Death Valley and its mountain shadow mystery

Robert Spurlock took this image of a mountain shadow, falling across Death Valley, and posted it on EarthSky’s Facebook page. This is the shadow of Ubehebe Peak falling on Racetrack Playa – beloved Death Valley landmarks. Here are those landmarks again, this time from a high vantage point. Robert’s image is wonderful in itself, especially when you consider how fast the shadow was moving: about 20 feet per second by his estimation. But the green line on the shadow’s edge also provided an enjoyable mystery.

Notice the green line on the edge of the shadow. Robert didn’t recall seeing the green line at the time he took the picture, but, he said, he was fumbling with his gear, and the shadow’s boundary was, after all, moving very fast across the desert sand.

Afterwards, I asked two experts about this green line and, while we never got a complete answer, their speculations are interesting.

Larry Sessions – who blogs about atmospheric phenomena for EarthSky – said:

I don’t know for sure, but I strongly suspect that it is a diffraction effect originating in the sunlight passing the ridge or mountain casting the shadow. Normally diffraction effects are seen on a much smaller scale, but this strikes me as a likely explanation. I don’t recall ever seeing something quite like this, but in fact there is something to reflect back the light, that being the sand. The physical size and shapes of the sand grains, along with their transparency, may be just right to allow an internal reflection of the green light. In that case it would be due to an effect similar to the internal reflection that causes a rainbow, except here the drops are really grains of sand. Or it could be a combination of diffraction from the edge of the mountain that forms the shadow, and internal reflection from the sand grains.

Plus, Larry added:

Another effect to be seen in the photo is called the “glory,” which in this case is evident as a slight glow around the head in Robert’s shadow.

Do you see what might well be a glory around Robert’s head? It’s very subtle – just a faint glow. Mountain climbers sometimes see glories, too, around their own heads, as they look opposite the sun. You might see a much more noticeable glory than the one in Robert’s picture while traveling in an airplane. You’ll look down and see the glory on the clouds below. The shadow of the airplane will be in the dead center of the glory.

Larry didn’t feel 100% sure about his answer regarding the green line on the shadow boundary, though. He suggested I contact Les Cowley in the U.K., who runs the amazing and beautiful website called Atmospheric Optics. I’ve been a fan of this website for years, and you will be, too, so it was an honor for me to have a reason to contact Les. He kindly agreed to look at Robert’s picture. He pondered it for a few days. And here’s what Les, who is surely a world expert on this sort of thing, finally said:

Usually if I leave an optical puzzle to fester in the head for a few days a solution will come. Not this time. I do not know the cause. Shadow edges of large objects like houses or mountains are diffuse because of the 0.5 degree angular diameter of the sun. Diffraction at that scale is insignificant and undetectable. Effects from the sand grains are a (slight) possibility but I would like to see separate confirmation images showing it.

My golden rule with these things (sorry to sound pompous!) is that odd effects need to be seen with the unaided eye plus 2-3 hand held camera shots. The latter helps to eliminate camera lens and internal reflection effects.

Meanwhile, Robert Spurlock, who took the photo, is a guy who spends a lot of time outside and who takes a lot of photos. Robert said:

I’ve been photographing weird things for 40 years and only have had “camera artifacts” when pointing at the sun, not away. In this pic there is nothing able to reflect light back into the lens…..I have played around with contrast, exposure, brightness, and saturation fixes and the line remains (and stays green) throughout. Hmmmmm. I love a mystery, I guess there is only one thing to do…..Go back to the Racetrack and repeat the experiment/experience with friends, yeah!

Robert wanted to point out, also, that in the far distance of the upper right, you can see a sliding stone out on the playa.

Robert added:

I love this place!

Now that we get to enjoy your photo of the shadow of Ubehebe Peak falling on Racetrack Playa in Death Valley, Robert … we all love it, too. Thank you.

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