The second full moon to occur within a single calendar month, a comparatively rare event that happens roughly once every two to three years because the lunar cycle, at about 29.5 days, is close to but shorter than most calendar months. Despite the name, a blue moon is not actually blue in color; genuinely blue-tinted moons are a separate, much rarer effect caused by smoke or ash particles of a particular size scattering red light out of the moon's reflected glow.
Facts
PeriodicityUnder the modern calendar definition, a second full moon falling within one calendar month occurs about seven times every 19 years, or roughly once every two to three years 1 First DescribedThe modern once in a blue moon calendar definition traces to James Hugh Pruett's 1946 Sky and Telescope article on the second full moon in a calendar month, though the term itself had already appeared in the 1937 Maine Farmers Almanac 1 Identification
Distinguishing AppearanceA calendrical blue moon does not actually look blue; a genuinely blue tinted moon is a separate and much rarer effect that requires volcanic ash or large scale smoke particles of just the right size to scatter red light out of the moon's reflected glow 1 Sources
1. Wikipedia - Blue Moon
Introduction
A calendrical blue moon (by Pruett's definition) is predictable and relatively common, happening 7 times in every 19 years (i.e. once every 2 or 3 years).
True blue moons section
if volcanic eruptions or large-scale fires release particles into the atmosphere of just the right size to preferentially scatter red light
History section
second full moon in a calendar month
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