blue moon

Last Saturday, it was a Blue Moon. I didn't know that was anything more than a silly saying, but now I do. Lunations for lunatics.

A blue moon can refer to the third full moon in a season with four full moons, or the second full moon of a calendar month. 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.