Blue Moon

A blue moon refers to either a second full moon in a calendar month, a third full moon in a season containing four, or a moon that appears blue due to atmospheric effects. The term "blue moon" originated in modern American folklore and is not connected to Native American lunar tradition, but rather was popularized by amateur astronomer James Hugh Pruett in 1946. Calendrical blue moons occur due to the shorter time between successive full moons (29.5 days) being shorter than the average calendar month, and occur about 7 times every 19 years. The phrase "once in a blue moon" originated in the 1810s and was first recorded in 1833, while the use of "blue moon" to refer to a specific calendrical event dates from 1937, when the Maine Farmers' Almanac first used the term.learn more on wikipedia