The True History of 'Once in a Blue Moon,' From Insult to Astronomy

The phrase rolls off the tongue so naturally that most people never stop to wonder where it came from. Once in a blue moon has come to mean something that happens rarely but not impossibly — a once-every-few-years kind of event. The actual history behind those five words is stranger and more accidental than almost anyone knows, woven together from Renaissance sarcasm, a catastrophic volcanic eruption, and a magazine misprint from the 1940s.

When "Blue Moon" Meant the Impossible

The phrase did not originally mean rare. It meant flatly impossible.

In 16th-century England, calling something a blue moon was the equivalent of saying when pigs fly, an absurdity invoked to mock gullibility. A 1528 anti-clerical pamphlet used the idea of a blue moon to ridicule people who would believe anything their superiors told them. The moon was not blue, had never been blue, and the entire point was that it never would be.

That meaning held for nearly three centuries. Somewhere in the early 1800s, though, the phrase began to soften from impossible to improbable. The first recorded use of blue moon to mean a very long period of time appeared in 1821 in a London street-life chronicle called Real Life in London, where it was already fresh enough slang that the author felt obliged to add a footnote explaining it to readers. A dozen years later, in 1833, a British literary magazine published the exact phrase we still use today, describing something that happens very rarely but is not entirely out of the question.