Meaning
Describes something that happens very rarely or almost never. It functions as a fixed adverbial measure of rarity - 'we see each other once in a blue moon' - and, despite the astronomical backstory, in ordinary speech it makes no claim about the moon itself. Informal and fixed in form; usually tucked around a verb of frequency rather than used as a standalone exclamation. Regional use: General English.
Origin
'Blue moon' began as a term for an outright absurdity, on a par with the moon being made of green cheese. The Phrase Finder records William Barlow writing sarcastically in 1528 that if the priests 'saye the mone is belewe' we must believe it, and John Frith pairing the blue moon with green cheese in 1529. The shift to the modern 'rarely' sense is recorded in Pierce Egan's Real Life in London (1821), and the American Heritage Dictionary dates the rare-event use of 'blue moon' to roughly 1815-1825. A separate strand is astronomical: the Maine Farmers' Almanac had since 1819 labelled the third full moon in a four-moon season a Blue Moon, and in 1946 the amateur astronomer James Hugh Pruett misread that rule in Sky & Telescope and described a Blue Moon as the second full moon in a calendar month. Pruett's error, later spread by a Trivial Pursuit question, became the popular definition. The moon can literally look blue after major volcanic eruptions such as Krakatoa in 1883, but this is generally treated as unrelated, since the earliest uses of 'blue moon' mean impossibility rather than rarity.
Research Sources
References used to check this entry’s meaning, history, or documented forms.
Variants
- Once every blue moon
- Once in a very blue moon
- Twice in a blue moon
Usage Examples
- We only get snow deep enough to close the schools once in a blue moon here.
- He phones his cousins once in a blue moon, usually around the holidays.
- Once in a blue moon the board rejects the chair's proposal, and today was one of those days.