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If you look closely enough, you’ll see the night sky is actually a little green. SciShow Space explains the science behind the phenomenon known as airglow.

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The night sky is full of all sorts of things: the Moon, stars, some planets, sweeping swaths of the Milky Way. But if you took away all of that stuff, you'd think the sky would be just dark. I mean, night is black, right?

Not really. If you look closely enough, you'll see that the night sky is actually a little green. If you're far away from light pollution and your eyes have adjusted properly, you can see green light as a faint glow, usually close to the horizon and sometimes with streaks and swirls in it. But, it's much more obvious from orbit, where the dark side of Earth looks like it has its very own green bubble, with some wispy red on the outside. This ghoulish sheen kind of sounds like an aurora, but it's very different. 

It's called airglow, and it happens all over the world. We've talked about other aspects of the sky's colors, like why it's blue during the day and what causes auroras, and like those phenomena, airglow had to do with how the sun's energy interacts with our atmosphere.

It begins during the day time, when ultraviolet radiation from the sun hits molecules of oxygen in the air. When that happens, some those molecules can't handle the extra energy and they split into individual oxygen atoms. Not only that, this excess energy also causes the electrons in each of those atoms to jump into a higher orbital around their nuclei.

When atoms become excited like this, they also become really unstable, like I get when get really excited. So, when night comes and the atoms aren't getting bombarded by sunlight anymore, they're going to release this excitement however they can, and the most stable way to do that is to combine with another oxygen atom. The odds of that happening are better where there is more oxygen around, obviously.

So, up to about 95 km above Earth's surface, the atmosphere is dense enough for these worked-up oxygen atoms to find others of their kind. When they combine, these atoms release the energy they got from the sun, emitting it at a wavelength that's just barely in the visible spectrum, a very faint blue that's hard to see from the ground.

But, if you go a little higher in the atmosphere, you'll find that things get more complicated. Between 95 and 100 km, you're almost at the border between Earth's atmosphere and space, where there's much less oxygen. Atoms up here are less likely to find a partner to pair with, and the longer they remain unstable, the greater the chance is that they'll release their energy in the next best way, by dropping their electrons back down a notch, to a slightly less excited state.

When this happens, they unleash some of their energy, but at a lower wavelength than those other atoms, emitting light smack in the middle of the green part of the electro-magnetic spectrum. So, it's the excitement of the oxygen atoms at the very edge of space that creates the green airglow you can see in the night sky.

But if you could look at Earth from orbit, you'd see there's still another layer, a wide band of red above the green.  This red band occurs just above a 100 km in altitude, the outermost limits of Earth's area envelope, where some of those less excited but still unstable oxygen atoms finally wind up.

Up here, even though they literally went to the ends of the Earth, they're never gonna find another atom to combine with. So eventually their electrons finally drop all the way down from their less-excited state to their stable, or ground state. And this final jump releases energy yet again, although less than before, so it emits light in the red part of the spectrum, creating the ribbon of scarlet that hangs over the green airglow.

So basically, the colors that you can see in the nighttime sky, whether in orbit or on the ground, are the result of impatient oxygen atoms at the edge of space, looking to get rid of some pent-up energy.

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