Tonight, if you look up in the sky, you’ll find a rare sight—one that occurs "once in a blue moon."

As in, you'll see an actual blue moon, which is (colloquially at least) when a second full moon occurs within the same calendar month. This occurs roughly every 2.7 years, according to—hence the famous phrase. The last time we earthlings got to witness what's known as a blue moon was August 2012, and the next time won’t be until January 2018.

Despite what the name suggests, the moon won’t be changing color. According to the National Aeronautics and Space Administration (NASA), the color of the moon will likely appear to be pale grey and white, just like the one you’re used to seeing on any other night.

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It used to be that a "blue moon" meant "the third full moon in a season that contained four of them," reports. The reason the definition changed to our current understanding of the term was all the result of a misunderstanding, according to NASA. In the 1940s a magazine called Sky & Telescope misunderstood the definition, printing an article that stated the "second [full moon] in a month, so I interpret it, is called Blue Moon."

Further complicating things, a moon that appears blue in color does actually happen. NASA explains that the moon can look blue to us if say, a major volcanic eruption occurs and blasts a ton of particles in the air. Forest fires can cause it, too. Who knew?

Today's blue moon actually rose this morning at 6:43 EDT, but let's face it, most of us missed it. So make sure to look for it tonight. And if you can't see it, the Slooh Community Observatory will be streaming it online at 8:30 EDT.

While the moon may not look all that different, you’ll be witnessing an event that only happens…well, once in a blue moon. And on top of that you might even notice a few weird body effects.

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