Sure, we’re all familiar with sunsets, and how they appear to turn the entire sky close to the horizon red at night.
But it turns out it isn’t just the Sun, and it isn’t just the sky. If you look at the Moon at either Moonrise or Moonset, guess what color it appears to be? (Even in urban settings!)
Too difficult to tell? Let’s find you a better picture done with time-lapse photography.
What’s going on to cause this? Why do things which aren’t normally red appear red when you look at them on the horizon, from the Sun to the Moon to the sky itself?
This is all the atmosphere’s fault. After all, things look exactly the same on the Moon’s horizon as they do directly overhead:
So why so different here? What exactly does our atmosphere do? Well, the simple answer is that it scatters light. Not all light equally, though. The atmosphere is better at scattering blue light away, which means that blue light gets dispersed all throughout the sky pretty easily. But red light is more likely to pass directly through, which is why things appear redder on the horizon: more of the bluer light gets scattered away, while the red light comes (mostly) through to you. See this dramatic photo of the sky just after sunset from an altitude of 500 meters:
But there’s another huge effect, as you’ve probably guessed. After all, this doesn’t work when the Sun is directly overhead, or even close to it, but it works very well on the horizon. The Earth is pretty big, having a diameter just under 13,000 kilometers. But the Earth’s atmosphere is very thin. In fact, the troposphere (where air is breathable), which contains nearly 80% of the Earth’s air, only extends 17 km up on average. Like I said, thin, at least compared to the Earth.
Going through 17 km of air doesn’t have such a dramatic effect on light; about 84% of it still gets through. But when you look at the horizon, the light has a lot farther to go through that atmosphere. Think about it!
So, pretty much, only the red light gets through. And this very same phenomenon not only explains why the sky looks red at sunset, it also explain why the Moon looks red during a lunar eclipse! Because the only light that gets through is the little bit of red light that makes it all the way through the Earth’s atmosphere and onto the Moon:
So if you’ve got a message to send hundreds of kilometers through the air, send it in red, please. The atmosphere is just too strong for blue (or even green) signals to get through very far. Thanks a lot, Nitrogen gas.