The blue moon rises over people gathered on Glastonbury Tor in Somerset, England Friday, July 31, 2015

Photo by Matt Cardy/Getty Images

While most years have 12 full moons, this year has 13.  A blue moon, which refers to the second of two full moons appearing in the same calendar month, was seen Friday night for the first time since 2012.  The phenomenon happens every two years on average, according to NASA, and the next one isn’t due until 2018.  Once in a blue moon: photos of the lunar event from around the world.  //via pbs

It isn’t blue… so where did the term “blue moon” come from? Read here.

Blue moons are very rarely blue. Most are pale gray and white, resembling a moon on any other night.  A truly blue colored moon can occur on rare occasions, according to NASA, with most being spotted after volcanic eruptions.

What is a blue moon? video