Crepuscular rays are parallel!

Bad Astronomy
The entire universe in blog form
Nov. 2 2011 7:00 AM

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:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

Advertisement

[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



Related posts:


TODAY IN SLATE

Culturebox

The Ebola Story

How our minds build narratives out of disaster.

The Budget Disaster That Completely Sabotaged the WHO’s Response to Ebola

PowerPoint Is the Worst, and Now It’s the Latest Way to Hack Into Your Computer

The Shooting Tragedies That Forged Canada’s Gun Politics

A Highly Unscientific Ranking of Crazy-Old German Beers

Education

Welcome to 13th Grade!

Some high schools are offering a fifth year. That’s a great idea.

Culturebox

The Actual World

“Mount Thoreau” and the naming of things in the wilderness.

Want Kids to Delay Sex? Let Planned Parenthood Teach Them Sex Ed.

Would You Trust Walmart to Provide Your Health Care? (You Should.)

  News & Politics
Politics
Oct. 22 2014 9:42 PM Landslide Landrieu Can the Louisiana Democrat use the powers of incumbency to save herself one more time?
  Business
Continuously Operating
Oct. 22 2014 2:38 PM Crack Open an Old One A highly unscientific evaluation of Germany’s oldest breweries.
  Life
Gentleman Scholar
Oct. 22 2014 5:54 PM May I Offer to Sharpen My Friends’ Knives? Or would that be rude?
  Double X
The XX Factor
Oct. 22 2014 4:27 PM Three Ways Your Text Messages Change After You Get Married
  Slate Plus
Tv Club
Oct. 22 2014 5:27 PM The Slate Walking Dead Podcast A spoiler-filled discussion of Episodes 1 and 2.
  Arts
Culturebox
Oct. 22 2014 11:54 PM The Actual World “Mount Thoreau” and the naming of things in the wilderness.
  Technology
Future Tense
Oct. 22 2014 5:33 PM One More Reason Not to Use PowerPoint: It’s The Gateway for a Serious Windows Vulnerability
  Health & Science
Wild Things
Oct. 22 2014 2:42 PM Orcas, Via Drone, for the First Time Ever
  Sports
Sports Nut
Oct. 20 2014 5:09 PM Keepaway, on Three. Ready—Break! On his record-breaking touchdown pass, Peyton Manning couldn’t even leave the celebration to chance.