That band of deep blue in the sky opposite a sunrise or sunset? It’s the shadow of the planet.
Okay, this is another one of those things that might be obvious to everyone except me… but I literally found out about this just today. So, keeping in mind xkcd’s Lucky Ten Thousand, now I’m passing it on to you.
When the sun is low on the horizon, look to the sky opposite it. Assuming clear weather and a long enough vista, you’ll see a band of dark blue stretching across the horizon. It’s the shadow of the Earth itself, cast up into the sky by the rising or setting sun.
Above that band is a pink band called the Belt of Venus: sunlight reflected back in the opposite direction by particles in the atmosphere. Both disappear quickly as the sun rises enough to wash them out, or sets to send the sky into darkness.