why is it called the blue moon

The term "blue moon" has multiple meanings and origins. Traditionally, it referred to the third full moon in a season that has four full moons, rather than the usual three. This definition came from the Maine Farmers' Almanac in the 19th and early 20th centuries, where full moons were named for activities appropriate to the time of year, and an extra full moon in a season was called a blue moon to keep the naming system in place. The more modern, popular meaning of a blue moon is the second full moon in a calendar month. This definition arose from a misinterpretation published in a 1946 astronomy magazine, which misunderstood the original Farmers' Almanac explanation. Despite the misunderstanding, this meaning has widespread acceptance today. The phrase "once in a blue moon" originally came from an expression meaning something very rare or impossible. In rare atmospheric conditions, such as after volcanic eruptions, the moon can actually appear bluish, which contributed to the phrase's evolution. In summary, "blue moon" refers to a rare extra full moon either seasonally or monthly, with historical roots in calendar calculations and folklore about its rarity and occasional actual blue appearance in the sky.