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31 December 2009 Blue Moon with partial lunar eclipse

A blue moon is an “extra” full moon in years that have thirteen full moons. Most years have twelve full moons which occur approximately monthly, but in addition to those twelve full lunar cycles, each solar calendar year contains an excess of roughly eleven days compared to the lunar year of 12 lunations. The extra days accumulate, so every two or three years (7 times in the 19-year Metonic cycle), there is an extra full moon. Lunisolar calendars have rules on when to insert such an intercalary of embolismic (“leap”) month, and what name it is given; e.g. in the Hebrew calendar the month Adar is duplicated. The term “blue moon” comes from folk lore. Different traditions and conventions place the extra “blue” full moon at different times in the year.

• In calculating the dates for Lent and Easter, the Clergy identify the Lent Moon. It is thought that historically when the moon’s timing was too early, they named an earlier moon as a “betrayer moon” (belewe moon), thus the Lent moon came at its expected time.^[1]
• Folklore gave each moon a name according to its time of year. A moon which came too early had no folk name – and was called a blue moon – bringing the correct seasonal timings for future moons.
• The Farmers’ Almanac defined blue moon as an extra full moon that occurred in a season; one season was normally three full moons. If a season had four full moons, then the third full moon was named a blue moon.
• Recent popular usage defined a blue moon as the second full moon in a calendar month, stemming from an interpretation error made in 1946 that was discovered in 1999.^[2] For example, December 31, 2009 was a blue moon according to this usage.

A “blue moon” is also used colloquially to mean “a rare event”, especially in the phrase “once in a blue moon”.

Info : Wikipedia