#BAFact: Face east just after sunset. That dark band across the horizon? The Earth's shadow on the sky.

The Earth's air isn't perfectly transparent. There's haze, aerosols, and just junk floating in it. When the Sun is up, it's lit up like everything else. If an object passes between that junk and the Sun, you can see the shadow of that object cast on the floating material, just like you see the shadow of a tree cast on the ground.

At sunset, the horizon of the Earth itself can get between the Sun and that stuff, casting its shadow on the sky itself! Above that shadow line the stuff is lit by the setting Sun, but below it the Sun is blocked by the Earth. This casts a darkish band across the eastern horizon at sunset, called the Belt of Venus, and it's generally visible if you have a clear view to the east. It's easiest to see in summer, when generally speaking there's more haze in the air.

You can see it in the video below, and I have more info on the blog:  
http://blogs.discovermagazine.com/badastronomy/2011/10/30/time-lapse-crater-lake/

I'll leave you with a question: why is the name for this phenomenon ironic? :)
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