(Newsweek) 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.
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