Once in a blue moon

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From the MIT International Student Office:

Blue moons are best known from the phrase “once in a blue moon,” which means “extremely rarely.” The first recorded use of this idiomatic phrase is in an anti-clerical flyer in 1528, published by William Roy and Jeremy Barlowe. In reference to the clerical corruptions, one said in Old English, “O churche men are wyly foxes […] Yf they say the mone is blewe / We must beleve that it is true / Admittynge their interpretacion.” The context is not one hundred percent clear; but a number of websites interpret this as a reference to priests who required laymen to believe in their statements regardless of how false or ridiculous these were.

A current example would be: “Once in a blue moon I go to a concert, only when there is a singer I really like.”


No matter the actual origin and meaning of the idiom, blue moons are indeed rare.  Since one occurs between 9:37 PM and 4:00 AM tonight (Philly time), and there won't be another till 2037, hie thee to an open spot where you can look toward the east to see it.  Mind you, it won't just be a blue moon, it will be super.

Should we call it a "blue supermoon", a "super blue moon", or…?


Selected readings

[h.t. June Teufel Dreyer]


  1. Robert Coren said,

    August 30, 2023 @ 9:51 am

    This is very likely an example of the Recency Illusion, but I've been hearing "once in a blue moon" (with the meaning "very rarely") all my life, but it seems to me that it's been used to specify the second full moon in a calendar month for only a couple of decades. This is a moderately rare event, and it refers to the moon, so I guess that makes some kind of sense. but since there's no reason why such a moon would look in any way "blue", it seems a little forced to me.

    (Of course the point of the original phrase is that the moon is never actually blue.)

  2. Denis Mair said,

    August 30, 2023 @ 9:52 am

    The quote from the flyer is old-fashioned, but it is not "Old English"; it is probably not even "Middle"(i.e., Chaucerian).

  3. mg said,

    August 30, 2023 @ 10:49 am

    @Denis Mair – Oy! Just sent an email to that office asking for a correction, as an embarrassed alum.

  4. Observer said,

    August 30, 2023 @ 10:58 am

    A blue moon is an astronomical phenomenon where two full moons occur in the same month. Because one lunar cycle is 29.53 days, a blue moon happens roughly every 2-3 years. The next blue moon will actually be in December 2028 or January 2029 (depending on your time zone) and coincide with the New Year's Eve lunar eclipse.

    The August 2023 blue moon is different because it is also a supermoon, where the moon is at perigee (its closest approach to Earth). The next blue moon(s) that will also be a supermoon will be in January 31 and March 1, 2037. (There will be 2 blue moons in 2037 because there will be no full moons in February that year.)

  5. Terry K. said,

    August 30, 2023 @ 11:06 am

    The next blue moon (by the 2nd full moon in a month definition) is May 31, 2026. They happen every 3 years or so (a little less), though sometimes you get two in one year, January and March, because of February only having 28 days.

    What doesn't happen till 2037 is the combination of a blue moon and a supermoon.

  6. Taylor, Philip said,

    August 30, 2023 @ 11:14 am

    "hie thee to an open spot where you can …" — surely "hie thee to an open spot where thou canst … ".

  7. RfP said,

    August 30, 2023 @ 11:15 am

    @Robert Coren:

    This is very likely an example of the Recency Illusion…

    Yes, it apparently is—although there’s a slight wrinkle in that people have relatively recently misconstrued the original, seasonal meaning—as explained in the Blue moon article in Wikipedia:

    The term has traditionally, in the Maine Farmer's Almanac, referred to an "extra" full moon, where a [tropical] year which usually has 12 full moons has 13 instead. The "blue moon" reference is applied to the third full moon in a season 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 of 19 years). The author of a March 1946 article in Sky & Telescope 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 in a single solar calendar 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 an event happens rarely, like the blue moon itself. An example in a sentence: "My sister lives in another state, so I only see her once in a blue moon."

  8. David Marjanović said,

    August 30, 2023 @ 11:22 am

    (Of course the point of the original phrase is that the moon is never actually blue.)

    Probably not in the British Isles or most of the US – but "never" is not quite true. Wikipedia: "The phrase in modern usage has nothing to do with the actual color of the Moon, although a visually blue Moon (the Moon appearing with a bluish tinge) may occur 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.[1]"

    Green occurs, too.

  9. Peter Taylor said,

    August 30, 2023 @ 5:04 pm

    Robert Coren wrote:

    This is very likely an example of the Recency Illusion, but I've been hearing "once in a blue moon" (with the meaning "very rarely") all my life, but it seems to me that it's been used to specify the second full moon in a calendar month for only a couple of decades.

    The magazine Sky and Telescope, which takes a significant part of the blame for this usage, has a much more recent article which suggests that the phrase only spread beyond New England farmers and North American astronomy nerds in the 1980s, and beyond North America in 1988. That's not much more than a couple of decades ago.

    This is a moderately rare event, and it refers to the moon, so I guess that makes some kind of sense. but since there's no reason why such a moon would look in any way "blue", it seems a little forced to me.

    Since it seems to originate from a desire to give every full moon a name ("Harvest Moon", "Egg Moon", "Moon Before Yule", etc.) it is more than a little forced; and since its main purpose nowadays seems to be to provide newspaper editors with padding on a semi-regular basis, there's no reason that you shouldn't refuse to "[admit] their interpretation".

  10. Yerushalmi said,

    August 31, 2023 @ 4:21 am

    My brother pointed out to me once that a blue moon occurs exactly as frequently as the Jewish leap month, which is brilliant.

  11. Terry Hunt said,

    August 31, 2023 @ 5:57 am

    @ David Marjanović — I have myself seen a green moon. It was around 1973, give or take a year, and I assumed at the time that it was caused by a fine haze of smoke in the air, more detectable by smell than sight, from a woodland fire in the vicinity.

    I conjecture that the "blue" of "blue moon" was, long ago, some rarer word, which morphed into a familiar one in the same way that "L'Infanta de Castille" supposedly became "Elephant and Castle" (which may be apocryphal, though I believe there are many similar instances in English), but what word it might have been I have no idea.

  12. Rodger C said,

    August 31, 2023 @ 9:38 am

    the phrase only spread beyond New England farmers and North American astronomy nerds in the 1980s

    Born 1948 here. Definitely not the case.

  13. RfP said,

    August 31, 2023 @ 11:43 am

    I’m not sure how X/Twitter links work in the comments here, but with any luck, here’s a working link to an amazing composite of multi-colored moons taken by “Marcella Giulia Pace, Italian astrophotographer who took ten years to capture 48 colours of the moon #WomensArt (via Amazing Physics)”:


  14. M. Paul Shore said,

    August 31, 2023 @ 11:53 am

    I'm happy to report that the three words "in Old English" have been stricken from the MIT International Student Office webpage quoted in the original post. It's particularly nice to know that a few non-native-English-speaking international students may have been at least vaguely set straight on what is and isn't Old English before they, in some cases, move on to locations outside the Anglosphere. (Students residing more or less permanently within the Anglosphere have a longer time available, and more opportunities, to learn about such things.)

    Regarding the identity of the language of the quotation, Wikipedia seems, in its "Middle English" article, to regard the period 1500 to 1540 as the period of transition from Late Middle English to Early Modern English. (That period was, among other things, close to the center of the main portion of the Great Vowel Shift.)

    By the way, is there anyone else here who thinks that right now the Anglosphere may be about to transition, or has already started to transition, from Late Modern English to Post-Modern English? Note that I'm referring to typical oral language, not the language of educated writing. (I have some specific thoughts as to why this might be the case, which are too lengthy and too off-topic to go into here.)

  15. Victor Mair said,

    August 31, 2023 @ 1:54 pm


    Thanks for the spiral of the colors of the moon!


  16. Rodger C said,

    September 1, 2023 @ 12:53 pm

    I learned, a good while ago, that Middle English ended about a hundred years earlier than teh Wiki asserts: sometime between Chaucer and Malory. Would anyone really say that Malory wrote Middle English?

  17. Rodger C said,

    September 1, 2023 @ 12:53 pm

    Let alone Wyatt?

  18. Philip Anderson said,

    September 1, 2023 @ 3:13 pm

    @Rodger C
    I think some people want to align the linguistic transition from Middle to Modern English with the transition from the Middle Ages to the Modern Era, which in Britain is conventionally dated to 1485, and the start of the Tudor dynasty (in Europe, 1492 is more significant).

  19. Michael Watts said,

    September 2, 2023 @ 9:37 am

    English as spoken on the internet seems to be losing the EN-form of verbs, leading to constructions like "I should have went [somewhere]", but I see no real reason to call this a more transitional period than other periods.

  20. @Michael Watts said,

    September 5, 2023 @ 10:01 am

    I was born in the 1960s and, for as long as I can remember, there have been people who use 'went' instead of 'gone' in such instances (and it's usually "I should of went"). I heard it frequently long before there was such a thing as the internet. (I'm Irish but there's nothing uniquely Irish about it.) However, until the advent of the internet, I just never had any reason to see what emerges when those people put their words in writing.

  21. Pamela said,

    September 5, 2023 @ 10:33 pm

    I'll raise my hand, yes, the language seems to be in a period of transition, or we already transited. it may have happened when people got the idea from New at Six that "impact" is a transitive verb. It's not a matter, I don't think, of people just ignoring standards. As Michael points out (if I'm understanding it), there are venerable dialect differences that nevertheless preserve intelligibility rather than degrading it. "Went" and "gone" are like that in some dialects, I think, as is "aks" for "ask" (and "aks" is actually the original word), and where I grew up we had an all-purpose but very clear very verb, "spect," that so far as I know has never been accepted as standard anywhere. But in what passes for acceptable published English, objective case is on the way out; it is possible that a majority don't know what it is let alone how to use it. "Than" is totally unmoored from logic; "different than" is accepted as standard written English even though "different" is an absolute and cannot be "than" except when comparing one difference to another. The last vestiges of gender are gone (who today would write that a nation or a ship is a "she"?). Nobody seems aware that "man" was a neuter word for an adult (so they don't know where the word "woman" came from) and are wracking their brains trying to think of a jazzy neuter word for an adult. Declensions have developed lacunae. Strong verbs are weaklings; people think the past imperfect is now past perfect, or you can just swap in any apparently past form of a verb regardless of the conditions you actually mean to describe. "we sung" "we have saw" "we have ran" "we swum" (I actually think people say this by analogy to "we snuck," not knowing that "snuck" is not a word) are all over the place. One is expected to accept unintentional opacity and ambiguity as informality. But nobody fixed the spelling, which futurists a half century ago thought would be the first thing to get space-aged.

  22. Taylor, Philip said,

    September 7, 2023 @ 3:23 pm

    Altough there are many aspects of Pamela¹s comment on which I would like to comment, I will restrict myself to just one. Pamela asks « who today would write that […] a ship is a "she" ? ». I would.

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