Originally, in a season in which four full moons occurred, the blue moon was the third of these.
In 1946, an error in the magazine *Sky and Telescope* led to the term referring to the second blue moon in a month.
Today, the phrase 'blue moon' is used to refer to an event that happens quite rarely. Interestingly, by either of the definitions given above, it would ean something that occurs on average every 2.5 years.
There'll be a blue moon in a few weeks.
I watch soap operas once in a blue moon.
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