blue moon

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a 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. (archaic) Something absurd.
    • 1528, Rede Me and Be Not Wrothe, pamphlet
      Yf they say the mone is belewe, we must believe that it is true.
  3. (obsolete) The third full moon in a quarter that contains four rather than the usual three full moons.
  4. (idiomatic) (by extension) A long time.
  5. A flavor of sweet ice cream that is popular in the Midwestern U.S. and is a component of Superman ice cream.


Mistake in Sky & Telescope magazine in 1946, retracted nearly 60 years later.


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a blue moon

  1. The second of two full moons that occur in the same calendar month.

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