It is not rare, it is not blue, and the modern definition comes from a magazine getting it wrong in 1946
Blue Moon holds a 3.0 from thirteen reviews, which is not a rating and which we will not use.
But a blue moon is a bookkeeping error.
The arithmetic
A lunar month is about twenty-nine and a half days.
A calendar month is about thirty and a half.
Which means the moon runs slightly fast against the calendar, and the discrepancy accumulates – and roughly every two and a half years, a single calendar month ends up containing two full moons.
That is a blue moon, in the modern popular definition.
It is not rare. It happens every two or three years, reliably, and it is entirely predictable.
And that definition is a mistake
Which is the genuinely good part.
The older meaning was different and more subtle – it referred to the third full moon in a season containing four, which is a rule from an old almanac and which is considerably harder to explain.
And in 1946, an article in an astronomy magazine attempted to explain that rule and got it wrong.
The writer described it as the second full moon in a calendar month.
Which was incorrect. And which was then repeated, and picked up, and used in a radio quiz, and spread – and it is now, comprehensively, what the phrase means.
An error in a magazine in 1946 redefined a piece of folk astronomy for the entire English-speaking world.
The Prat’s astronomical desk regards this as the single best-documented example in seven postcodes of a mistake becoming the truth by being repeated.
The tellable version wins.
And it is not blue
Obviously.
Although the moon can appear blue, genuinely and physically, and it has been recorded.
It requires particles in the atmosphere of a very specific size – around one micron – which scatter red light and let blue through.
Large volcanic eruptions do it. Krakatoa did it, and the moon was reported as blue for a considerable period afterwards. Enormous forest fires have done it.
So a genuinely blue moon is real, and it is a sign that something has gone very badly wrong somewhere.
The Prat’s atmospheric desk would note that the moon is far more commonly reported as red – during an eclipse, when the only light reaching it has been bent through the entire ring of Earth’s sunrises and sunsets at once.
Which is a genuinely beautiful fact. The red on the eclipsed moon is every dawn and every dusk on the planet, arriving at the same moment.
Verdict
Thirteen reviews. Not a rating.
Go, be the fourteenth, and look up.
Once in a blue moon
Which is the phrase, and which is older than either definition.
It appears in the sixteenth century – and at that point it did not mean rarely. It meant never.
To say a thing would happen when the moon turned blue was to say it was absurd, impossible, and would not occur.
And the phrase softened, over four hundred years, from impossible to merely infrequent – which is a substantial demotion and which nobody noticed happening.
Seven postcodes and seven hundred and forty-nine articles. The word describes something difficult, and then it does not.
Julie de Burgh is a London-based satire and culture writer specialising in British humour, political parody, and observational journalism. Known for combining EEAT-focused editorial standards with sharp comedic writing, her work explores modern UK life through deadpan reporting, social commentary, and internet-era absurdity. Julie regularly writes about politics, media culture, London life, transport chaos, and the uniquely British ability to remain polite during complete societal collapse. Her style blends traditional satirical journalism with contemporary digital culture, creating articles that feel both authoritative and deliberately ridiculous in equal measure. [email protected]
Someone’s going to frame this in their London office. It will be the most honest decoration about how London actually functions.