The Sky That Chooses

The Sky That Chooses

Nineteenth century. A scientist sits at his desk. His name was John William Strutt, but everyone knew him as Lord Rayleigh. He is thinking about one question, a question people had been asking for thousands of years. Why is the sky blue? He took a sheet of paper and wrote down a formula. For the first time, someone explained it so precisely that textbooks today say almost the same thing. But you and I won't need paper. We're going to play.

— Maybe the sky is just painted? Somebody took a brush and painted it blue?

Good guess! But then where did the paint come from? And why doesn't it drip? No, it's trickier than that. The sky isn't painted. The sky chooses, all by itself, which color to show you.

Look. The Sun sends light down to us. And that light isn't just "white". It's actually a mix of all the colors at once. Red, orange, yellow, green, blue, violet. All together. Mix all your markers and you get a dark muddy mess. But mix all the colors of light, and you get almost white. Amazing, right?

— Wait. All the colors at once make white? That's not fair.

Not fair, I agree! But that's just how light works. So this white light travels down from the Sun, toward us. And on the way, it runs into... air.

And air isn't emptiness. It's full of tiny molecules. So small that you can't see them even in the strongest microscope. And when light hits these molecules, some of it scatters in all directions. But different colors scatter differently.

— I know! Red is a big heavy ball, so it flies right past. Blue is small and light, so it gets knocked away!

Lovely idea! But light has no weight. Red isn't heavier than blue, and they travel at the same speed. So it isn't about weight. Then what is it? Light is a wave, like a ripple on water. And every color has its own stride: red has a long stride, blue has a short one. An air molecule is far smaller than either of them. The short blue wave makes the molecule shake faster, and the faster it shakes, the more light it throws around. To the right, to the left, up, down. About four or five times more blue gets scattered than red. The long red wave rolls right past, almost undisturbed.

So blue light spreads across the whole sky, in every direction. And wherever you look up, there's a blue ray coming at you. That's why the whole sky looks blue.

— But what about violet? Its stride is even shorter. So shouldn't the sky be violet?

Oh, now that's the right question! Violet does scatter even more than blue. That's true. But there are two catches. First: the Sun sends out less violet than blue. Second: our eyes are not very sensitive to violet. So the eye ends up seeing more blue than violet. The sky is blue, not violet. The eye is a tricky thing.

— And why is the sunset red? It's the same Sun!

Same Sun. But different geometry! In the morning and at midday, the Sun is high. Its light takes a short way down through the atmosphere. Blue scatters across the sky, and the sky is blue. But in the evening, the Sun sits low, just above the edge of the Earth. And its light has to cross the atmosphere on a very long path, almost sideways, through the whole thick layer of air. Along the way, the blue gets scattered off in every direction, and it never reaches your eyes. Only the long waves arrive, the red and the orange. And that's your sunset.

— And space? The sky up there should be blue in the daytime, right?

No. In space, the sky is black. Even with the Sun right there, blindingly bright, the sky around it is black. Because in space there's almost no air. No molecules. Nothing to scatter the blue light. It flies straight on and never bounces off anything. So if you're not looking straight at the Sun, all you see is darkness.

A blue sky is a gift from the atmosphere. Not from light. Not from the Sun. From air.

— And what if the air is dirty? Smog, dust?

Then the sky changes color! Big particles of dust and smoke scatter light differently, not so picky about color. And the sky turns whitish, gray, murky. You can see it after a big fire or a dust storm. On Mars, for example, rusty dust hangs in the air all the time, and in the daytime the sky there is a reddish yellow. But the Martian sunset is bluish, because the dust there lets blue light through to the eye better than the other colors.

— So a blue sky only exists on Earth?

Well, on planets with the right kind of atmosphere, maybe something similar. But Earth's sky is special. It's blue, that particular blue, because our air is clear and made of tiny molecules, much smaller than a wave of light.

And now, an experiment you can do in your own kitchen. Take a glass of clean water. Shine a flashlight at it from the side. The water is clear, so the beam goes straight through. Now add a little milk to the water, just a few drops. Stir. And shine the flashlight from the side again. The water turns slightly bluish along the sides. That's the milk particles scattering the blue light, just like air molecules do. And if you look through the water at the flashlight itself, it looks yellowish or orange. The blue rays have gone off to the sides. Only the red and orange reached you. A sunset in a glass.

Today, on your way home, look up at the sky. If it's clear and blue, then right now, billions of blue rays are scattering in every direction, every second. If the sky looks pale or whitish, then something in the air is getting in the way of their game. And in the evening, find the sunset and tell it, "Got it."

That was "Oh, My Guide." If you're curious how anything else works, you can order a tour on it. Your backyard, the pyramids, a rhinoceros. Grown-ups know where to find me.

If you leave with more questions than you came with, that was the idea. The answers and the map live at moygid dot online.