Bad Astronomy

Crepuscular rays are parallel!

When you go outside at sunset, many times you’ll be greeted with spectacular rays of light and shadow stretching across the sky. These are called crepuscular rays, and are caused by clouds blocking the sunlight, their long shadows cast on haze and other particulates floating in our air.

Those rays fan out, spreading away at different angles… but that’s an illusion! The rays are parallel, and I offer this photograph as proof:

[Click to penumbrenate.]

That shot was taken on October 18, 2011, by an astronaut on board the International Space Station as it passed over India. Towering cumulonimbus clouds threw their long shadows back, away from the Sun. Note that the shadows from different clouds are parallel to each other! That’s because the Sun is very far away compared to the distance between the clouds.


Here’s a picture I found on Flickr showing what we see from the ground, though (it’s not of the same clouds, but just a typical display of crepuscular rays). The fanning out of the rays is actually an illusion, caused by perspective! It’s precisely the same thing that makes railroad tracks or long roads appear to converge in the distance. Things farther away look smaller, so the parallel rails of a railroad track appear to get closer together as you look farther away. For railroad tracks you look down to see this; for cloud shadows you look up! Other than that, they’re the same.

So why do the shadows in the first picture look parallel? It’s because the astronaut was looking straight down on the clouds and shadows, so his distance to any part of the shadow was roughly the same; the shadow near the cloud and way downstream (so to speak) were both about the same distance away from him. That negates the perspective effect, and the shadows are revealed for what they truly are: parallel.

Astronauts have said it for years, but it bears repeating: exploring space gives you perspective. And in this case, it’s literally true.

Image credit: NASA; Elsie, Esq.’s Flickr Stream

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