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 a second full moon in a month of the common calendar. The phrase has nothing to do with the actual color of the moon, although a literal ‘blue moon’ (the moon appearing with a tinge of blue) may occur in certain atmospheric conditions: e.g., when there are volcanic eruptions or when exceptionally large fires leave particles in the atmosphere.

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