Crepuscular rays

The entire universe in blog form
Aug. 2 2006 11:02 PM

Crepuscular rays

A few weeks ago I was walking home with Mrs. Bad Astronomer and the Little Astronomer after taking the Bad Dog to the park, and we had all remarked on how lovely the rising Moon was. The Sun was just at the horizon, the Moon was on the opposite side of the sky, nearly full, and the air around the Moon was reddish and pretty.

As the sky darkened, I noticed a pattern of light and shadow pushing its way through the sky. It was low contrast, and it took me a moment to figure out what I was seeing, but then it snapped into place: crepuscular rays! These are the beams of light you can sometimes see fanning out from the setting Sun. The sunlight is broken up by mountains or clouds on the horizon in front of the Sun, and if the air is hazy enough you can see the beams superposed on the sky.

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

It's common enough to see the rays coming from the sunset, but the sky has to be just right for the rays to reach all the way across the sky. It has to be hazy, but not too hazy or else the light gets absorbed, and the rays fade out. This night things were just so, and I could see the rays coming together opposite the Sun in the sky.

I was able to get a picture:

I had to stretch the contrast to bring the rays out, and even then they're subtle; the eye is much better at seeing contrast than a camera is.

That image was taken with me facing roughly southeast. When I turned a little to my right and faced more south, here's what I saw:

That was a visual treat. The rays were obvious, but not splashy, and the Moon rising in and among them was very pretty.

Notice anything odd about those two pictures? The rays appear to be coming from two different directions! In the first picture, they are oriented upper-left to lower-right. In the second, they are upper-right to lower-left. That's because the area of the sky in the first picture is east of the anti-solar point (the point on the horizon directly opposite the Sun), and the area in the second picture is west of it. The rays are converging on the anti-solar point, so they look like they come from different directions on either side of that point.

I know that sounds confusing, but in fact you've seen this before! This is all an illusion due to perspective. It's like looking at railroad tracks while you're standing on them. They appear to converge in the distance due to perspective:

The rail on the left points to the upper right, and the rail on the right points to the upper left. See? Same thing with crepuscular rays, but on a somewhat larger scale.

I bet not too many people have seen crepuscular rays converging on the anti-solar apex (say that five times fast!), let alone the Moon rising entwined in them. But they are one of the many, many rewards you get in life if you keep your eyes open, and your head up.

TODAY IN SLATE

Medical Examiner

Here’s Where We Stand With Ebola

Even experienced international disaster responders are shocked at how bad it’s gotten.

It Is Very, Very Stupid to Compare Hope Solo to Ray Rice

The U.S. Is So, So Far Behind Europe on Clean Energy

Even if You Don’t Like Batman, You Might Like Gotham

Friends Was the Last Purely Pleasurable Sitcom

The Eye

This Whimsical Driverless Car Imagines Transportation in 2059

Politics

Meet the New Bosses

How the Republicans would run the Senate.

A Woman Who Escaped the Extreme Babymaking Christian Fundamentalism of Quiverfull

Nobody Knows How Many Gay Married Couples Live in America—Not Even the Census Bureau

  News & Politics
Politics
Sept. 22 2014 6:30 PM What Does It Mean to Be an American? Ted Cruz and Scott Brown think it’s about ideology. It’s really about culture.
  Business
Moneybox
Sept. 22 2014 5:38 PM Apple Won't Shut Down Beats Music After All (But Will Probably Rename It)
  Life
Outward
Sept. 22 2014 4:45 PM Why Can’t the Census Count Gay Couples Accurately?
  Double X
The XX Factor
Sept. 22 2014 7:43 PM Emma Watson Threatened With Nude Photo Leak for Speaking Out About Women's Equality
  Slate Plus
Slate Plus
Sept. 22 2014 1:52 PM Tell Us What You Think About Slate Plus Help us improve our new membership program.
  Arts
Brow Beat
Sept. 22 2014 7:46 PM Azealia Banks’ New Single Is Her Best in Years
  Technology
Future Tense
Sept. 22 2014 6:27 PM Should We All Be Learning How to Type in Virtual Reality?
  Health & Science
Medical Examiner
Sept. 22 2014 4:34 PM Here’s Where We Stand With Ebola Even experienced international disaster responders are shocked at how bad it’s gotten.
  Sports
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.