Smithsonian.com

The last “blue moon,” seen during a partial lunar eclipse in December 2009. Image via Wikimedia Commons/Codybird

This Friday, look to the night sky and you’ll see what’s referred to as a “blue moon”—the last time you’ll get the chance to glimpse this phenomenon until 2015. Those expecting to see a moon that’s actually an unusual color, though, will be disappointed. The term simply refers to the unusual occurrence of a second full moon within one calendar month, and since we already had a full moon on August 2, this will be a blue moon, the first seen in the United States since December 2009.

Every 29.53 days, the moon undergoes a complete phase cycle, as the portion of its surface that’s illuminated by the sun shifts from entirely within our line of sight (a full moon) to entirely hidden on the “far” side of the moon, away from the earth (a new moon, which is completely dark). Since 29.53 days is relatively close to the lengths of the months in our calendar, most months only have one full moon. Our calendar, however, is based off our motion around the sun, not the phases of the moon, so the periods don’t match up exactly.

As a result, every 2.7 years, two full moons are squeezed into one month. It’s the same way that, if you get a paycheck every two weeks, you’ll occasionally end up getting three paychecks within one month, since two 14-day pay periods (28 days) don’t match exactly with the calendar month. That’s what’s happening on Friday—the moon will reach its second full phase of August at precisely 9:58 a.m. EDT on the the 31st—and will happen next in July 2015.

If the moon won’t actually be blue Friday, then why the colorful name? Although it’s frequently cited as a piece of old folklore, Philip Hiscock, a professor of folklore at Memorial University in Canada, writes in Space & Telescope that this isn’t the case. Hiscock writes that it’s “a truly modern piece of folklore, masquerading as something old.”

Originally, in the early 1900s in places such as the Maine Farmer’s Almanac, the term “blue moon” was used to refer to a related phenomenon, when four full moons occurred within a given season, instead of the typical three. However, in 1946, amateur astronomer James High Pruett incorrectly interpreted the term in an article he wrote in Sky & Telescope using the meaning we know today. The mistake was repeated several times—notably, in 1980 on the NPR show Star Date—and eventually the new definition stuck, along with a common misattribution to traditional folklore, which “appeals to our modern sensibilities, including our desire to have plausible origins,” Hiscock writes. Since then, the term has been appropriated for everything from a novel to a butterfly to the widely popular Belgian white-style beer.

On very rare occasions, the moon actually can appear blue, if particulate matter of the right size is suspended in the atmosphere and interacts with light reflecting off the moon. “If there’s been a recent forest fire or volcanic eruption that pumped significant smoke or ash into the upper atmosphere, it is possible for the moon to take on a bluish hue,” says Space.com. Specifically, if the ash or other particles are roughly 1 micron wide (1 millionth of a meter), they will scatter red wavelengths of light, allowing other colors to pass through and reach the earth. This can cause the moon to appear blue or greenish and has happened several times in recent history, such as during the June 1991 eruption of Mount Pinatubo in the Philippines, which caused the phenomenon to occur in several places around the world.

Perhaps this accounts for the most commonly used meaning of the term, “once in a blue moon,” which refers to something that happens on a very infrequent basis. Unlike the blue moon that you can see Friday night and can count on like clockwork every 2.7 years, seeing a moon that’s actually in blue color will be more difficult. To do so, you might have to be patient for quite a while—and wait around for a massive volcanic explosion.