Look Up! 'Once in a Blue Moon' Happens for Real on May 21

A blue moon captured on August 31, 2012 in Nottingham, England, by Simon Smith. Image credit: Simon Smith via NASA

Look up at the sky Saturday night, May 21, and you will be treated to a bona fide "blue moon"—that of "once in a …" fame. Will the show blow your mind? Probably not, but it will give you a new appreciation for the Moon's orbit of the Earth and the speed at which said orbit takes place. It's not often that you can tilt your neck a mere 15 degrees and learn something new, so here's your big chance.

WHAT IS A BLUE MOON? 

Last year, we were treated to the total eclipse of a red harvest super moon, the result of which was a giant Moon in the night sky that turned an alarming shade of red. It was a celestial event you won't see for another 17 years, and it might have spoiled you a bit for everything that follows until then.

Here is the bad news about Saturday's blue moon: the Moon will not turn blue.

The contemporary definition of a "blue moon" is when two full moons appear in the night sky in a single calendar month, as opposed to one. This happens once every two-and-a-half to three years.

So why blue and not once in a periwinkle moon or cerulean moon? The "blue" in this moon's name came not originally from color, but rather is thought to be a modernization of the Old English word belewe, which means "to betray." (As in: There are supposed to be 12 full moons every year, and this 13th moon betrays that notion.)

A MUDDLED HISTORY

The phrase "blue moon" did not always refer to the number of full moons in a single month. That is a relatively modern notion—the result of an error in a 1946 issue of Sky & Telescope magazine (a respected publication not often associated with mistakes). Previously, "blue moon" referred to seasons. As the Farmers' Almanac explains it: "If there are twelve full moons a year, and four seasons, that means there are three full moons per season. When the odd fourth full moon appears, that portion of the season was once called a 'blue moon.'"

It is fair to ask why that 13th full moon only sometimes appears in a calendar year. The reason involves the duration of lunar phases. A complete moon phase cycle as viewed from here on Earth lasts not 30 or 31 days, but 29.5305882. Carry that number for 12 months and you get 354, or 11 days shy of a year. Around the three-year mark, those 11 days add up to an extra full moon in a single calendar year. This is one of those years. If there's anything you've been putting off, Saturday, May 21 might be the day to get it over with. You can then spend the next three years saying you only do it "once in a blue moon," and you're free from obligation until May 18, 2019, when the blue moon will return.