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.