Blue Moon: The Folklore of the Extra Full Moon

Somebody at the bar points at the sky and announces that tonight's moon is a blue moon, and you look up, and it is stubbornly, ordinarily grey-white. You feel mildly cheated. You are entitled to: almost everything about the blue moon, including its definition, is a misunderstanding that people decided to keep.

The phrase is old and it originally meant an absurdity. A 1528 English pamphlet scoffs that if churchmen say the moon is blue, we must believe it is true — the point being that only a fool would. From there "once in a blue moon" drifted into meaning something vanishingly rare, with no astronomy attached at all.

How a magazine invented the modern blue moon

The definition everyone now recites — the second full moon in a calendar month — is a twentieth-century error. The Maine Farmers' Almanac used "blue moon" for something fussier: the third full moon in a season that contains four, a bookkeeping trick that kept the traditional moon names aligned with the calendar. In 1946 an amateur astronomer named James Hugh Pruett, writing in Sky and Telescope, misread the almanac's rule and described the blue moon as the second full moon in a month. The mistake was cleaner than the original, so it spread — helped along in 1980 when the radio program StarDate repeated it, and again when a Trivial Pursuit card enshrined it in the 1980s. Sky and Telescope later published a cheerful confession of the whole chain of custody. By then the wrong definition had won, and even almanac loyalists concede there are now simply two kinds of blue moon.

By either definition the event arrives roughly every two to three years, which is rare-ish rather than miraculous. Occasionally the calendar overdelivers: 1999 managed two blue moons, in January and March, because February went without a full moon entirely.

When the moon has actually turned blue

The moon can literally appear blue, and the recipe is documented. After Krakatoa erupted in 1883, ash particles of just the right size scattered red light and let blue through, and observers around the world reported blue and green moons for many months. In September 1950, smoke from enormous muskeg fires in Alberta and British Columbia drifted across the Atlantic and turned the moon — and in places the sun — blue over Britain. So the honest answer to "can the moon be blue" is yes, provided something large is on fire.

Omens, wishes and the betrayer moon

The folklore layer came later, as it usually does. Modern pagan and folk-magic traditions treat a blue moon as a night when spells and wishes carry extra weight, and some older readings frame an extra full moon as an omen of disorder, a month with too much moon in it. This is folklore, freely admitted as such by most people who practice it. One popular etymology claims "blue" descends from an Old English word for betrayer, referring to a moon that falsely signals the timing of Lent; linguists treat that story with polite suspicion. Nothing about a blue moon differs physically from any other full moon, apart from the paperwork. The moon illusion will still make it look huge on the horizon, and people will still swear the night felt strange — which says more about us than about the moon, and is not entirely uninteresting for that.

Under this page, readers have logged the odd things that happened to them on blue moon nights, and the jury has been voting on which accounts hold up. Add your verdict, or your own night.