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Hank: You know those pictures that your parents and your friends and your parents' friends are always posting all over social media? The ones that have some sort of inspirational quote, plus a picture where the Sun seems like it's using beams of light to break through a wall of clouds?

Those trails of light are called crepuscular rays, and they're actually pretty common. If you look at the sky on a partly cloudy day, especially if it's around dawn or dusk, odds are you'll see those rays for yourself - though you probably won't see any inspirational quotes floating around in the air.

Crepuscular rays happen when sunlight passes through air that has a bunch of stuff in it. Sunlight is always bouncing off of stuff in the air; the way it bounces depends on what it's hitting. Nitrogen in the air, for example, scatters the bluer parts of the Sun's light, which is why the sky is blue.

Crepuscular rays happen when sunlight hits bigger particles, like dist and water, which bounce all the light's colors pretty equally. And when we see all the colors of light at once, we call that white light, which is why a lot of crepuscular rays look white.

Sometimes the rays don't look white, which just means that the light hitting them wasn't white to begin with. So crepuscular rays around sunset will be redder, since sunlight looks pretty red by that point.

But crepuscular rays aren't always visible whenever there's a lot of dust or moisture in the air. There also needs to be something blocking some of the sunlight. Without anything in the way, we wouldn't be able to see the paths of individual rays, no matter how much stuff was in the air. Light would be bouncing off of everywhere equally, and everything would look equally lit-up.

These two things - particles in the air and something to block parts of the sunlight - are why clouds are so common in those Facebook pictures. Clouds are denser pockets of water vapor and dust that are really good at blocking sunlight, and the area around clouds also tend to have some dust and water; so if some light does manage to break through a hole in the clouds, it hits a whole bunch of material that scatters it.

But you don't need clouds. Mountains and buildings can also create crepuscular rays. And if the sun is at the horizon, like at dawn or dusk, you can get crepuscular rays that stretch all the way across the sky. So intense.

Like all crepuscular rays, they look like they're radiating out from the Sun, like a bunch of lines coming from a single point. But the ones right above you will look parallel. And the rays going behind you, directly opposite where the sun is, all meet again where they are called anticrepuscular rays. And they look like they're radiating out from some invisible sun on the darker side of the horizon.

So what's actually happening? Are the rays parallel, or do they spread out from the Sun?

Well, both.

The sun is so far away that when its light gets to Earth, it's all pretty much going in the same direction. So all of the sunlight causing crepuscular rays is virtually parallel, which means that the rays themselves are always parallel lines. They just look like they’re meeting at the Sun because of perspective, which makes things look smaller when they’re far away, just like how train tracks look parallel when you’re standing next to them, but they look like they’re getting closer together the farther away they are from you.

So the beginnings of crepuscular rays, closer to the Sun, are like those distant train tracks that seem like there’s hardly any space between them. Then, the rays directly above you are like the tracks right by your feet, so they look parallel. But the anticrepuscular rays are also really far away from you -- they’re just as far as the original ones were, so they look like they come together just like the original crepuscular rays did.

If you could see crepuscular rays from space, you’d see them for the parallel lines that they really are. And this is a thing that we know, because astronauts have seen crepuscular rays from space -- and this is what they see. So the next time someone puts up some photos of supposedly inspirational crepuscular rays, don’t forget to congratulate them on their excellent picture of dusty light.

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