What Is a Blue Moon, Actually?
Every couple of years, news sites announce a "blue moon" and explain, confidently, that it means the second full moon in a calendar month. Here's the fun part: that definition comes from a magazine mistake made in 1946 — and the phrase itself is four hundred years older than that, with a completely different meaning.
"Once in a blue moon" means very rarely. The phrase starts in 1528 as a joke about absurdity — "the moon is blue" was the era's "pigs can fly." Over centuries it softened from "never" to "almost never." The astronomy definition (second full moon in a month) is the newest layer, born from a 1946 Sky & Telescope article that misread an almanac.
1528: the moon as an absurdity
The oldest known "blue moon" appears in a satirical pamphlet from 1528, taking aim at the clergy of the day:
"Yf they saye the mone is belewe / We must beleve that it is true." — William Roy & Jeremy Barlow, Rede Me and Be Nott Wrothe, 1528
Translation: if the powerful tell you the moon is blue, you're expected to nod along. A blue moon here isn't rare — it's impossible, a thing only a fool would accept. That's the seed of the idiom: the moon being blue as shorthand for "that simply doesn't happen."
From "never" to "almost never"
Language loves to soften. Over the following centuries, "blue moon" slid from impossible to merely improbable, and by the 1800s "once in a blue moon" was in common use meaning what it means today: so rarely you'd remember the occasion. The same drift happened to "when pigs fly" cousins in other languages — hyperbole erodes into idiom with enough use.
1946: the mistake that became the definition
Now for the astronomy. The Maine Farmers' Almanac had a quirky old rule: a season normally has three full moons, but occasionally gets four — and when that happened, the almanac called the third full moon of that season the "blue moon" (to keep the other moons' traditional names in their proper slots).
In 1946, an article in Sky & Telescope magazine misread that almanac rule and stated a blue moon was the second full moon in a calendar month. The error was repeated, picked up by a radio quiz show in the 1980s, then by Trivial Pursuit — and today the mistake IS the mainstream definition. The magazine later ran a cheerful correction admitting its role. Too late: the moon had sailed.
We love this story because it's a rare case where we can watch a "fact" being born from an error in real time, citations and all — the same mechanism we catch in our fake origins roundup, except here the fake definition won so completely that astronomers now use it with a shrug.
When the moon really turns blue
One more layer: the moon can literally look blue. After the 1883 eruption of Krakatoa, ash particles of just the right size scattered red light and left moons looking distinctly blue across much of the world — for months. Large forest fires have done the same on a smaller scale. So a genuinely blue moon is real, unpredictable, and much rarer than either calendar definition — which makes the original 1528 joke weirdly accurate after all.
For the record-keeping on this one, Sky & Telescope has owned up to its part in the story, Merriam-Webster covers both senses, and the Phrase Finder traces the citations back to 1528. If you enjoy phrases where the "boring" truth beats the legend, try break the ice — or see how a real line got dark in deadline.
Quick FAQ
What does "once in a blue moon" mean?
Very rarely — something that almost never happens.
What's the astronomy definition of a blue moon?
Popularly, the second full moon in a calendar month (roughly every 2–3 years). That definition traces to a 1946 magazine error; the older almanac rule was the third full moon in a four-moon season.
How old is the phrase?
The first known use is 1528, where "the moon is blue" was an example of an obvious absurdity.
Can the moon actually be blue?
Yes — after major volcanic eruptions (like Krakatoa in 1883) or big forest fires, ash and smoke particles can make the moon appear blue.
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The IdiomJar team researches phrase origins from dictionaries, digitized newspaper archives, and the work of etymologists — and clearly labels anything that's folklore rather than fact. Read more about how we work.