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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A blue moon is the second full moon that appears in a calendar month. The period from one full moon to another is approximately 29 and a half months with 31 days are more likely to witness a blue moon, whereas February, with its 28 days (29 days in leap years), cannot host a blue moon. On average, a blue moon appears once every 33 months or full moons, totaling 41 occurrences per century, or roughly seven instances every 19 years. A more uncommon phenomenon is the occurrence of two blue moons within the same calendar year, happening approximately four times per century.[1] A blue moon can also refers to a seasonal blue moon, the third full moon in an astronomical season (period between a solstice and equinox) that has four full moons.[2][3]
"Once in a blue moon" is a traditional phrase meaning "very rarely indeed".[4] 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.[5][6]
The 1937 Maine Farmer's Almanac is the earliest instance that has been found for using blue moon as a calendrical term.[7] The historical explanation given there was spurious.
The Moon usually comes full twelve times in a year, three times in each season... However, occasionally the moon comes full thirteen times in a year. This was considered a very unfortunate circumstance, especially by the monks who had charge of the calendar. It became necessary for them to make a calendar of thirteen months, and it upset the regular arrangement of church festivals. For this reason thirteen came to be considered an unlucky number. Also, this extra moon had a way of coming in each of the seasons so that it could not be given a name appropriate to the time of year like the other moons. It was usually called the Blue Moon... In olden times the almanac makers had much difficulty calculating the occurrence of the Blue Moon and this uncertainty gave rise to the expression "Once in a Blue Moon". [8]
The term 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.[9]