The Blue Moon is an astronomical event only occurring every two or three years, and the full moon we enjoyed on August 31 was one.
The definition of a “blue moon” is the third full moon in a season that has four full moons. Typically, there are three full moons in a season, twelve per year. However, the lunar cycle is slightly less than a calendar month, so the days accumulate and every couple of years there is an “extra” full moon.
Now we know that “once in a blue moon” is quite a long time.
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