Blue moon
Metaphor for a rare event, or lunar calendrical term / From Wikipedia, the free encyclopedia
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A blue moon is defined by the Oxford English Dictionary (OED) as (1) A moon (real, depicted, or imagined) that appears blue; (2) (Since c1820) a long or indefinite length of time; a rarely recurring period or event (“once in a blue moon”); (3) (Since 1946) a second full moon in a calendar month, or formerly (from 1937) the third full moon in a season containing four full moons.[1]
The calendrical meaning of "blue moon" (3) is unconnected to the other meanings. It is often referred to as “traditional”,[2][3] but since no occurrences are known prior to 1937 it is better described as an invented tradition or “modern American folklore”.[4] The practice of designating the second full moon in a month as "blue" originated with amateur astronomer James Hugh Pruett in 1946.[5] It does not come from Native American lunar tradition, as is sometimes supposed.[6][7]
The moon - not necessarily full - can sometimes appear blue due to atmospheric emissions from large forest fires or volcanoes, though the phenomenon is rare and unpredictable (hence the saying “once in a blue moon”).[8][9][10] A calendrical blue moon (by Pruett's definition) is predictable and relatively common, happening 7 times in every 19 years (i.e. once every 2 or 3 years).[1] Calendrical blue moons occur because the time between successive full moons (approximately 29.5 days) is shorter than the average calendar month.[11] They are of no astronomical or historical significance, and are not a product of actual lunisolar timekeeping or intercalation.