It's very dark at Paranal but there's still background light that we can't do anything about: the atmosphere itself glows at night. It can be surprisingly bright. Often it's green, when oxygen atoms are glowing. It can be red, too, when nitrogen is responsible. And it can be orange, when sodium atoms are being excited. Tonight, it was extremely orange, looking a lot like streetlights on clouds, except there were no clouds, and there are definitely no streetlights near here. It got really strong while I was taking a time lapse and you can see huge waves in the upper atmosphere rippling.
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