Blue moon
Common name for one of the full moons in a year with 13 full moons / From Wikipedia, the free encyclopedia
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"Once in a blue moon" is a traditional phrase meaning "very rarely indeed".[1] Webster's Dictionary (1949) defined a blue moon as meaning, "A phenomenon never, or very rarely, seen; hence, a rarely recurring occasion."[2] However, since the mid-20th century the term blue moon has commonly been applied to the occurrence of a second full moon within a single calendar month, something that happens every two or three years, thus obscuring the original meaning of the term.
The use of blue moon to mean a specific calendrical event dates from 1937, when the Maine Farmers' Almanac gave a definition slightly different from the one now in common use, and was followed by a 1946 article in Sky and Telescope which led to the current usage, popular since the 1980s.[3][4]
The phrase in modern usage has nothing to do with the actual color of the Moon, although a visually blue Moon (the Moon appearing with a bluish tinge) may occur under certain atmospheric conditions—for instance, if volcanic eruptions or fires release particles in the atmosphere of just the right size to preferentially scatter red light.[5] This is a genuinely uncommon occurrence, more in accord with the original meaning of the phrase "once in a blue moon".