Language doesn't always reward accuracy; sometimes it rewards what spreads.
Language rarely travels in a straight line, and the phrase 'once in a blue moon' is a quiet testament to that truth. What began in the 16th century as an expression for the flatly impossible drifted, century by century, into a description of the merely rare — and then, in 1946, a single misreading of an almanac accidentally wrote the definition most of the world now carries. The story of the blue moon is, in the end, a story about how meaning is made not by the careful but by the many.
- A phrase born to describe impossibility has spent five centuries quietly reinventing itself, arriving in modern mouths meaning something far more modest: rarity.
- The tension at the heart of the story is a 1946 mistake — a writer misread an almanac and published the wrong definition, and the error spread faster than the truth ever could.
- Two competing definitions now coexist: the 1937 original, which counts the third full moon in a four-moon season, and the 1946 misinterpretation, which simply names the second full moon in a calendar month.
- The public has largely settled on the wrong answer, a reminder that language is shaped by circulation, not correction.
- On December 31, 2028, a blue moon, a supermoon, and a total lunar eclipse will converge in a single night — a super blood blue moon that may finally justify the phrase's oldest, most extreme meaning.
When someone says they do something once in a blue moon, they mean rarely. But the phrase began its life meaning something far stronger: the impossible. In the 16th century, saying 'the moon is blue' was a way of calling something absurd, beyond the reach of reality. Over the following centuries, that impossibility softened into improbability, and improbability softened into rarity, until the phrase arrived in modern English carrying only a faint echo of its original force.
The 20th century gave the phrase a more literal shape. In 1937, the Maine Farmer's Almanac offered the first formal definition: a blue moon was the third full moon in a season containing four instead of the usual three. It was a precise, considered answer. But nine years later, a writer for Sky and Telescope misread the almanac and published a different claim — that a blue moon was simply the second full moon in any calendar month. It was a mistake, born from unclear writing and careless reading.
The mistake won. The 1946 misinterpretation spread through popular culture and became the definition most people carry today. Language, it turns out, doesn't always reward accuracy — it rewards what travels well. By either definition, a blue moon arrives roughly once every year or two, rare enough to feel meaningful but not so remote as to feel mythical.
That may change on December 31, 2028, when a blue moon will coincide with a supermoon and a total lunar eclipse — a convergence that will make the moon appear larger than usual and turn it a deep, blood-red color. The three phenomena arriving together in a single night is the kind of event that earns the weight of the phrase's oldest meaning. When that night comes, the sky will offer something that genuinely, in every sense of the word, happens once in a blue moon.
When someone mentions doing something once in a blue moon, they mean it happens rarely—maybe once a year, maybe less. But the phrase carries a strange history, one that began as a description of the impossible and only gradually became what we use it for today.
The trail starts in the 16th century, when "the moon is blue" was an expression applied to things that could never happen. It was a way of saying something was absurd, impossible, beyond the realm of reality. Over the following centuries, the language shifted. The impossible became merely improbable. The improbable became rare. By the time the phrase settled into modern English as "once in a blue moon," it had shed its original meaning entirely and simply meant something that occurred infrequently.
The transformation accelerated in the 20th century, when astronomy and almanac-making gave the phrase a literal anchor. In 1937, the Maine Farmer's Almanac published a formal definition: a blue moon was the third full moon in a season that contained four full moons instead of the usual three. This was the first time anyone had tried to pin down exactly what a blue moon actually was. But the definition didn't stick universally. Nine years later, in 1946, an author writing for Sky and Telescope magazine misread what the Farmer's Almanac had said. The author interpreted it differently, claiming that a blue moon was simply the second full moon occurring in any single calendar month. It was a mistake—a misunderstanding born from unclear writing.
Yet that error became the standard. The wrong definition caught on with the public and has remained the dominant understanding ever since. Today, when most people hear "blue moon," they're thinking of the 1946 misinterpretation, not the original 1937 definition. Language doesn't always reward accuracy; sometimes it rewards what spreads.
Both definitions, however, point to something genuinely rare. A blue moon by either measure occurs roughly once every year or two—infrequent enough to feel special, but not so rare that you'll never witness one in your lifetime. The phrase, which began as hyperbole for the impossible, now describes something that is merely uncommon.
But 2028 will offer something that truly deserves the weight of that old language. On December 31st of that year, a blue moon will rise. That alone would be noteworthy. But this blue moon will also be a supermoon, meaning the moon will be at its closest point to Earth and will appear noticeably larger in the sky. And it will occur during a total lunar eclipse, which will turn the moon a deep reddish color—what astronomers call a blood moon. The convergence of all three phenomena in a single event is genuinely rare, the kind of thing that justifies the original 16th-century language about impossibility. When that night arrives, the sky will offer something that truly happens once in a blue moon.
The Hearth Conversation Another angle on the story
So the phrase started meaning something was impossible? That's the opposite of what it means now.
Exactly. In the 1500s, saying "the moon is blue" was like saying "when pigs fly." It was a way of dismissing something as absurd. But over centuries, the language softened. Impossible became rare.
And then the Maine Farmer's Almanac tried to make it literal?
Right. In 1937, they said a blue moon was the third full moon in a four-full-moon season. It was an attempt to give the phrase an actual astronomical meaning.
But that's not what people think it means today.
No. In 1946, someone at Sky and Telescope magazine misunderstood the definition and said it was the second full moon in a month instead. That was wrong, but it spread.
So we've all been using the wrong definition for eighty years?
Not wrong exactly—just based on a misreading. Language doesn't care about original intent. It cares about what people actually use.
What makes 2028 special, then?
That's when you get all three things at once: a blue moon, a supermoon, and a total lunar eclipse. That's when the old language about impossibility actually fits again.