Crepuscular rays – a tale of sunbeams diverging in the blue sky

Anti-crepuscular rays appear to converge in the eastern sky directly opposite the sun shortly after sunset on July 4. The pink rays are reddened sunbeams piercing through gaps in distant clouds; the dark 'rays' are cloud-shadowed regions. The dark band at bottom is the Earth's shadow rising. Photo: Bob King

It’s the 4th of July and I’m waiting for the fireworks to start. In the west the crescent moon is still shining, but a mass of clouds is slithering up from the horizon, hinting at the storm that will hit an hour after the show is over. That’s when I look over my shoulder to the east and spot a delicate series of choice anti-crepuscular rays.

It’s a mouthful of a word — anti-kree-PUSS-cue-ler — but you’re probably already familiar with them in another guise. Crepuscular rays are beams of sunlight that radiate from the sun when it shines through openings in the clouds. Sunbeams are visible because they’re reflected by dust, water droplets and even the air itself. The word ‘crepuscular’ refers to the hours around dawn or dusk, when the phenomenon is more often noticed.

Rays of sunlight streaming through clouds form lovely fans of light and shadow called crepuscular rays. Photo: Bob King

The shafts of sunlight bursting forth from a cloud formation appear both ‘dark’ and light. The bright shafts are sunbeams reflecting off dust and air; the dark ones are shadows caused by clouds blocking the sunlight. Together they form spectacular radiant fans of light that seem to herald the coming of the gods. On good days anyway.

Anti-crepuscular rays form the very same way as crepuscular ones, the only difference being the beams and shadows stretch clear across the sky to the opposite horizon. They’re visible around the time of sunset in the eastern sky and at sunrise in the western sky.  Be aware, they’re usually fainter than crepuscular rays, so you’ll need to look around a bit to see them.

Railroad tracks appear to converge in the distance to our eyes.

Notice that the rays appear to diverge or radiate from their source, the sun. In truth, the rays of light are nearly parallel, yet they appear to spread out because of how we perceive objects at a distance. The farther away something is, the smaller and more foreshortened it appears. Railroad tracks are a good example. Although we know the two rails remain separate and parallel, they appear to come together and almost touch in the distance. This is exactly what happens when viewing anti-crepuscular rays. Parallel light beams appear to converge at the anti-solar point directly opposite the sun.

From orbit, we see cloud shadows and sunlight passing them are nearly parallel. Credit: NASA

Take a look at the sunset photo of clouds and their shadows taken by astronauts on the Discovery space shuttle in 1998. There you can see how nearly parallel the shadows and beams are. A photo taken at the same time from the ground would show a grand display of crepuscular rays amid towering cumulus clouds and their extension as anti-crepuscular rays at the anti-solar point.

Crepuscular rays are everywhere. Here, trees stand in for clouds and a light fog provides a reflecting medium. Credit: Mila Zinkova

Crepuscular rays can form in many ways. All that’s required is a light source, a medium to reflect that source and something to cast a shadow. I’ve seen crepuscular rays of illuminated dust from sunlight shining through small holes in our window shade. Sunlight or moonlight in fog will also do the trick.

Light and shadow are constantly at play around us. The more we notice them, the more delightful the world becomes.

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