Crepuscular rays

By Phil Plait | August 2, 2006 11:02 pm

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.

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.

CATEGORIZED UNDER: Astronomy, Cool stuff, Science

Comments (13)

  1. eddie

    WOW! You just brought back a spectacular memory for me.

    When my daughter was somewhere around six or seven years old, we were driving home from visiting relatives through a rural area of South Georgia, and saw the most incredible display of crepuscular rays reaching across the sky.

    At the time, I didn’t even know what they were, having never encountered such a sight. We stopped and got out of the car to marvel at those suckers. As I recall (and this was some 15 years ago) they were very well-defined, and followed the “railroad track” model to a tee.

    I made it a point to find out what they were, and my daughter and I talk about that sight to this very day.

    Thanks for the bringing this memory to the surface again, BA. It was one of those special moments that only a parent can truly understand.

  2. Evolving Squid

    When I was a little squid, my family used to call those effect “glory rays” because every illustration of God or the power of God or similar divine-type of interaction always has those rays radiating out from the sun/sky/face of god/whatever.

    I think I was a teen before I learned the word “crepuscular” :)

  3. “Glory rays” is a bad name for the crepuscular effect, since there is a quite different but also very neat phenomenon called the glory. I saw my first glory on a plane flying back from Tokyo to Chicago; in a roundabout way, glories played a big role in nuclear physics, because seeing one at the Ben Nevis weather observatory prompted C. T. R. Wilson to invent the cloud chamber!

  4. I saw a great glory while flying once. I could see the shadow of the plane in the exact center (as it must be) of the glory as we flew over a cloud. We were descending, so as we got lower the shadow of the plane got bigger, while the glory stayed the same size. Weird. Eventually we got close enough to the cloud that I could tell the shadow of the plane was off-center– I was sitting near the back of the plane, so that part was centered for me. After another minute we plunged into the cloud deck, right into the glory.

    There’s just so much to see!

  5. Max Fagin

    But of course, Bart Sibrel knows the truth that the crepuscular rays really are converging. An so are the railroad tracks . . . Ha!!

  6. Evolving Squid

    I’ve seen that “glory” phenomenon, but didn’t know it had a name. heh. I learn something new every day…

  7. Evolving Squid

    Glory does sound better than “that rainbowy shadow in the middle effect thingy”

  8. The descrepencies in these so-called “photographs” serve to prove that not only has the Bad Astronomer never actually seen the sky, but that the sky doesn’t really exist at all! It’s all part of a government conspiracy, wherein they project these images upon a screen surrounding the Earth to keep us from seeing the giant alien overlords studying us. It’s the only explanation that makes any sense, people! None of your fancy talk about “perspective” and “train tracks” and “reality” can convince me otherwise.

  9. David Vanderschel

    The most spectacular sunset I have ever seen in my life occurred a couple decades ago. I was in Cozumel on a boat returning from scuba diving. The sun set directly behind a puffy cumulus cloud that was fairly far out on the horizon. The rays into which the cloud divided the sunlight were bright and colorful all the way across to where they reconverged in the east. (There was also an interesting effect resulting from the fact that there was a sliver of visibility underneath the cloud.)

  10. Heh. I saw the most phenomenal sunset of my life just two weeks ago. I wrote a four page essay about it — yes, seriously — and now I just need to figure out what to do with it. It’s too long to post here. But the sunset was quite a sight.

  11. nice blog about the ray – check out my Sunset Railway image and blog.

    :) Marichu


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