![December 2009 partial lunar eclipse-cropped](https://upload.wikimedia.org/wikipedia/commons/3/37/December_2009_partial_lunar_eclipse-cropped.jpg)
A blue moon is an additional
full moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic coordinate system, ecliptic longitudes of the Sun and Moon opp ...
that appears in a subdivision of a year: the third of four full moons in a
season
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 pol ...
.
The phrase in modern usage has nothing to do with the actual color of the
Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
, although a visually blue Moon (the Moon appearing with a bluish tinge) may occur under certain atmospheric conditions—for instance, if
volcanic eruption
Several types of volcanic eruptions—during which lava, tephra (ash, lapilli, volcanic bombs and volcanic blocks), and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often ...
s or fires release
particles
In the physical sciences, a particle (or corpuscule in older texts) is a small localized object which can be described by several physical or chemical properties, such as volume, density, or mass.
They vary greatly in size or quantity, from su ...
in the
atmosphere
An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A s ...
of just the right
size
Size in general is the Magnitude (mathematics), magnitude or dimensions of a thing. More specifically, ''geometrical size'' (or ''spatial size'') can refer to linear dimensions (length, width, height, diameter, perimeter), area, or volume ...
to preferentially
scatter red light.
Definition
The term has traditionally, in the ''Maine Farmer's Almanac'', referred to an "extra" full moon, where a year which usually has 12 full moons has 13 instead. The "blue moon" reference is applied to the third full moon in a
season
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 pol ...
with four full moons,
thus correcting the timing of the last month of a season that would have otherwise been expected too early. This happens every two to three years (seven times in the
Metonic cycle
The Metonic cycle or enneadecaeteris (from grc, ἐννεακαιδεκαετηρίς, from ἐννεακαίδεκα, "nineteen") is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recu ...
of 19 years).
The author of a March 1946 article in ''
Sky & Telescope
''Sky & Telescope'' (''S&T'') is a monthly American magazine covering all aspects of amateur astronomy, including the following:
*current events in astronomy and space exploration;
*events in the amateur astronomy community;
*reviews of astronomic ...
'' attempted to decipher the traditional practice of the editors of the ''Maine Farmers' Almanac'' by examining old issues of the almanac. Without enough almanacs to see the correct pattern, he conjectured the wrong rule for 'blue moons', which led to the modern colloquial misunderstanding that a blue moon is a second
full moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic coordinate system, ecliptic longitudes of the Sun and Moon opp ...
in a single
solar calendar
A solar calendar is a calendar whose dates indicate the season or almost equivalently the apparent position of the Sun relative to the stars. The Gregorian calendar, widely accepted as a standard in the world, is an example of a solar calendar.
T ...
month, with no link to the order it occurs in a season. The phrase "once in a blue moon" is also used idiomatically, which means once after a long time. An example in a sentence: "My sister lives in Alaska, so I only see her once in a blue moon."
Owing to the rarity of a blue moon, the term "blue moon" is
used colloquially to mean a rare event, as in the phrase "once in a blue moon".
One
lunation
In lunar calendars, a lunar month is the time between two successive syzygies of the same type: new moons or full moons. The precise definition varies, especially for the beginning of the month.
Variations
In Shona, Middle Eastern, and Europ ...
(an average lunar cycle) is 29.53
day
A day is the time period of a full rotation of the Earth with respect to the Sun. On average, this is 24 hours, 1440 minutes, or 86,400 seconds. In everyday life, the word "day" often refers to a solar day, which is the length between two so ...
s. There are about 365.24 days in a
tropical year
A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time fro ...
. Therefore, about 12.37 lunations (365.24 days divided by 29.53 days) occur in a tropical year. In the widely used
Gregorian calendar
The Gregorian calendar is the calendar used in most parts of the world. It was introduced in October 1582 by Pope Gregory XIII as a modification of, and replacement for, the Julian calendar. The principal change was to space leap years dif ...
, there are 12
months (the word ''month'' is derived from ''moon'') 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
The Metonic cycle or enneadecaeteris (from grc, ἐννεακαιδεκαετηρίς, from ἐννεακαίδεκα, "nineteen") is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recu ...
), there is an extra full moon in the year. The extra full moon necessarily falls in one of the four seasons, giving that season four full moons instead of the usual three, and, hence, a "blue" moon.
* In
calculating
A calculation is a deliberate mathematical process that transforms one or more inputs into one or more outputs or ''results''. The term is used in a variety of senses, from the very definite arithmetical calculation of using an algorithm, to th ...
the dates for
Lent
Lent ( la, Quadragesima, 'Fortieth') is a solemn religious observance in the liturgical calendar commemorating the 40 days Jesus spent fasting in the desert and enduring temptation by Satan, according to the Gospels of Matthew, Mark and Luke ...
and
Easter
Easter,Traditional names for the feast in English are "Easter Day", as in the '' Book of Common Prayer''; "Easter Sunday", used by James Ussher''The Whole Works of the Most Rev. James Ussher, Volume 4'') and Samuel Pepys''The Diary of Samuel ...
, Catholic
clergy
Clergy are formal leaders within established religions. Their roles and functions vary in different religious traditions, but usually involve presiding over specific rituals and teaching their religion's doctrines and practices. Some of the ter ...
identified a "Lenten moon". Historically, when the moons arrived too early, they called the early moon a "betrayer" (''belewe'') ''moon'', so the Lenten moon came at its expected time.
*
Folklore
Folklore is shared by a particular group of people; it encompasses the traditions common to that culture, subculture or group. This includes oral traditions such as tales, legends, proverbs and jokes. They include material culture, ranging ...
named each of the 12 full moons in a year according to its time of year. The occasional 13th full moon that came too early for its season was called a "blue moon", so the rest of the moons that year retained their customary seasonal names.
* The ''Maine Farmers' Almanac'' called the third full moon in a season that had four the "blue moon".
* 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, 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.
* Using the common 1946 ''Sky & Telescope'' misunderstanding, when one calendar month has two full moons; the second one is called a "blue moon". 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, so that the second one in each month is called a "blue moon", and the month of February, with only 28 or 29 days, has no full moon. Under this misinterpretation, certain tropical years with 13 full moons, which would normally be said to have one blue moon, can be said to have two blue moons, making blue moons appear to be more frequent.
Etymology
![Blue moon of 31](https://upload.wikimedia.org/wikipedia/commons/5/51/Blue_moon_of_31.08.2012.jpg)
The earliest recorded English usage of the term ''blue moon'' is found in an anti-clerical pamphlet (attacking the Roman clergy, and Cardinal
Thomas Wolsey
Thomas Wolsey ( – 29 November 1530) was an English statesman and Catholic bishop. When Henry VIII became King of England in 1509, Wolsey became the king's almoner. Wolsey's affairs prospered and by 1514 he had become the controlling figur ...
in particular) by two
converted Greenwich friars, William Roy and Jerome Barlow, published in 1528 under the title ''Rede me and be not wrothe, for I say no thynge but trothe''.
The relevant passage reads:
It is not clear from the context that this refers to
intercalation; the context of the passage is a dialogue between two priest's servants, spoken by the character "Jeffrey" (''a brefe dialoge betwene two preste's servauntis, named Watkyn and Ieffraye''). The intention may simply be that Jeffrey makes an absurd statement, "the moon is blue", to make the point that priests require laymen to believe in statements even if they are patently false.
"Betrayer moon" speculation
In 2007, Joe Rao, Skywatching Columnist at
Space.com
Space.com is an online publication focused on space exploration, astronomy, skywatching and entertainment, with editorial teams based in the United States and United Kingdom. The website offers live coverage of space missions, astronomical discov ...
, stated that many years previously he had speculated in
''Natural History'' magazine that the "Blue Moon Rule" might derive from ''belewe'', which he described as an
Old English
Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...
word meaning 'to betray', because the extra full moon "betrays the usual perception of one full moon per month."
He then added that his speculation had been "innovative", but "completely wrong".
[ In 2009, a less detailed version of this speculation was mentioned in ]Farmers' Almanac
''Farmers' Almanac'' is an annual American periodical that has been in continuous publication since 1818. Published by Geiger of Lewiston, Maine, the ''Farmers' Almanac'' provides long-range weather predictions for both the U.S. and Canada. The ...
.
''Maine Farmers' Almanac'' blue moons
In the 19th and early 20th centuries, the ''Maine Farmers' Almanac
Maine () is a state in the New England and Northeastern regions of the United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Canadian provinces of New Brunswick and Quebec to the northeast and nort ...
'' 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 were given to each lunation in a season. The seasons used were those of the mean tropical year
A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky of a celestial body of the Solar System such as the Earth, completing a full cycle of seasons; for example, the time fro ...
, 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). When a season has four full moons the third is called the "blue moon" so that the last can continue to be called with the proper name for that season.
''Sky and Telescope'' calendar misinterpretation
The March 1946 ''Sky and Telescope
''Sky & Telescope'' (''S&T'') is a monthly American magazine covering all aspects of amateur astronomy, including the following:
*current events in astronomy and space exploration;
*events in the amateur astronomy community;
*reviews of astronomi ...
'' 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
''StarDate'' is a science radio program of The University of Texas at Austin McDonald Observatory, broadcast on over 360 radio stations. It is a daily guide to the night sky and breaking astronomical news. Typically heard without formal introdu ...
'' on January 31, 1980 and in a question in the ''Trivial Pursuit
''Trivial Pursuit'' is a board game in which winning is determined by a player's ability to answer trivia and popular culture questions. Players move their pieces around a board, the squares they land on determining the subject of a question t ...
'' game in 1986.
Whole 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 volcanoes have also turned the moon blue. People saw blue moons in 1983 after the eruption of the El Chichón
El Chichón, also known as Chichonal, is an active volcano in Francisco León, Chiapas, Francisco León, north-western Chiapas, Mexico. El Chichón is part of a geologic zone known as the Chiapanecan Volcanic Arc. El Chichón is a complex of dome ...
volcano in Mexico, and there are reports of blue moons caused by Mount St. Helens
Mount St. Helens (known as Lawetlat'la to the indigenous Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United St ...
in 1980 and Mount Pinatubo
Mount Pinatubo is an active stratovolcano in the Zambales Mountains, located on the tripoint boundary of the Philippine provinces of Zambales, Tarlac and Pampanga, all in Central Luzon on the northern island of Luzon. Its eruptive history w ...
in 1991.[Blue Moon](_blank)
. science.nasa.gov (July 7, 2004). In the Antarctic diary of Robert Falcon Scott
Captain Robert Falcon Scott, , (6 June 1868 – c. 29 March 1912) was a British Royal Navy officer and explorer who led two expeditions to the Antarctic regions: the ''Discovery'' expedition of 1901–1904 and the ill-fated ''Terra Nov ...
for July 11, 1911 his entry says, "... the air thick with snow, and the moon a vague blue".
On September 23, 1950, several muskeg
Muskeg (Ojibwe: mashkiig; cr, maskīk; french: fondrière de mousse, lit. ''moss bog'') is a peat-forming ecosystem found in several northern climates, most commonly in Arctic and boreal areas. Muskeg is approximately synonymous with bog or ...
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 1micrometre
The micrometre ( international spelling as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is a unit of length in the International System of Unit ...
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 many particles slightly wider than the wavelength of red light (0.7 micrometer)—and no other sizes present. It 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 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.
Blue moons between 2009 and 2037
The following blue moons occur between 2009 and 2021. These dates use UTC as the time zone; exact dates vary with different time zones.
Seasonal
Using the ''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 on:
* November 21, 2010
* August 20, 2013
* May 21, 2016
* May 18, 2019
* Aug 22, 2021
Calendar
Unlike the astronomical seasonal definition, these dates are dependent on the Gregorian calendar and time zones. These blue moons cannot occur in February.
Two full moons in one month (the second of which is a "blue moon"):
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
A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to the plane of the Earth' ...
visible in some parts of the world), only in time zones west of UTC+05
UTC+05:00 is an identifier for a time offset from UTC of +05:00. This time is used in:
As standard time (Northern Hemisphere winter)
''Principal cities: Islamabad, Karachi''
South Asia
*Pakistan - Time in Pakistan
As standard time (year-rou ...
* 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
UTC+07:00 is an identifier for a time offset from UTC of +07:00. In ISO 8601 the associated time would be written as . It is 7 hours ahead of UTC, meaning that when the time in UTC areas is midnight (00:00), the time in UTC+07:00 areas would ...
* 2012: August 2 and 31, only in time zones west of and including UTC+10
UTC+10:00 is an identifier for a time offset from UTC of +10:00. This time is used in:
As standard time (year-round)
''Principal cities: Brisbane, Gold Coast, Vladivostok, Khabarovsk, Port Moresby, Dededo, Saipan''
North Asia
*Russia – Vl ...
* 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
A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to Ecliptic, the plane of t ...
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 31
* 2026: May 1 and 31
* 2028: December 2 and 31 (total lunar eclipse
A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to Ecliptic, the plane of t ...
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
* 2031: September 1 and 30
* 2034: July 1 and 31
* 2037: January 1 and 31 (total lunar eclipse
A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to Ecliptic, the plane of t ...
visible in some parts of the world), plus March 2 and 31
The next time New Year's Eve
In the Gregorian calendar, New Year's Eve, also known as Old Year's Day or Saint Sylvester's Day in many countries, is the evening or the entire day of the last day of the year, on 31 December. The last day of the year is commonly referred to ...
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
A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to Ecliptic, the plane of t ...
.
See also
* Black moon Black Moon may refer to:
* Black moon, one of four astronomical events involving new or dark moons
* Black Moon (person) (c. 1821–1893), Lakota (American Indian) headman
* Black Moon or Lilith (hypothetical moon), a hypothetical natural satelli ...
* Blood moon
* Wet moon
''Wet Moon'' is a series of graphic novels by Sophie Campbell and published by Oni Press. Primarily set in the fictional southern American college town of Wet Moon, the series stars a large cast of characters, most in their late teens and early ...
* Strawberry moon
References
External links
Once in a blue moon meaning and origin
by The Idioms Dictionary
Information about the August 2012 Blue Moon
"Folklore of the Blue Moon"
by Philip Hiscock
"What's a Blue Moon?"
by Donald W. Olson, Richard T. Fienberg, and Roger W. Sinnott, ''Sky & Telescope''. Article explaining that originally a blue moon meant the 3rd full moon in a season of 4 full moons, and how the "2nd in a month" error began.
{{DEFAULTSORT:Blue Moon
Calendars
Moon myths
Full moon