why is blue moon called blue moon

A blue moon is called a "blue moon" because the term originally referred to something very rare or impossible, deriving from a 16th-century expression "the Moon is blue." The modern popular meaning of a blue moon is the occurrence of a second full moon within a single calendar month, which happens infrequently, roughly once every two to three years. There is also an older definition from the Maine Farmers’ Almanac that calls a blue moon the third full moon in a season that has four full moons. The term "blue moon" may have initially arisen from rare atmospheric conditions, such as after volcanic eruptions (e.g., Krakatoa in 1883), when the moon can appear bluish due to dust particles scattering light. However, this literal blue appearance is rare and not related to the calendrical meanings. The more common usage today as the second full moon in a month originated from a misunderstanding published in an astronomy magazine in the 1940s but has become widely accepted folklore. The phrase thus symbolizes a rare event, hence "once in a blue moon" meaning very seldom.