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What You Need to Know About Tonight’s Blue Moon

Why is it blue?

A blue moon happens infrequently enough that we have an idiom based around it. But look out tonight, and you'll see a full moon. But you'll maybe notice that it's just as grey as always. So what's up with calling it a blue moon? Are you just trying to mess with us?

A blue moon, unlike a blood moon or a harvest moon, is not named for the color it takes on, but rather the fact that it's a fourth full moon in a given season that typically has three. The moon has a rotation period between 27 and 29 days with some variation in between, so while a year typically has around 12 full moons, a blue moon is the term for the appearance of a 13th. The other full moon was on July 1.

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While it seems like a boring thing, it is actually fairly rare, happening approximately once every three years, with the next set for January 2018 (and the last almost three years ago in August 2012). While it's been traditionally known as a seasonal variation, it's instead become mistakenly known as the second full moon in a given month. As Scott Neuman at NPR points out, this definition of a "blue moon" first appeared in 1946 in Sky & Telescope magazine, and was a complete editorial mistake that gained steam into a commonly held misconception. He also points out that the moon can appear blue given the right conditions, which is mostly "a volcanic eruption" that scatters the light enough to give it a bluish tint.

Don't go out and take your binoculars or telescope and just point it directly at the Moon. Not without proper filters, at least. The Moon is reflecting incoming sunlight and is a high-albedo (reflection) object, it can get washed out very easily and even hurt your eyes a little to look at. But most telescope eyepieces can fit a moon filter, which acts a bit like a pair of lenspiece sunglasses, dimming your field of view. With that in, you can make out some brilliant lunar features, even with a basic 70 mm telescope. Craters, lunar seas, and other features can be made out, and with enough work, you can even get a smartphone photo of the full moon.

There are some other night sky opportunities out there tonight to pair with a brilliant second full moon. In some parts of the mid-Atlantic, the International Space Station will be flying overhead at about 9:25 PM. (The exact time ranges by city but it's within about a five minute span.) A good pair of binoculars could make for a decent sighting of the space habitat, though it's unlikely you'll see Scott Kelly waving back. But to see when you might see it overhead, NASA has a Spot the Station website.

Depending on the time you look, you may also be able to see Venus and Jupiter, though they'll both set in the early evening. However, Saturn will be out throughout the night sky, and a basic telescope can make out the rings. The second largest planet in the solar system will make an arc across the sky throughout the night, moving over the moon at some point. For a guide to seeing it, check out this EarthSky post. (Mars is in opposition and thus not visible currently.)

So go outside tonight, weather permitting. It's pretty obvious which one is the Moon, and something like this won't happen for another two-and-a-half years, so might as well enjoy it now. (Resisting temptation to say, "It only happens once in a blue moon!")