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A blue moon photographed during the December 2009 lunar eclipse.


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blue moon (plural blue moons)

  1. (rare) The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere.
  2. An extra full moon that appears in a subdivision of a year, namely:
    1. The third full moon in a quarter that contains four rather than the usual three full moons.
    2. The second of two full moons that occur in the same calendar month.
  3. (idiomatic, by extension) A long time.
  4. A bright blue, sweet-flavored ice cream that is popular in the Midwestern U.S. and is a component of Superman ice cream.

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