Why a Blue Moon's Not Really Blue

At first, "blue moon" was slang for something that was flat-out impossible. But, over time the meaning of this idiom changed to refer to things that were either rare or highly unlikely. This explains the modern phrase "once in a blue moon." Nowadays, when a person says that something occurs "once in a blue moon," they mean it doesn't happen very often — but it's not impossible.

In the 1800s, this expression received yet another meaning. It takes the moon 29.53 days to complete a full rotation around the earth. In the process, the moon goes through all its phases. Therefore, each calendar season — spring, summer, fall and winter — typically sees three full moons apiece, assuming one full moon in a month. But every so often, a single month will have two blue moons. During the 19th century, some stargazers began to refer to the third full moon in a season which sees four of them altogether as a "blue moon." The Maine Farmer's Almanac popularized this definition.

Time for a quick aside. You might be wondering why the third moon in a season with four was singled out here. Why wasn't that fourth one also called a "blue moon?" The answer boils down to naming conventions. Again, most years see 12 full moons in total. Many cultures have given names to those which appear at specific points in the year. For example, in America, the last full moon of the winter is called the "worm moon." Now if the winter season in a given year were to see four full moons, calling the last one a "blue moon" would disrupt this linguistic status quo.

Right, then: Back to the Maine Farmer's Almanac. From 1932 to 1957, the (now-defunct) publication championed that whole "third moon's the charm" definition and used it to pinpoint the date of the next seasonal blue moon and subsequent seasonal blue moons.

The waters were later muddied in 1946, when an astronomer named James Hugh Pruett wrote an article about blue moons for Sky & Telescope magazine. In it, he misinterpreted an excerpt from the 1937 edition of the Maine Farmer's Almanac. This led him to conclude — erroneously — that a blue moon is the second full moon in a calendar month.

Pruett's blunder went on to have a life of its own. One 1980 episode of the radio program "StarDate" repeated his mistaken idea about what a blue moon is. After that, this new definition turned up on a Trivial Pursuit card and in a children's nonfiction book. Yesterday's misprint can become tomorrow's accepted wisdom: Most people now subscribe to Pruett's definition. So too, for that matter, does the Merriam-Webster dictionary.

Thanks to the kerfuffle, astronomy fans are left with two competing schools of thought about what constitutes a "blue moon." The rival definitions now go by different names. The third full moon in a season with four of them is called a "seasonal blue moon." Meanwhile, the second full moon in a calendar month is called a "monthly blue moon" or calendrical blue moons. The latter phenomenon occurs once every two to three years.

You'll note that neither definition has anything to do with the moon's coloration. By virtue of its surface geology, Earth's natural satellite usually looks gray. And during lunar eclipses, the refraction of sunlight can give it a rusty red appearance. But does the moon ever turn blue?

Yes, but only under certain circumstances. In the past, there've been documented instances of smoke and ash from massive forest fires and volcanic eruptions sending particles which filter out red light into the atmosphere. When this happens, the moon takes on an azure appearance. In short, if you ever see a moon that's literally blue, it's because something very destructive is happening down on Earth. Pretty steep price to see one.