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The Spokesman-Review Newspaper
Spokane, Washington  Est. May 19, 1883

Blue moon will close out month of July

Dan Vergano USA Today

Famed in slang and song, a blue moon is not really blue. But, as the expression “once in a blue moon” would indicate, it is pretty rare.

And it’s making an appearance this weekend.

A blue moon – the nickname for the second full moon in the same month – rises Saturday night, an astronomical oddity with a name based on a botched bit of modern folklore.

Two full moons rarely occur in the same calendar month – only about once every 33 months – because full moons fall 29 1/2 days apart. The last time this happened was in November 2001, according to the U.S. Naval Observatory.

The term “blue moon” to describe something rare originated centuries ago. Folklorists have traced references to poems from Shakespeare’s time. Playing on the notion that the moon was made of green cheese, poets joked that a never-seen “blue” moon was made of well-aged blue cheese.

Years later, a Maine farmer’s almanac started using the expression to mean the third full moon in a season. That meaning held until 1946, when a mixed-up reference in Sky & Telescope magazine defined a blue moon as the second full moon in a month.

The magazine admitted to its “Blue Moon blooper” five years ago. But in the intervening years the mistake made its way into the Trivial Pursuit board game, according to folklorist Philip Hiscock of Canada’s Memorial University of Newfoundland, embedding the definition in popular imagination.

While a full moon is usually a familiar white or occasionally orange, it can look blue under the right atmospheric conditions, says astronomer Tony Phillips of NASA’s Marshall Space Flight Center in Huntsville, Ala. Volcanic eruptions, forest fires, even snowstorms can give the moon a bluish tinge, he says, especially around sunset.

The most famous occurrence of blue-tinted moons came in 1883, when dust from the volcano Krakatoa in Indonesia colored the atmosphere for two years.

“It’s fire season across the western U.S., so it’s not inconceivable,” Phillips adds. “Of course, most often that turns the moon a more reddish color. It all depends on the dust.”