once in a blue moon

The popular story

Ask where once in a blue moon comes from and most people reach for astronomy. They point either to the rare occasions when the moon really does appear bluish – tinted by high ash or smoke after a great eruption or a vast forest fire – or to the calendar curiosity now called a “blue moon”, meaning a second full moon falling within a single calendar month. On this view the phrase simply borrows a genuinely infrequent celestial event and presses it into service as a byword for rarity.

It is a reasonable guess, and both of those blue moons are real. But the history runs the other way. The saying is older than the astronomy usually invoked to explain it, and its “blue” began not as a colour report at all, but as a flat statement of the impossible.

What the record shows

The oldest relevant use points not to rarity but to absurdity. In the anti-clerical pamphlet Rede me and be nott wrothe, printed at Strasbourg in 1528 by William Roy and Jerome Barlow, two speakers mock the blind credulity that, they complain, the clergy demand of ordinary people:

Yf they saye, the mone is belewe, / We must beleve that it is true.

Here “the moon is blue” is a byword for a manifest falsehood – the sort of claim only a fool would swallow, in the same family as later sayings like “when pigs fly”. Nobody thought the moon was ever really blue; that was the whole point. To do something “when the moon is blue” was, in effect, to do it never, and the phrase carried a sting of ridicule aimed at anyone gullible enough to be told otherwise.

The exact modern phrase, with its softened meaning of “rarely”, is first recorded much later. The Athenæum of 16 November 1833, reviewing the London musical stage, uses it casually and without any gloss:

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.

That the writer feels no need to explain the phrase, and expects the reader to catch its sense at once, shows it was already well established in speech. Word historians connect it to the older expression once in a moon – that is, once a month, and by extension “now and then” or “occasionally” – with “blue” added as a fanciful, essentially meaningless intensive rather than as a description of the moon’s colour. The blue, in other words, is decorative emphasis, the same impulse that gives us “in a blue funk” or “once in a coon’s age”.

The verdict

This is an attested idiom, with two firmly dated staging posts: the 1528 precursor “the moon is blue”, meaning an absurdity or impossibility, and the 1833 first record of “once in a blue moon” in its present sense of “rarely”. What is not supported is the widespread assumption that the phrase was coined from the astronomical or calendrical “blue moon”. The familiar calendar sense – a second full moon in a month – is itself a relatively modern development, popularised through a twentieth-century misreading of an American almanac and spread by a magazine article, and so it cannot possibly be the source of a phrase already in comfortable use in print in 1833. The idiom lent its name to the astronomy, not the other way round.

What we don’t know

The precise route between 1528 and 1833 is not fully mapped, and the shift from “impossible or absurd” to “merely infrequent” was gradual rather than datable to a single moment or author. We also cannot say who first attached “blue” to the older “once in a moon”, only that the compound was current and unremarkable by the 1830s. These gaps are entirely ordinary for a colloquial phrase that lived for generations in speech before anyone thought it worth writing down.

How it changed

The trajectory is one of steady softening. In the sixteenth century a blue moon was an impossibility, and to promise something “when the moon is blue” was a polite way of refusing it outright. Over time the emphasis slid from impossible to merely very seldom, helped along by the older “once in a moon” with its natural monthly rhythm suggesting infrequency rather than never. By the early nineteenth century the phrase had settled into the mild, good-humoured sense of rarity we use now, shedding almost entirely the sharp edge of mockery it once carried against the credulous.

Usage today

Today once in a blue moon is a gentle, faintly wistful way of saying “hardly ever”: we clear out the loft once in a blue moon, or hear from an old friend once in a blue moon. It carries no astronomy and no insult, only the sense of something pleasantly or regrettably rare. When a genuine astronomical “blue moon” turns up in the newspapers every few years, it keeps the picture fresh in people’s minds – but the meaning we actually reach for was fixed in the language long before that calendar sense had been invented.

The phrase is a good case study in how idioms and the facts behind them can drift apart and then be reconnected by mistake. Speakers reach for “once in a blue moon” to mean simple rarity, with no thought of eruptions, ash or the timing of full moons; the astronomical “blue moon” is a separate and much later coinage that happens to share the words. When a magazine or news bulletin ties the two together – explaining the saying by reference to the calendar event – it is offering a neat story that the dates flatly contradict. Knowing the real order of things does not change how we use the phrase, but it does inoculate us against a tidy explanation that gets the history exactly backwards.