The Blue Moon in the Sky Tonight Isn't as Rare as It Sounds

Blue Moon
A full moon, known as the blue moon, is seen next to the Statue of Liberty in New York on July 31. REUTERS/Eduardo Munoz

In the evening of Friday July 31, a blue moon will rise in the sky. It's an event supposed to happen so infrequently it has become an idiom for the exceptional, but the reality is that, by cosmological standards, a blue moon is hardly rare at all.

"It's all relative," says Jacqueline Faherty, an astronomer at the American Museum of Natural History. "Something that happens every two to three years I don't consider to be that rare."

A blue moon refers to the second full moon during a harvest month—and the name is something of a misnomer. "There's no color change for it at all," says Faherty. "The color of the moon is solely dependent on what kinds of particles are in the atmosphere." In fact, if the moon were to appear blue, there would be cause for some concern as it would be indicative of a "massive volcanic eruption," says Faherty. The eruption of Krakatoa in 1883, for example, colored the moon blue for the two following years.

The popular meaning of blue moon has evolved over the years. The English usage of the phrase "once in a blue moon" can be traced back to 1528, used in the same way as "on the twelfth of never" to describe a rare event, according to the History Channel. The phrase came about because an actual blue moon—like that caused by Krakatoa—is an exceedingly rare phenomenon. Then, the phrase began to be used as a calendrical measurement. For example, in the 1800s, the Maine Farmers' Almanac used blue moon to describe the third full moon in a season with four full moons instead of the regular three. Then, in 1946 Sky and Telescope magazine used the phrase erroneously to signify the second moon of the calendar month. But the mistake caught on, and today, remains the most widely accepted definition.

The first full moon of this month was July 2; there are 29.5 days between each full lunar cycle. Tonight's full moon may not be blue, or even particularly rare—but if you like, there's no harm in using it as an excuse to splurge a little on dinner, or maybe order that second martini. After all, it is "only once in a blue moon."