what is the blue moon

A blue moon is an additional full moon that appears in a subdivision of a year, either the third of four full moons in a season or the second full moon within a single calendar month. The phrase "blue moon" has nothing to do with the actual color of the Moon, although a visually blue Moon (the Moon appearing with a bluish tinge) may occur under certain atmospheric conditions, such as volcanic eruptions or fires releasing particles in the atmosphere of just the right size to preferentially scatter red light. The earliest recorded English usage of the term "blue moon" is found in an anti-clerical pamphlet published in 1528, which used the phrase to mean something absurd and untrue. Blue moons are not actually blue, but they can appear reddish or orange during a lunar eclipse. Blue moons occur once every two to three years.