"Only once in a blue moon is there controversy over exactly what constitutes a blue moon. But it's not a blue moon in the strict definition of the 19th Century Farmer's Almanac. According to the almanac, a true blue moon is the third full moon in a season that has four full moons. This also happens about every three years, but on a different cycle from the blue moon that occurs tonight. Confused? So is the American Heritage Dictionary, which lists both definitions of the blue moon."
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