5:11 pm
Tue August 20, 2013

What Makes A Blue Moon Blue?

If you were looking up at the skies on Tuesday night, you might have noticed a big, bright full moon. What you might not have realized is just how rare that full moon was. It wasn't blue in color, but it was a true Blue Moon. 
Full moons occur, on average, every 29.5 days. If you're standing on Earth, this means the sun is directly opposite the moon. 

At 9:45PM Tuesday look up! If skies are clear, you will see a big, bright Blue Moon.
At 9:45PM Tuesday look up! If skies are clear, you will see a big, bright Blue Moon.
Credit mark roy from Thursday Island, Torres Strait, Australia

A Blue Moon typically refers to the unusual occasion where there is a second full moon in a calendar month. But as Sky & Telescope Magazine senior contributing editor Kelly Beatty explained, that’s not the original meaning of a Blue Moon.

"We have folklorish names for full moons, like the Hunter's Moon, the Harvest Moon, and so forth and every once in a while we get an extra full moon inserted into that sequence," he said. "And so the Maine Farmers Almanac solved that problem by calling the third full moon in a particular season the Blue Moon, and that kept everything in sync."

Tuesday's Blue Moon was a real Blue Moon — the third full moon of a rare four in this summer's season.

How did the meaning of the term Blue Moon change? Beatty admitted that Sky & Telescope is responsible for the flip.

"In 1946 one of our authors said the second full moon in a month is called the blue moon. That’s wrong. And it might have stayed obscure, but in 1980 a radio program called "Earth and Sky" picked up on it and it became the accepted definition," he said. "Sky & Telescope came clean a few years ago, but you know, the genie's out of the bottle."

Blue Moons really do only occur once in a blue moon. The next time we’ll see a "twice in a month" Blue Moon won’t be until July of 2015, and the next third full moon of four in a season won’t occur until May of 2016.

If your curiosity is piqued, you can learn more about the Blue Moon saga here.

And if you'd like some background music for Blue Moon viewing, you can't do much better than Billie Holiday: