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.

When the Moon Was Actually Blue

Meanwhile, in the physical world, the moon was literally turning blue on rare occasions — and the cause was science.

The most documented instance followed the 1883 eruption of Krakatoa in Indonesia, one of the largest volcanic explosions in recorded history. The eruption injected enormous quantities of fine ash and dust into the upper atmosphere.

Crucially, the particles were approximately one micron wide, a size that triggers a phenomenon called Mie scattering, in which particles scatter specific wavelengths of light rather than all wavelengths equally. The volcanic dust filtered out the longer, redder wavelengths while allowing shorter blue wavelengths to pass through. For nearly two years after Krakatoa, observers across the globe reported seeing the moon with a distinctly blue tint.

Similar effects have been documented after major forest fires and other large volcanic events throughout history. The moon can appear genuinely blue when the right-sized particles are suspended in the upper atmosphere in sufficient concentration. It is rare, but it is real physics.

The Accident That Created a New Definition

The astronomical definition of a blue moon most people know today was born from a misreading.

For centuries, older almanacs such as the Maine Farmers' Almanac used what is now called the seasonal definition: a blue moon was the third full moon in a season containing four. Since most seasons have three full moons, a fourth was unusual enough to earn its own name.

In 1946, an article in Sky and Telescope magazine attempted to explain this rule and got it wrong, stating instead that a blue moon was simply the second full moon within a single calendar month. The mistake was simpler and catchier than the original. It spread through astronomy hobbyist circles and was reinforced when the board game Trivial Pursuit adopted the popular definition in the 1980s, cementing it in culture so thoroughly that today both definitions are accepted. Astronomers use either.

Under the calendar definition, a blue moon occurs roughly every two to three years, since the lunar cycle of 29.5 days occasionally allows two full moons to fall within the same month. Under the seasonal definition, the frequency is similar, arising when a season's alignment catches four full moons rather than the usual three.

What It Looks Like in the Sky

A blue moon is not visually different from any other full moon. It will not appear blue unless volcanic ash or wildfire smoke particles of precisely the right size happen to be suspended in the atmosphere above you. What makes it notable is its relative scarcity in the calendar, and the five centuries of linguistic and scientific history that trail behind the name.

Every full moon carries its own story. The June 2026 Strawberry Moon drew its name from the wild strawberry harvest calendars of Algonquin tribes. The blue moon's name came from somewhere stranger: a satirist's insult, a catastrophic Indonesian volcano, and an astronomy writer's honest mistake. According to NASA's skywatching resources, the next calendrical blue moon will not arrive until 2028, making the phrase feel appropriately earned.

The history of once in a blue moon is itself a blue moon kind of story: the kind that only comes around once in a very long while.

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