The most literal meaning of '' blue moon '' is when the moon ( not necessarily a full moon ) appears to a casual observer to be unusually bluish , which is a rare event . The effect can be caused by smoke or dust particles in the atmosphere , as has happened after forest fires in Sweden and Canada in 1950 and 1951 , and after the eruption of Krakatoa in 1883 , which caused the moon to appear blue for nearly two years . Other less potent volcanoes have also turned the moon blue . People saw blue moons in 1983 after the eruption of the El Chichón volcano in Mexico , and there are reports of blue moons caused by Mount St. Helens in 1980 and Mount Pinatubo in 1991 . In the Antarctic diary of Robert Falcon Scott for July 11 , 1911 his entry says , " ... the air thick with snow , and the moon a vague blue " . On that date the moon phase would have looked full
eruption of Krakatoa in 1883
1883 eruption of Krakatoa
Blue moon