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My life improved dramatically when I finally understood why the sky is blue, like really why.

Basically, it's because of Rayleigh Scattering (and Mie Scattering)

"What the hell is Rayleigh Scattering? What does that mean? That doesn't mean anything?" one might reasonably ask.

Well, basically, blue and violet light are scattered much more randomly than the longer, redder wavelengths. This results in the longer wavelengths making it to your eyes directly, whereas the blue and violet (though the sun doesn't emit much violet compared to blue) may take a more complicated path, and end up reaching your eyes appearing to have come from some random point in the sky.

This is why the sun appears yellow during the day, or even orange or red at sunrise and sunset, because even more blue light gets filtered out by randomly bouncing down to the surface or back off into space when it has to move through a bigger mass of air to reach you. However, from space, the sun is just plainly white.

So basically, a bunch of white light is coming down into the atmosphere, and it's all getting scattered, but the degree to which it's scattered is determined by 1. how much air it has to go through to make it to you in a straight line from the sun and 2. how long its wavelength is.

If the sky scattered all the wavelengths equally, it'd look like this. Kind of white-grey.

Rayleigh scattering also works on distant objects too, for instance these far off mountains appear blue because the originally green light reflected off of them has had its blue and violet light scattered by the air in between you and them.

The atmosphere isn't like a blue filter, letting only the blue light through, if it were, EVERYTHING would be blue.

And the atmosphere isn't absorbing and glowing blue either, or is the atmosphere itself blue, like a cloud of blue smoke. It's doing none of these things.

All the wavelengths are still coming in, and white light is still the vast majority of that light. However, when in the particular case that it gets scattered off an air particle and reaches your eyes, you see it as blue.

Frankly, the air scatters violet light more this way than it does blue, however the sky doesn't appear violet merely because the sun itself isn't putting out as much violet light than it is blue light, so we see blue.

Well, you might then also still wonder, "if all the objects are getting a little more blue coming in from every angle, shouldn't everything be a bit more... blue? Shouldn't it accumulate, even if the effect is small?" And you'd be right to ask this if you didn't account for all of the blue light that gets bounced right back into space.

This is why you can well, see the atmosphere from space. All of that blue light is just wandering back off into space from the atmosphere, and that effect sort of nullifies the previous conjecture.

Now here's the really REALLY cool bit about Rayleigh Scattering. If the white light has to travel through a LOT of air to reach you, like at sunrise and sunset, the light starts to lose a considerable amount to the ground and to space, and it becomes orange or golden...

...But when the sun is even lower beneath the horizon, most of the shorter wavelengths are shooting right off into space, but all of the shorter wavelengths are still bouncing around. So everything appears blue. They call it blue hour.

The blue light goes wild, whereas the redder light takes a more direct path. Here's a case where the shadow side of a mountain is being illuminated by the blue light, it's golden and blue hour right next to each other.

Finally, the last cool tidbit of fun info for you about Rayleigh Scattering is that on Mars, there's so little air that a small halo around the sun at sunrise and sunset actually becomes blue for the same reason as why OUR sky is blue, but it can only happen on Mars at those times because Mars' atmosphere is so thin and pathetic.

It can sometimes seem like we take some facts for granted, and some things are true for a very sort of complicated set of reasons, but it's not magic, it's actually rather mundane, and it's fun to ask questions. Really dumb questions, really obvious ones. If we don't we just get old and afraid to ask.

Sometimes it really just feels great to KNOW why the sky really is blue, to genuinely really understand why that's the case is a rewarding thing by itself.

@starwall
I’ve seen pictures of it from space before but somehow it never registered that it was a different color...

@starwall
It’s those cheap phone cameras they use in space, of course

@Paradox @starwall Mars is suffering from an old battle scar. Poor guy can't see as well as he used to.

@starwall :blobaww: this is rad, thanks yo! I thought this was something I 'totally vaguely understood' but like - there are whole layers I didn't know about and it makes sunsets especially like ~10% cooler now.

@starwall should we point and laugh at mars’ weak and ineffectual atmosphere?

@KitsuneAlicia @starwall idk if you knew this, but there is no white light wavelength. It's just a combination of various visible wavelength such that the red, green, and blue cones in the human eye get equally activated

@KitsuneAlicia @starwall so an individual photon can't be white, but I suppose 3 or more together can be

@starwall In other words, the higher energy blue & violet rays are like a kid who was just fed candy. XD

@KitsuneAlicia also sugar rushes are myths but we'll debunk that another day

@starwall I look forward to the thread so that I can finally tell off people who criticize me for eating too much sweets. XD

@KitsuneAlicia sugar crashes are still very real though, even though sugar rushes are basically a placebo.

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