireball meteors. In his article on Blue Moons, he mentioned the 1937 Maine almanac and repeated some of Lafleur's earlier comments. Then he went on to say, "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."
  5. ^ a b The New Encyclopaedia Britannica, Volume 16. Chicago. 2005. p. 602.{{cite book}}: CS1 maint: location missing publisher (link)
  6. ^ Gibbs, Philip (May 1997). "Why is the sky blue?". math.ucr.edu. Retrieved November 4, 2015. ... may cause the moon to have a blue tinge since the red light has been scattered out.
  7. ^ Philip Hiscock (August 24, 2012). "Once in a Blue Moon". Sky & Telescope. Retrieved May 27, 2019.
  8. ^ Roy, William; Barlow, Jerome (1845). Rede Me and Be Nott Wrothe. Chiswick: Charles Whittingham.
  9. ^ "belewe". Oxford English Dictionary. Oxford University Press. September 2023. Retrieved May 4, 2024. belewe, variant of blue, adj. and n.
  10. ^ "Is the "blue" in "blue moon" a reference to betrayal?". StackExchange. Retrieved May 4, 2024. Neither the Oxford English Dictionary (OED), nor the Online Etymology Dictionary provide any support for the idea that the term "blue moon" has any connection to Old English belǽwan "to betray". To the contrary, the OED indicates that the "blue" in "blue moon" is derived from the familar color word, which is a loanword from French (although French in turn got the word from a Germanic language). The OED does indicate that the color word blue was spelled "belewe" in some Middle English manuscript or manuscripts.
  11. ^ Joe Rao, Space.com Skywatching Columnist (May 25, 2007). "The Truth Behind This Month's Blue Moon". Space.com. Retrieved May 4, 2024. Many years ago in the pages of Natural History magazine, I speculated that the rule might have evolved out of the fact that the word "belewe" came from the Old English, meaning, "to betray." "Perhaps," I suggested, "the second full Moon is 'belewe' because it betrays the usual perception of one full moon per month." But as innovative as my explanation was, it turned out to be completely wrong.
  12. ^ "What is a Blue Moon And When Is The Next One?". Farmers’ Almanac. Retrieved May 4, 2024. One explanation connects it with the word belewe from Old English, meaning, "to betray." Perhaps, then, the Moon was "belewe" because it betrayed the usual perception of one full Moon per month? That would make sense.
  13. ^ "What is a blue moon and how often does it occur?". Royal Museums Greenwich. Retrieved May 4, 2024. Quite where the term blue moon came from is unclear. It may be a mispronounciation of the disused word "belewe" which means 'to betray'. This may be a reference to the betrayal of the usual idea of having one full moon in each month or perhaps the "betrayal" by the Moon of worshippers attempting to determine the position and duration of Lent in the calendar year.
  14. ^ a b c Shelley, Percy Bysshe (1816). Alastor; or, The Spirit of Solitude, and Other Poems. London.
  15. ^ Egan, Pierce (1905). Real Life in London. London: Methuen. p. 163.
  16. ^ Grose, Francis; Egan, Pierce (1823). Grose's Classical Dictionary of the Vulgar Tongue. London.
  17. ^ Grose, Francis (1994). The 1811 Dictionary of the Vulgar Tongue. London: Senate.
  18. ^ https://www.projectpluto.com/bluemoon.htm
  19. ^ Wallis, Faith. Bede: The Reckoning of Time (PDF). Liverpool: The University Press. p. 53.
  20. ^ Sinnott, Roger W.; Olson, Donald W.; Fienberg, Richard Tresch (May 1999). "What's a Blue Moon?". Sky & Telescope. Retrieved March 1, 2024.
  21. ^ Hiscock, Philip (August 30, 2012). "Folklore of the 'Blue Moon'". International Planetarium Society. Retrieved March 1, 2024.
  22. ^ Sawyer Hogg, H (1950). "Out of Old Books (Blue Sun)". Journal of the Royal Astronomical Society of Canada. 44: 241. Retrieved May 3, 2024.
  23. ^ Gibbs, Philip (May 1997). "Why is the sky blue?". math.ucr.edu. Retrieved November 4, 2015. ... may cause the moon to have a blue tinge since the red light has been scattered out.
  24. ^ a b c Symons, George James (1888). The Eruption of Krakatoa, and Subsequent Phenomena. London: The Royal Society.
  25. ^ a b Blue Moon Archived March 23, 2010, at the Wayback Machine. science.nasa.gov (July 7, 2004).
  26. ^ Minnaert, M: "De natuurkunde van 't vrije veld" 5th edition Thieme 1974, part I "Licht en kleur in het landschap" par.187; ISBN 90-03-90844-3 (out of print); also see ISBN 0-387-97935-2
  27. ^ Scott Lingley (July 28, 2015). "The year the sun turned blue". Phys.org. Retrieved May 27, 2019.
  28. ^ Scott, Captain R. F. (July 2019). "Scott's Last Expedition". The Journals of Captain R. F. Scott. 12. Retrieved June 22, 2022.
  29. ^ Bowling, S. A. (1988-02-22). Blue moons and lavender suns Archived March 21, 2012, at the Wayback Machine. Alaska Science Forum, Article #861
  30. ^ "Month | Define Month at Dictionary.com". Dictionary.reference.com. Retrieved August 25, 2013.
  31. ^ Plait, Phil There is also a myth that it came from a calendar printer who printed the phases of the moon in white. When it came to pass that there were two full moons in the same month, they colored the second one blue. "Today's full moon is the 13th and last of 2012".{{cite news}}: CS1 maint: multiple names: authors list (link)
  32. ^ Hocken, Vigdis; Kher, Aparna. "What Is a Blue Moon and When Is the Next One?". Time and Date. Retrieved April 26, 2024.
  33. ^ Davis, Lance D. (August 20, 2021). "August 2021 Brings Rare Seasonal 'Blue Moon' - Watch the Skies". blogs.nasa.gov. NASA. Retrieved April 26, 2024.
  34. ^ Espanek, Fred. "Eclipses and the moon's orbit". NASA. Retrieved March 10, 2016.
  35. ^ "Blue moon | Astronomy, Lunar Phases & Folklore | Britannica". Encyclopedia Britannica. Retrieved April 26, 2024.
  36. ^ Harper, David (2018). "The Double Blue Moon of 2018". Obliquity. Retrieved January 22, 2022.
  37. ^ "Sunday's Full Moon is "Blue" by a Different Definition". August 19, 2021.
  38. ^ Giesen, Jurgen. "Blue Moon". Physik und Astromonie. Retrieved January 17, 2009.

External links[edit]

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