Blue Moon Definition
noun
The third full moon in a three-month calendrical season that has four full moons.
American Heritage
A very long time.
Webster's New World
The second of two full moons occurring in the same month.
American Heritage
Other Word Forms of Blue Moon
Noun
Singular:
blue moonPlural:
blue-moonsOrigin of Blue Moon
Sense 2, probably from the rare occurrence whereby the moon appears blue from high amounts of dust in the atmosphere, as from a volcanic eruption
From American Heritage Dictionary of the English Language, 5th Edition
Mistake in Sky & Telescope magazine in 1946, retracted nearly 60 years later.
From Wiktionary
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