They happen about…once in a blue moon

By Nolan Feeney / Time.com and Time.com
July 29, 2015
Advertisement
Credit: Getty Images/Imagebroker RF

Blue moons aren’t actually blue, but they are as rare as the saying goes.

The second full moon in a month is commonly referred to as a blue moon—we had a full moon on July 2, and we’ll have another this Friday on July 31—but there’s a more complicated definition according to the U.S. Naval Observatory:

This article originally appeared on time.com.