Blue moon

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An ESA radio telescope located at the New Norcia Station, Western Australia, tracking the "super blue moon" of August 30, 2023. The blue colour is an effect of the camera. Chasing Chandrayaan and the super blue Moon ESA25063943.gif
An ESA radio telescope located at the New Norcia Station, Western Australia, tracking the "super blue moon" of August 30, 2023. The blue colour is an effect of the camera.

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. [2]

Contents

The calendrical meaning of "blue moon" is unconnected to the other meanings. It is often referred to as “traditional”, [3] [4] but since no occurrences are known prior to 1937 it is better described as an invented tradition or “modern American folklore”. [5] The practice of designating the second full moon in a month as "blue" originated with amateur astronomer James Hugh Pruett in 1946. [6] It does not come from Native American lunar tradition, as is sometimes supposed. [7] [8]

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”). [9] [10] [11] 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). [2] Calendrical blue moons occur because the time between successive full moons (approximately 29.5 days) is shorter than the average calendar month. [12] 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.” [13] 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”) [14] with belǽwan (Old English “to betray”) [15] ) led to a false etymology for the calendrical term that remains widely circulated, despite its originator having acknowledged it as groundless. [16] [17] [18]

Percy Bysshe Shelley’s poem "Alastor" (1816) [19] mentioned an erupting volcano [19] :7 and a “blue moon / Low in the west.” [19] :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.”) [20] 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.’” [21] An earlier (1811) version of the same dictionary had not included the phrase, so it was likely coined some time in the 1810s. [22] "Once in a blue moon" is recorded from 1833. [2]

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 (if it is not a coinage by the editor, H. P. Trefethen) is unclear.” [2] 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". [4]

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. [23]

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." [2] 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. [24] [25]

Several songs have been titled "Blue Moon", seen as a "symbol of sadness and loneliness." [25]

Visually blue moon

The moon (and sun [26] ) 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. [10] According to the Encyclopaedia Britannica, 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).” [9]

A Royal Society report on the 1883 Krakatoa eruption [27] gave a detailed account of “blue, green, and other coloured appearances of the sun and moon” seen in many places for months afterwards. [27] :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. [27] :173 The report authors suspected that green moons were a contrast effect, since in those cases the surrounding sky was seen as red. [27] :203

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. [28]

The moon looked blue after forest fires in Sweden and Canada in 1950 and 1951, [29] 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. [28] [30]

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". [31]

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. [32]

Calendrical blue moon

A calendrical "blue moon" during the December 2009 lunar eclipse December 2009 partial lunar eclipse-cropped.jpg
A calendrical "blue moon" during the December 2009 lunar eclipse

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 (1887-1957). [2] 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. [33] 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.” [34]

In 1999 folklorist Philip Hiscock presented a timeline for the calendrical term. [35] 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 (21 November 1915, 22 August 1918, 21 May 1921, 20 February 1924, 21 November 1934, 22 August 1937, and 21 May 1940). Six years later, Laurence J. Lafleur (1907-66) quoted the almanac in the U.S. magazine Sky & Telescope (July 1943, page 17) in answer to a reader’s question about the meaning of “blue moon”. Then James Hugh Pruett (1886-1955) quoted it again in Sky & Telescope (March 1946, p3), 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 (with Pruett’s definition) 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 (“What is a blue moon? When there are two full moons in a month, the second one is called a blue moon. It is a rare occurrence.”) [36] In 1986 it was included as a question in Trivial Pursuit (likely taken from the children’s book), 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.” [37] [38] 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”, [39] “true” [40] or “traditional” [41] rule (though of course no tradition of it exists prior to 1937). Blue moons by Pruett’s definition are sometimes called “calendar blue moons”. [42] 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 (March 21). 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”. [43]

Blue moon dates

The table below has blue moon dates and times (UTC) calculated according to Pruett’s “calendar” rule (second full moon in a calendar month) and two versions of the “seasonal” rule (third full moon in a season with four). The Maine rule uses equal-length seasons defined by the dynamical mean sun, and is presumed to have been the original rule of Trefethen. [38] The “astro-seasonal” rule uses the unequal astronomical seasons defined by the apparent sun. All calculations are by David Harper. [44]

The fourth column shows blue moon dates that were actually printed in the Maine Farmers’ Almanac, as found by Olson, Fienberg and Sinnott in 1999. They studied issues published between 1819 and 1962, and found that all mentions occurred between 1937, when H.P. Trefethen introduced the term, and 1956, when Trefethen’s editorship ended (consistent with it being Trefethen’s own invention). Occasional discrepancies between the Maine rule and the almanac’s printed dates can be ascribed to clerical errors or miscalculation. In one case (August 1945) Trefethen appears to have used the apparent rather than mean sun. [37]

The table shows that in 200 years there are 187 full moons that could be called "blue" by some definition - an average of nearly one per year. Two Pruett blue moons can occur in a single year (1915, 1961, 1999, 2018, 2037, 2094). 1915 had four blue moons (two Pruett, one Maine, one astro-seasonal). 1934 and 2048 have three (one of each type).

Despite the 187 blue moons appearing across the 200 years in this table, only 146 years have any of these 3 types of blue moons, leaving 54 years (thus averaging just over 1 year in every 4) which have none of the 3 rules represented in that calendar year.

While not totally unexpected (given the overlapping frequencies of these 3 rules), it so happens there are not any 2 sequential years (at least within these 200) wherein none of the 3 types of blue moon occur.

Conversely, despite the preponderance of years with blue moons (of at least 1 type) occurring in this 200-year range, there are no instances of more than 4 sequential years having a blue moon, of any of these 3 types -- i.e. at least 1 year out of every 5 sequential years has none of the 3 types appearing.

YearPruett rule [45] Maine rule [46] Astro-Seasonal [46] Almanac [37]
1901Jul 31 at 10:35
1902May 22 at 10:46May 22 at 10:46
1904Mar 31 at 12:45
1905Feb 19 at 18:52Feb 19 at 18:52
1906Nov 30 at 23:07
1907Aug 23 at 12:15Aug 23 at 12:15
1909Aug 31 at 05:06
1910Aug 20 at 19:14Aug 20 at 19:14
1912May 30 at 23:29
1913Feb 21 at 02:03May 20 at 07:18
1915Jan 31 at 04:42
1915Mar 31 at 05:36Nov 21 at 17:36Aug 24 at 21:40Nov 21
1917Sep 30 at 20:31
1918Aug 22 at 05:02Aug 22 at 05:02Aug 22
1920Jul 30 at 23:17
1921May 21 at 20:15May 21 at 20:15May 21
1923Apr 30 at 21:31
1924Feb 20 at 16:07May 18 at 21:52Feb 20
1925Oct 31 at 17:15
1926Aug 23 at 12:38Aug 23 at 12:38summer
1928Aug 31 at 02:35
1929May 23 at 12:50Aug 20 at 09:42spring
1931May 31 at 14:31
1932Feb 22 at 02:07May 20 at 05:09winter
1933Dec 31 at 20:53
1934Mar 31 at 01:15Nov 21 at 04:26Aug 24 at 19:37Nov 21
1936Sep 30 at 20:59
1937Aug 22 at 00:47Aug 22 at 00:47Aug 22
1939Jul 31 at 06:37
1940May 21 at 13:33May 21 at 13:33May 21
1942Apr 30 at 21:58
1943Feb 20 at 05:45May 19 at 21:13Feb 20
1944Oct 31 at 13:36
1945Nov 19 at 15:13Aug 23 at 12:03Aug 23
1947Aug 31 at 16:33
1948May 23 at 00:37Aug 19 at 17:32May 23
1950May 31 at 12:44
1951May 21 at 05:45May 21 at 05:45May 21
1952Dec 31 at 05:06
1953Nov 20 at 23:12Aug 24 at 20:21Nov 20
1955Oct 31 at 06:03
1956Aug 21 at 12:38Aug 21 at 12:38Aug 21
1958Jul 30 at 16:46
1959May 22 at 12:56May 22 at 12:56
1961Jan 31 at 18:46
1961Apr 30 at 18:41Nov 22 at 09:44
1962Feb 19 at 13:18May 19 at 14:32
1963Nov 30 at 23:55
1964Nov 19 at 15:43Aug 23 at 05:25
1966Aug 31 at 00:13
1967May 23 at 20:22Aug 20 at 02:27
1969May 31 at 13:17
1970May 21 at 03:38May 21 at 03:38
1971Dec 31 at 20:18
1972Nov 20 at 23:07Nov 20 at 23:07
1974Oct 31 at 01:20
1975Aug 21 at 19:48Aug 21 at 19:48
1977Jul 30 at 10:52
1978May 22 at 13:17May 22 at 13:17
1980Mar 31 at 15:13
1981Feb 18 at 22:58Feb 18 at 22:58
1982Dec 30 at 11:31
1983Nov 20 at 12:29Aug 23 at 14:59
1985Jul 31 at 21:40
1986Aug 19 at 18:54Aug 19 at 18:54
1988May 31 at 10:54
1989Feb 20 at 15:32May 20 at 18:16
1990Dec 31 at 18:36
1991Nov 21 at 22:56Nov 21 at 22:56
1993Sep 30 at 18:54
1994Aug 21 at 06:47Aug 21 at 06:47
1996Jul 30 at 10:35
1997May 22 at 09:13May 22 at 09:13
1999Jan 31 at 16:05
1999Mar 31 at 22:50
2000Feb 19 at 16:27Feb 19 at 16:27
2001Nov 30 at 20:49
2002Nov 20 at 01:34Aug 22 at 22:29
2004Jul 31 at 18:05
2005Aug 19 at 17:53Aug 19 at 17:53
2007Jun 30 at 13:48
2008Feb 21 at 03:30May 20 at 02:11
2009Dec 31 at 19:11
2010Nov 21 at 17:27Nov 21 at 17:27
2012Aug 31 at 13:56
2013Aug 21 at 01:44Aug 21 at 01:44
2015Jul 31 at 10:41
2016May 21 at 21:14May 21 at 21:14
2018Jan 31 at 13:27
2018Mar 31 at 12:35
2019Feb 19 at 15:53May 18 at 21:11
2020Oct 31 at 14:47
2021Nov 19 at 08:57Aug 22 at 12:02
2023Aug 31 at 01:35
2024Aug 19 at 18:25Aug 19 at 18:25
2026May 31 at 08:44
2027Feb 20 at 23:23May 20 at 10:59
2028Dec 31 at 16:48
2029Nov 21 at 04:02Aug 24 at 01:51
2031Sep 30 at 18:56
2032Aug 21 at 01:46Aug 21 at 01:46
2034Jul 31 at 05:54
2035May 22 at 04:25May 22 at 04:25
2037Jan 31 at 14:01
2037Mar 31 at 09:53
2038Feb 19 at 16:09May 18 at 18:23
2039Oct 31 at 22:36
2040Nov 18 at 19:05Aug 22 at 09:09
2042Aug 31 at 01:59
2043Aug 20 at 15:04Aug 20 at 15:04
2045May 30 at 17:51
2046May 20 at 03:15May 20 at 03:15
2048Jan 31 at 00:13Nov 20 at 11:19Aug 23 at 18:06
2050Sep 30 at 17:31
2051Aug 22 at 01:34Aug 22 at 01:34
2053Jul 30 at 17:03
2054May 21 at 15:16May 21 at 15:16
2056Mar 31 at 10:22
2057Feb 19 at 11:56May 18 at 19:02
2058Oct 31 at 12:51
2059Nov 19 at 13:09Aug 23 at 09:41
2061Aug 30 at 22:17
2062Aug 20 at 03:55Aug 20 at 03:55
2064May 30 at 10:34
2065May 20 at 02:05May 20 at 02:05
2066Dec 31 at 14:40
2067Mar 30 at 20:07Nov 20 at 23:49Nov 20 at 23:49
2069Sep 30 at 18:06
2070Aug 21 at 19:53Aug 21 at 19:53
2072May 31 at 22:15
2073May 21 at 10:02May 21 at 10:02
2075Apr 30 at 18:35
2076Feb 19 at 23:48May 18 at 17:38
2077Oct 31 at 10:36
2078Nov 19 at 12:52Aug 23 at 08:11
2080Jul 31 at 19:12
2081Aug 19 at 11:15Aug 19 at 11:15
2083May 31 at 09:41
2084May 20 at 02:36May 20 at 02:36
2085Dec 30 at 23:57
2086Nov 20 at 20:12Nov 20 at 20:12
2088Sep 30 at 15:25
2089Aug 21 at 06:15Aug 21 at 06:15
2091Jul 30 at 11:58
2092May 21 at 10:00May 21 at 10:00
2094Jan 31 at 12:35
2094Apr 30 at 13:54
2095Feb 19 at 06:59May 19 at 09:21
2096Oct 31 at 11:13
2097Nov 19 at 13:03Aug 22 at 23:52
2099Aug 30 at 17:55
2100Aug 19 at 21:29Aug 19 at 21:29

Remarks

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. So the date of the full moon falls back by nearly one day every calendar month on average. Each calendar year contains roughly 11 days more than the number of days in 12 lunar cycles, 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 (however defined), giving that season four full moons instead of the usual three. [47] [48] [49]

Given that a year is approximately 365.2425 days and a synodic orbit is 29.5309 days, [50] 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 Pruett blue moon to occur. Or approximately once in 32.5 months on an average.

When there are two Pruett blue moons in a single year, the first occurs in January and the second in March or April. [51] [52]

The next time New Year's Eve falls on a Pruett 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

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A season is a division of the year based on changes in weather, ecology, and the number of daylight hours in a given region. On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant. Various cultures define the number and nature of seasons based on regional variations, and as such there are a number of both modern and historical definitions of the seasons.

The Borana calendar is a calendrical system once thought to have been used by the Borana Oromo, a people living in southern Ethiopia and northern Kenya. The calendar has been claimed to be based on an earlier Cushitic calendar, developed around 300 BC found at Namoratunga. However, reconsideration of the Namoratunga site led astronomer and archaeologist Clive Ruggles to conclude that there is no relationship. The Borana calendar consist of 29.5 days and 12 months for a total 354 days in a year. The calendar has no weeks but name for each day of the month. It is a lunar-stellar calendar system.

<span class="mw-page-title-main">Supermoon</span> Full or new moon which appears larger

A supermoon is a full moon or a new moon that nearly coincides with perigee—the closest that the Moon comes to the Earth in its orbit—resulting in a slightly larger-than-usual apparent size of the lunar disk as viewed from Earth. The technical name is a perigee syzygy or a fullMoon around perigee. Because the term supermoon is astrological in origin, it has no precise astronomical definition.

Maya astronomy is the study of the Moon, planets, Milky Way, Sun, and astronomical phenomena by the Precolumbian Maya Civilization of Mesoamerica. The Classic Maya in particular developed some of the most accurate pre-telescope astronomy in the world, aided by their fully developed writing system and their positional numeral system, both of which are fully indigenous to Mesoamerica. The Classic Maya understood many astronomical phenomena: for example, their estimate of the length of the synodic month was more accurate than Ptolemy's, and their calculation of the length of the tropical solar year was more accurate than that of the Spanish when the latter first arrived. Many temples from the Maya architecture have features oriented to celestial events.

<span class="mw-page-title-main">Astronomical basis of the Hindu calendar</span> Applied astronomy of ancient India

The Hindu calendar is based on a geocentric model of the Solar System. A geocentric model describes the Solar System as seen by an observer on the surface of the Earth.

References

  1. "Chasing Chandrayaan and the super blue Moon". www.esa.int. The European Space Agency. August 31, 2023. Retrieved September 21, 2024. Note that the blue colour surrounding the Moon in these images is the result of atmospheric and camera effects. The Moon itself does not change colour.
  2. 1 2 3 4 5 6 "blue moon, n." Oxford English Dictionary. Oxford University Press. September 2023. Retrieved May 3, 2024.
  3. 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.
  4. 1 2 Hannikainen, Diana (August 29, 2023). "The Moon is "Blue" this Wednesday... Or is it?". Sky & Telescope. Retrieved May 8, 2024. The 1937 Maine Farmer's Almanac reveals the traditional meaning of "Blue Moon."
  5. "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!"
  6. "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."
  7. Barbuzano, Javier (January 1, 2023). "Native American Full Moon Names". Sky & Telescope. Retrieved May 20, 2024. While this name is not Native American, it is included here for completeness.
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  10. 1 2 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.
  11. Philip Hiscock (August 24, 2012). "Once in a Blue Moon". Sky & Telescope. Retrieved May 27, 2019.
  12. Harper, David (2023). "Once in a Blue Moon". obliquity.com. Retrieved May 14, 2024.
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  14. "belewe". Oxford English Dictionary. Oxford University Press. September 2023. Retrieved May 4, 2024. belewe, variant of blue, adj. and n.
  15. "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 familiar 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.
  16. 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.
  17. "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.
  18. "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.
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  23. Wallis, Faith. Bede: The Reckoning of Time (PDF). Liverpool: The University Press. p. 53.
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  25. 1 2 Hiscock, Philip (August 30, 2012). "Folklore of the 'Blue Moon'". International Planetarium Society . Retrieved March 1, 2024.
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  33. Hiscock, Philip (August 24, 2012). "Blue Moons, Origins and History". Sky & Telescope. Retrieved May 10, 2024. I've checked with several native speakers of Czech and found nothing like "blue Moon." Secondly, an e-mail correspondent told me several years ago that calendar printers would always print a full Moon in red except when it was the second one in a month; then it would be printed in blue! That sounded wonderfully plausible -— until I looked at older calendars and found none that were so.
  34. "Blue Moon". IMDb. September 1, 1997. Retrieved May 10, 2024.
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  36. McLoone, Margo; Siegel, Alice; Rosenbaum, Richard (1985). The Kids' World Almanac of Records and Facts, Volume 1. New York: World Almanac Publications. p. 67.
  37. 1 2 3 Olson, Donald W.; Fineberg, Richard Tresch; Sinnott, Roger W. (May 1999). "What's A Blue Moon". Digital Library. Sky & Telescope. Retrieved May 10, 2024.
  38. 1 2 Olson, Donald W.; Fineberg, Richard Tresch; Sinnott, Roger W. (July 27, 2006). "What is a Blue Moon in Astronomy?". Sky & Telescope. Retrieved May 10, 2024.
  39. McClure, Bruce (May 20, 2016). "How Often A Seasonal Blue Moon?". Earth Sky. Retrieved May 10, 2024. It's a seasonal Blue Moon: the third of four full moons to occur in a single season.
  40. "'True' Blue Moon Occurs Sunday, August 22nd". Sky & Telescope. August 19, 2021. Retrieved May 10, 2024. The full Moon of Sunday, August 22nd, will be a "Blue Moon" according to the original — but not the most popular — definition of the phrase.
  41. "Blue Moon for Halloween". Sky & Telescope. October 27, 2020. Retrieved May 10, 2024. Editors and contributors to Sky & Telescope have traced the traditional astronomical definition to the Maine Farmers' Almanac in the late 1930s.
  42. "Blue Super Moon 2023". The Free Press Journal. August 30, 2023. Retrieved May 10, 2024. There are two kinds of blue moons, but they don't actually look blue. One type is called a calendar blue moon, and the other type is a seasonal blue moon.
  43. "Zodiac-Seasonal Blue Moon Information". LunarLiving.org. May 10, 2024. Retrieved May 10, 2024. The zodiac blue moons can occur in any month between the 18th to the 23rd day of the month.
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  45. Harper, David (2024). "Blue moon calculator". obliquity.com. Retrieved May 14, 2024.
  46. 1 2 Harper, David (2024). "Four Full Moons in a Season". obliquity.com. Retrieved May 14, 2024.
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  50. Espanek, Fred. "Eclipses and the moon's orbit". NASA. Retrieved March 10, 2016.
  51. Harper, David (2018). "The Double Blue Moon of 2018". Obliquity. Retrieved January 22, 2022.
  52. Giesen, Jurgen. "Blue Moon". Physik und Astromonie. Retrieved January 17, 2009.