There are some previous questions on this site about the etymology of the phrase "blue moon" (What is the origin of the phrase "blue moon"? Any alternate phrase for it?, Why do we call some full moons "blue" when they're not?). But none of these posts deals with a surprising hypothesis currently mentioned on the Wikipedia article "Blue Moon": that the word "blue" here is somehow connected to the Old English verb belǽwan meaning "to betray":

The suggestion has been made that the term "blue moon" for "intercalary month" arose by folk etymology, the "blue" replacing the no-longer-understood belewe, 'to betray'. The original meaning would then have been "betrayer moon", referring to a full moon that would "normally" (in years without an intercalary month) be the full moon of spring, while in an intercalary year, it was "traitorous" in the sense that people would have had to continue fasting for another month in accordance with the season of Lent.

As far as I can tell, none of the references listed in the Wikipedia article seems to be a reliable source for etymological information, or to reference any more reliable source to back up this etymology.

The first linked reference is an article "What is a Blue Moon" by the staff of Farmers' Almanac that simply says

One explanation connects it with the word belewe from the Old English, meaning, “to betray.” Perhaps, then, the Moon was “belewe” because it betrayed the usual perception of one full Moon per month.

(Note the vague wording here—"One explanation connects it with"—as in the Wikipedia article—"The suggestion has been made").

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.

Does anyone know of any more scholarly etymological sources than the ones that Wikipedia references that discuss this hypothesis?

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    In the Western tradition, Lent is backdated 40 days, not counting Sundays, from Easter Sunday, which falls on the first Sunday after the first full moon after the vernal equinox. There could never be an "extra" month of fasting.
    – KarlG
    Mar 18, 2018 at 21:06
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    Wikrapedia strikes again.
    – choster
    Mar 20, 2018 at 0:51
  • 1
    @KarlG ebareis posts: << Correction: "Easter Sunday, which falls on the first Sunday after the first "new moon" (not "full moon") after the vernal equinox." >> Aug 14, 2019 at 18:14
  • The new moon has nothing to do with the Christian dating of Easter and only marginally with setting the Passover month of Nissan, with Passover falling on 14 Nissan. The equinoctal rule was finally set at the Council of Nicaea.
    – KarlG
    Aug 14, 2019 at 22:38
  • @EdwinAshworth: I'm not super interested in how the date of Easter is calculated, so I might be wrong, but my impression after glancing at the relevant Wikipedia article is that ebareis is incorrect.
    – herisson
    Aug 14, 2019 at 23:02

4 Answers 4


The relationship between "blue" and "betrayer" in describing the periodic extra (thirteenth) moon in one of every three (more exactly 2.7) solar years is based on folklore and is therefore poor in documentation. It appears to be a pun and/or a case of false friends, and it's unclear how old or recent their connection is.

On the historical spellings of "blue" and "betrayer":

The modern English word "blue" has seen various spellings, including:

The modern English word "betrayer" is a cognate of the verb "to betray".

  • The infinitive was at one time in Old English spelled "belæwan" (c 5th–12th century), per the Old English Translator (compiled by a hobbyist).

  • The first-person singular spelling of Old English "betray" is given above as "belæwe". This spelling matches "belewe", save for the Æ in place of the second E. Note that Æ has evolved into AE and then often simply to E in many English words.

From the above history, and knowing the non-standard history of pre-Modern English, one can see there was overlap in similar spellings of "blue" and "betray", despite their ultimate derivations being from two different PIE roots, allowing for the possibility of false-friend association and puns.

On the oft-claimed connecting word, "belewe":

The antiquated and obscure word "belewe" if often cited as a bridge between the terms "blue" and "betrayer":

  • Intat.net (1993-12-03): "The blue moon was originally defined by the Maine Farmers' Almanac as the third full moon in… [Ellipses here are from the source, and no further content or citation is given there.] Some have posited that the name comes from the Old English word belewe, meaning to betray, because the blue moon betrays the usual one full moon per…"

  • Unity Churchill Nursery (1999-03-15): "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."

  • Peter Geiger, Farmers' Almanac 2001 (2000-09-01): "[The beginning of this sentence is not visible in Google Books' preview] …meaning, to betray. Perhaps, then, the moon was belewe because it betrayed the usual perception of one full moon per month."

  • Joe Rao, Curso Objectivo (2003-02-01): "One likely explanation has to do with the Old English word belewe, meaning to betray. The moon, this theory states, is belewe because it betrays the usual perception of one full moon per month."

  • Joe Rao, Space.com via NBC News (2004-07-02): "The phrase 'once in a blue moon' was first noted in 1824 and refers to occurrences that are uncommon, perhaps even rare. … I myself once [no date or source given as to when or where exactly he means at this writing, though note he's the author of the previous quotation from a year prior] suggested 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.'"

  • Greg Alexander, Wikipedia (2008-02-05): "An alternative interpretation uses the other meaning of belewe (which can mean blue or betrayer)[citation here to the Farmers' Almanac website, but no quotation]. The Church was responsible for calculating … the date of Easter … the first Sunday after the [first] full moon [following spring equinox] (the egg moon). The preceding moon (Lent moon …) is also an important Christian time. In years where the Lent or egg moon would [appear] too early, the moon was a betrayer moon, [it] was not to be used to [commemorate] the death and resurrection of Christ. Thus the clergy would tell the people that it was not the Lent moon they were seeing, but a false moon—a betrayer moon."

  • Farmers' Almanac Staff, Farmers' Almanac (2009-08-24): "One explanation [no citation given] 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."

  • Buffalo Sunday News (2010-01-10): "Apparently the astronomical use of blue moon derives not from a color but from an earlier word, belewe, meaning betrayer. The calculation of the date of Easter and the associated Lenten period is determined by lunar cycles. In some years a reasonable date for this religious feast did not satisfy the clergy because of an extra seasonal full moon, so that moon was rejected as a betrayer—a belewe moon."

  • Dave Tabler, Appalachian History (2013-01-01): "…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. 'Yf they saye the mone is belewe, We must beleve that it is true,' says William Barlow, the Bishop of Chichester, in the Treatyse of the Buryall of the Masse, 1528. There are twelve full moons in most years, which means that three full moons occur during most seasons. One season in ten contains four full moons; hence the third moon that occurs in such a season would be the belewe, or blue, moon."

  • Husna Haq, Christian Science Monitor (2015-07-29): "Some [cited with a broken link] have posited that the name comes from the Old English word belewe, meaning to betray, because the blue moon betrays the usual one full moon per month norm, by which farmers once marked the changing of seasons."

  • Jon Austin, Sunday Express (2015-07-31): "…some [no citation] suggest the term blue refers to a betrayer moon from belewe in Old English."

  • Gordon Johnston, NASA Science (2018-01-29): "Originally, the term blue moon referred to the third full moon in a season that has four full Moons. The earliest known reference to a blue moon in the English language is the phrase '…the mone is blewe…' from a pamphlet published in 1528. Some writers [no citation in source at to who] speculate that blue came from belewe, the phrase meant betrayer moon, and that the name referred to how an extra moon in a season confused or betrayed the dates for Lent and Easter. The newer definition of a blue moon as the second full moon in a calendar month dates from 1946."

  • Jeff Parsons, Mirror (2018-02-01): "Normally, a year would have 12 moons, but when a moon appears for the 13th time—an unusual occurrence—it was referred to as a belewe moon, which means betrayer. The earliest English language reference is thought to be found in an anti-clerical pamphlet written in 1528."

From the above quotations, it's unclear whether Joe Rao originated the blue–belewe–betrayer connection, whether a different modern author drew this conclusion for Mr Rao and others to perpetuate, or whether the connection is indeed pre-modern.

  • As KarlG says, there’s no room for “extra” fasting. What if Easter maths is the other way round; a “traitor” moon leads the faithful to finish fasting early… an extra moon signals either a false end or even an early start to the fast? The Easter point, after all, is not X days of fasting, but which days. Else, what have “betrayal” or “traitor” to do with agriculture, please? “False” might describe an extra moon but kills any phonetic link. Jiggling back and forth, isn’t it true that primitive farmers depend on the moon, not a written calendar and certainly not a completed religious almanac? Mar 18, 2018 at 22:46
  • The lunar month is ~29.5 days. Most solar years see 12 full moons, but 29.5 x 12 is ~11.25 days short of ~365.25 days. If your lunar year always counts 12 moons (no leap moon), in a few years, you plant crops early according to solar seasons. The medieval new year was calibrated at spring equinox (Easter), and was preceded by fasting (Lent). An unexpected 13th moon before equinox could bring an extended fast. When the pattern was recognized, a leap moon was introduced.
    – 2540625
    Mar 18, 2018 at 23:18
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    May I ask please why the downvote?
    – 2540625
    Mar 18, 2018 at 23:52
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    Thank you for the helpful links. One thing that I also found helpful was to look at the dates of all of the relevant articles. The Buffalo Sunday News (January 10, 2010), NASA Science (January 29, 2018), The Atlantic (December 31, 2009), UK Mirror (1 FEB 2018), UK Express (Jul 31, 2015), and Christian Science Monitor (JULY 29, 2015) articles were all published after the Wikipedia article was edited to include this hypothesis about the etymology (on 5 February 2008, by an editor who cited the Farmers' Almanac article). So I think it is likely that they learned about this idea from Wikipedia.
    – herisson
    Mar 19, 2018 at 0:51
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    @jtheletter: no medieval calendar in the West was lunar except the Jewish religious calendar. The celebration of Easter was regulated at the Council of Nicaea in 325 to the Alexandrine rule of Sunday after the first full moon after the vernal equinox. It is a lunar date because of Ex 12. No other full moon mattered to the Church. And again, there is no possibility of an "extended fast." The New Year in the Middle Ages: some followed the so-called Annunciation style where the year began on 25 March. Others had other dates.
    – KarlG
    Mar 19, 2018 at 0:53

Blue moon is first recorded in the very early 18th century, and was quite literal and, unsurprisingly, it continues in the present:

1702–A moon (real, depicted, or imagined) that appears blue. On rare occasions the moon can appear distinctly blue owing to the presence of smoke or dust particles in the atmosphere.

1702 This exactly resembles our English Brimstone Butterfly..were it not for those black Spots, and apparent blue Moons in the lower Wings. J. Petiver, Gazophylacii vol. I. 16

1883 The red sun and the blue moon mean higher temperature and general rain. Science 24 August 252/1

1987 If a blue moon sat in the empty sky..would I regret knowing that its source was a rare rising of forest fire dust? BOMB Spring 74

Obviously, blue moons were not common and thus a figurative meaning arose:

2.a. colloquial. A long or indefinite length of time; a rarely recurring period or event.

1821 ‘How's Harry and Ben?—haven't seen you this blue moon.’ [Note] Blue Moon—This is usually intended to imply a long time. P. Egan, Real Life in London vol. I. xiv. 24

That there is a note would imply that the phrase was not well-known at that time.

That said, a short time later:

2.b. once in a blue moon: rarely, exceptionally. Cf. once in a moon at once adv., conj., adj., & n. Phrases P.4.

1833 We are no advocates for the eternal system of producing foreign operas to the exclusion of the works of English composers, but once in a blue moon such a thing may be allowed. Athenæum 16 November 780/1

It was not until the 1930s that the second moon in a calendar month got a name – and even then it was a specific “blue moon”

The “Farmer’s Almanac version appeared in 1937

3. Originally U.S. Originally: the third full moon in a season which (exceptionally) contains four full moons (each season, as defined by the mean sun, normally containing three full moons) (now historical). In later use: a second full moon in a calendar month.

As shown in Sky & Telescope (1999) May 36–8, the later use of the term originated in a misunderstanding of the source of quot. 1937 by the author of quot. 1946.

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, (i.e. two in one calendar month) one can occur in any month except February. This later sense gained currency from its use in a United States radio programme, StarDate, in 1980, and its inclusion in the game Trivial Pursuit in 1986.

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.

1937 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. There are seven Blue Moons in a Lunar Cycle of nineteen years. Maine Farmers' Almanac August 21/2

1946 Dr. L. J. Lafleur quotes an explanation found in the Maine Farmers' Almanac for 1937... Full moons of the year were given names..provided there was only one per month. These names were as follows: Moon after Yule, Wolf Moon, Lenten Moon, [etc.]... But 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.

At this stage we can confidently say that the idea of a long obsolete Old English verb (whose meaning seems to have no relation to the Moon or tradition) lying dormant for seven or eight centuries suddenly, in the 1930s, being linked to the second moon of a calendar month is nil.

  • The 1702 quote seems quite telling - it directly links "on rare occasions" to the colour blue, in a way that fits the idiom much better than something which recurs reliably every two to three years. The real mystery seems to be where the "extra moon in a season" sense came from - perhaps actually adapted from the idiom, by someone looking for a nice name?
    – IMSoP
    Sep 3, 2023 at 20:32

To me the numbers eleven and twelve are the key after all, with the core "leven" meaning left(over) in counting on one's fingers after ten: one left, two left. https://taalaandewandel.com/2021/02/13/waarom-elf-en-niet-eentien/

In Dutch the word "blijven" (Eng.: remain) goes back to "be-lijven", with Germanic origin. In the 13th century found as "bliuen". That comes close to blue and belewe. It has the same core lijven/leven/left. So the blue moon is the full moon that remains, is left over, after the twelfth. Or in a season after the third. See: https://www.etymologiebank.nl/trefwoord/blijven

Actually I found it in English etymology as well:

Blive, belive: From Middle English beliven, from Old English belīfan (“to remain”), from Proto-West Germanic *bilīban, from Proto-Germanic *bilībaną (“to remain”), from Proto-Germanic *bi- + Proto-Indo-European *leyp- (“to stick, glue”).

Cognate with West Frisian bliuwe (“to stay”), Dutch blijven (“to remain”), German bleiben (“to remain”), Danish blive (“to be, remain”). More at leave. Alternative forms

bilive, blive
bleve, bileve, bilave, blewe


Blewe is actually also an obsolete form of blue.

  • 1
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  • 1
    This is all very speculative. It's often easy to come up with multiple plausible etymologies for a word, but finding out which is true requires finding historical evidence, not just comparing surface features of words.
    – IMSoP
    Sep 3, 2023 at 8:36
  • As far as I know there is no definitive proof for the origin of "blue moon". So anything that has been written to suggest or propose a solution is speculation. Including this whole page. And then the similarities are in my opinion not "surface features". That a word for blue could be mistaken or used as pun for a word meaning remain or left over is well supported by the added links. I think they are substantial enough to make it for a "possible solution".
    – Ger
    Sep 3, 2023 at 12:30
  • Yes indeed; but in that case the only answer we can give with any authority is that we don't know - and, as other answers have, present what direct clues we do have. Otherwise, it's just a game of looking through dictionaries until you find a word with roughly the right letters and telling a Just So story about how it connects.
    – IMSoP
    Sep 3, 2023 at 18:54
  • Note that Greybeard's conclusion, if you accept it, would apply equally to this theory as it does to the one in the question: "At this stage we can confidently say that the idea of a long obsolete Old English verb (whose meaning seems to have no relation to the Moon or tradition) lying dormant for seven or eight centuries suddenly, in the 1930s, being linked to the second moon of a calendar month is nil."
    – IMSoP
    Sep 3, 2023 at 20:10

the term "blue moon" for "intercalary month" arose by folk etymology,

the "blue" replacing the no-longer-understood belewe, 'to betray'.

I shouldn't consider myself a scholarly source of course, but it is so glaringly obvious so that it might go as common sense knowledge.

If eleven, twelve contained as is usually thought an element leven, left (en.WT: eleven) and if the intercalary month was precisely those days that were left at the end of a period in calendar (see below), then "belewe" cannot be anything but related to this idiom. be- may be the common prefix. Moon is related to month. This is surface analysis.

It's conceivable that a sense "left" could also come to mean desertion, betrayal, though I do disagree with the -leven theory of eleven, twelve, because it seems literally backwards, counting down – looking forward it requires a sense of "extra" rather than "left".

There is a lot to say about superstition in numerology and religion.

  1. So Judas, one of the twelve apostle often painted as the twelfth in order, is the prototype of a betrayer. However, the unlucky number usually associated with this is thirteen and the origin of this superstition is not clear. For example:

A year with 13 full moons instead of 12 posed problems for the monks in charge of the calendars. "This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar of thirteen months for that year, and it upset the regular arrangement of church festivals. For this reason, thirteen came to be considered an unlucky number."[23]

[en.WP: Thirteen (number)]

  1. It is notable that time keeping was not an exact science. The two-faced Roman god Janus gave it's name to January, technically the eleventh after December (decem- "10") with New Year's occasionally in March. The year "consisted of ten months, beginning in spring with March and leaving winter as an unassigned span of days before the next year" (en.WP: January), "Plutarch's Parallel Lives recounts that Romulus's calendar had been solar but adhered to the general principle that the year should last for 360 days. Months were employed secondarily and haphazardly, with some counted as 20 days and others as 35 or more." "According to Livy, it was Numa Pompilius, the second king of Rome (715–673 BC), who divided the year into twelve lunar months (History of Rome, I.19)." (en.WP: Roman Calender)

It is notable, therefore, that Latin liba is a potential epithet of Janus, "which remains purely hypothetical." (en.WP: Janus), sacrifical cake, cp. λιπάω, libate.

Une des épithètes de Janus est dérivée de l'offrande qu'on lui présentait le plus souvent. Lydus raconte que Yarron, dans le quatorzième livre Rerum divinârum mentionnait cette épithète, et il la traduit par Ποπάνων. Je pense que la forme latine aura été Libo. Il était d'usage de présenter au dieu des liba (πόπανα) les jours des calendes.

[J. S. Speÿer, "Le dieu romain Janus", Revue de l'histoire des religions 26 (1892): 28; emphasis mine, @vectory]

Note also "the Saxon term Wulf-monath (meaning "wolf month")" (en.WT: January) corresponds approximately to Dutch Low Saxon elf, German Low German elf, eleve, ölve, ölven, Old Saxon ellevan (en.WT: PGem. *ainalif "eleven") and English one /w-/. Janus and January were sometimes equated with goddess Juno or IUNO, whose name is possibly cognate to Proto-West-Germanic *aiw-, it may still be difficult to connect the forms of Old Saxon ēo, ēu, ē and Old High German ēo, but there is no good reason either to think of a wolf month. Compare eve, "with a pre-1200 loss of the terminal '-n'" (en.WT) or rather advent, aventura. So, leave can be associated with hollidays, cp. German Urlaub.

  1. On another note, leap year is "Probably from a much older formation related to the Old Norse hlaup-ár (“leap year”), Old English mōnan hlȳp (“moon's leap”) (compare Latin saltus lunae used in reckoning lunar months on a 19-year cycle)." (en.WT: leap year)

The chief competitor of the system which Dionysius introduced into the West was that constructed in the fifth century by Victorius of Aquitaine, which held its ground in Gaul for nearly three hundred years. Both were based on the lunar cycle of nineteen years, but they differed in four points: the earliest permissible date of the vernal new moon, the earliest day after this on which Easter could be kept, the latest day on which Easter could fall, and the place in each cycle in which the lunar year should be shortened by one day (the saltus lunae).

[Poole, Reginald L. “The Earliest Use of the Easter Cycle of Dionysius.” The English Historical Review 33, no. 129 (1918): 57.]

Note that saltus lunae is Medieval not Classical Latin (etymonline: leap year) so it does not constitute independent evidence for leap year, inasmuch as a folk etymology is always subject to interpretation, because: "There was considerable confusion about dates and the systems upon which bishops, abbots, and secular courts depended in the eighth and ninth centuries, and it was a problem to say which calendar applied to any event, or just how it applied." (Stevens, Wesley M. Speculum 78, no. 1 (2003): 144) With German Schaltjahr ("leap year") in mind, we can safely identify saltus as a corrupted form of kalendae / calendae, cp. yell, see also shall and the irregular change of German sollen.

  1. L. linquo "leave" compares better to left (hand side) than levis, light, cp. sinister and sine (if you dpn't know what this means you can safely ignore it and head on over to etymonline).

en.WP: Twelve with one reference to OED ed. 1 and one reference to IEW *-lif, *lib

  • 1
    This is all just speculative, and not "obvious" at all. It's often easy to come up with multiple plausible etymologies for a word, but finding out which is true requires finding historical evidence, not just comparing surface features of words.
    – IMSoP
    Sep 3, 2023 at 8:38
  • Surely the "comparative method" would be to identify words with the same meaning in other languages, and then see what they had in common, not to compare it to other words with similar features, then guess at a connection in meaning. For example, a surface reading makes "flutter by" -> "butterfly" seem quite plausible, but an actual comparison among languages suggests it really is "butter" + "fly". Even if "belew" meaning either "betrayer" or "left" was still used, the connection to "blue moon" would be just as speculative as "flutterby".
    – IMSoP
    Sep 3, 2023 at 9:10
  • @IMSoP that's a completely irrelevant point, unless you are low key suggesting fly was related to leave. If you are saying that "blijven (Eng.: remain) is not "the same meaning" required in naming the reminder of a calculation the problem is clearly on your end.
    – vectory
    Sep 3, 2023 at 17:26
  • It was an example of how similarities that are "glaringly obvious" are as likely to lead to completely wrong folk etymologies as they are to lead to useful insight. It seems "glaringly obvious" that butterflies have nothing to do with butter, so we need to look elsewhere - but it turns out they do. In the same way, you may think it's "glaringly obvious" that the "blue" in "blue moon" is a corruption of a word for "remainder", but it's just as likely that it is complete coincidence, and maybe always was just the colour.
    – IMSoP
    Sep 3, 2023 at 18:51
  • "... it's just as likely that it is complete coincidence, and maybe always was just the colour." Haven't I said that "I do in fact disagree with the -leven theory"? Etymologists hate this trick!
    – vectory
    Sep 3, 2023 at 19:45

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