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The modern definition of a blue moon is the second full moon in a calendar month. Since the moon completes a full cycle approximately every 29.5 days, getting two full moons in a single month is somewhat rare, according to timeanddate.com.
The original definition of a blue moon was the third full moon in an astronomical season that had four. Amateur astronomer James Hugh Pruett made a miscalculation and claimed the second full moon in any month would be a blue moon by the original definition, timeanddate.com said.
This was later proven to be inaccurate, but calling the second full moon in any month a blue moon stuck.
The next blue moon -- by the modern definition -- won't take place until Jan. 31, 2018.
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