the authentic origin of ‘once in a blue moon’









This definition is from Slang and its Analogues Past and Present (1890), by the British lexicographer John S. Farmer (1854-1916):

Once in a blue moon, popular phrase: extremely seldom; an unlimited time; a rarely recurring period.

Synonym of once in a blue moon: every Preston Guild.




Preliminary remark: the phrase once in a blue moon does not refer to blue moon in the sense of a second full moon in a calendar month.


The phrase once in a blue moon is a development from an earlier expression, once in a moon, meaning once a month and by extension occasionally. It is first attested in The Breviary of Health (1547), by the physician and author Andrew Borde (circa 1490-1549):

Madnesse that doth infest a man ones in a mone the which doth cause one to be geryshe, and wauerynge wytted, nat constant, but fantastical
     in contemporary English:
Madness that does infest a man once in a moon, which does cause one to be wayward, and wavering-witted, not constant, but fantastical

The adjective blue seems to have been added as a meaningless fanciful intensive, and, as John S. Farmer explained in the above-mentioned dictionary:

A blue moon, like the Greek Kalends, is something which does not exist. A variant is ‘when two Sundays come in a week.’

The phrase once in a blue moon is therefore comparable to the emphatic expression blue blazes, attested in the early 19th century, in which:
– the plural noun blazes refers to the flames of hell;
– the adjective blue is simply an intensifier, whithout real meaning.

Nothing in the early attestations of blue moon supports the popular theory that it refers to the moon appearing, on rare occasions, distinctly blue owing to the presence of smoke or dust particles in the atmosphere, for example when there are volcanic eruptions or exceptionally large fires. In fact, as late as 14th August 1880, Notes and Queries published the following query from one of its contributors, F. Chance — a query to which no one replied in the affirmative:

A blue moon is, I suppose, a thing that does not exist, like the Greek calends and the horse marines, though in order that “once in a blue moon” may mean “extremely seldom,” as it undoubtedly does, the moon ought occasionally, though extremely rarely, to be seen of a blue colour. I cannot say, however, that I ever have seen it so or heard of its being so seen. Has anybody ever seen the moon look blue?

The term blue moon in the sense of a long or indefinite length of time is first recorded in Real Life in London, or the Rambles and Adventures of Bob Tallyho, Esq. and His Cousin, the Hon. Tom Dashall through the Metropolis (1821), by the sporting journalist and author Pierce Egan (1772-1849):

Their attention was at this moment attracted by the appearance of two persons dressed in the extreme of fashion, who, upon meeting just by them, caught eagerly hold of each other’s hand, and they overheard the following—‘Why, Bill, how am you, my hearty?—where have you been trotting your galloper?—what is your arter?—how’s Harry and Ben?—haven’t seen you this blue moon.’
[footnote:] Blue Moon—This is usually intended to imply a long time.

In Grose’s Classical Dictionary of the Vulgar Tongue, revised and corrected (1823 edition), Pierce Egan wrote:

Blue moon. In allusion to a long time before such a circumstance happens. “O yes, in a blue moon.”

The phrase once in a blue moon is first recorded in The Athenæum, Journal of English and Foreign Literature, Science, and the Fine Arts (London) of 16th November 1833. About the new opera Gustavus the Third, written by Mr Planché and produced at the Theatre Royal, Covent Garden, a critic wrote:

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.




The phrase once in a blue moon, which dates from the 19th century, does not refer to the astronomical reality conveyed by the term blue moon, which first appeared in the Maine Farmers’ Almanac (USA) of August 1937 in the sense of the third full moon in a season which exceptionally contains four full moons (as defined by the mean sun, each season normally contains three full moons). In March 1946, in the American magazine Sky and Telescope, J. H. Pruett, misunderstanding this source, applied the term blue moon to a 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. 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. With respect to the origin of the astronomical term blue moon, the Oxford English Dictionary (3rd edition – 2013) explains:

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.




According to a popular theory, blue in the 20th-century astronomical term blue moon refers to belewe in an isolated 16th-century text: according to this explanation, belewe is an “archaic”* word meaning betrayer — the idea being probably that the full moon in question “betrays” the normal calendar month. This theory is so ludicrous that it is awe-inspiring. First, belewe has never been attested in the sense of betrayer, but only as one of the written forms of three words: blue, belief and believe. Secondly, this explanation is based on a text dating from the 16th century, at a time when the astronomical notion did not exist. Finally, how can belewe mean at the same time blue and betrayer?

(* Those who propagate this asinine theory often prefer to say that belewe is an “Old-English” word, as this patina apparently gives it a semblance of authenticity.)

The text in question is Rede me and be nott wrothe for I saye no thynge but trothe (Read me and be not wroth [= angry] for I say nothing but truth – Strasbourg, 1528), a satire against Cardinal Thomas Wolsey (1474-1530) and the Roman Catholic clergy, in which William Roy and Jerome Barlow wrote, about churchmen:

Agaynst God they are so stobbourne,
That Scripture they tosse and tourne
After their owne ymaginacion:
Yf they saye, the mone is belewe,
We must beleve that it is true,
Admittynge their interpretacion.
     in contemporary English:
Against God they are so stubborn,
That Scripture they toss and turn
After their own imagination:
If they say, the moon is blue,
We must believe that it is true,
Admitting their interpretation.

This specific use of blue moon is only recorded in this particular text: Roy and Barlow probably chose the word blue simply because it rhymes with true.

In fact, in this text, the image of the blue moon is comparable to the phrase to believe that the moon is made of green cheese and variants, first recorded in 1529 and meaning to believe an absurdity. In A Dictionarie of the French and English Tongues (1611), Randle Cotgrave cited the following French proverb:

Il pense que les nues sont pailles d’airain. He thinks the clouds are brazen spangles; like our he thinks the moon is made of green cheese.


I have exposed several other folk etymologies, in particular in the following articles:
origin of ‘Indian summer’ and French ‘été sauvage’
The usual explanation of ‘Hobson’s choice’ is fallacious.
the authentic origin of ‘to rain cats and dogs’
Kilkenny cats
the authentic origin of ‘a pretty kettle of fish’
to buy a pig in a poke vs. to let the cat out of the bag
origin of ‘to buttonhole’ (to detain in conversation)
origin of ‘point-blank’
between the devil and the deep blue sea
meaning and origin of ‘the devil to pay’
origin of ‘to turn a blind eye’.

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