what causes a blue moon

A blue moon is a term used to describe the second full moon in a calendar month. However, the moon itself does not actually turn blue in color, except in rare cases. One such case is when there are tiny particles in the air, such as smoke or dust, that scatter away red wavelengths of light, causing the moon to appear blue. This can happen after volcanic eruptions or forest fires release particles in the atmosphere of just the right size to preferentially scatter red light. For example, in 1883, the eruption of the Indonesian volcano Krakatoa caused the moon to appear blue for nearly two years. Other volcanic eruptions, such as the 1983 eruption of El Chichon volcano in Mexico and the eruptions of Mt. St. Helens in 1980 and Mount Pinatubo in 1991, have also been known to cause blue moons.