A big anti-crepuscular surprise

You may not be familiar with the grandiose Latin term crepuscular, but you've undoubtedly seen this most wonderful of atmospheric displays. The sun, partly hidden behind a friendly cumulus cloud, seems to send our rays of glory, fanning out in unconstrained abandon across the sky. God's rays, they are often called. And you can see why. You would almost expect a choir of angels to burst into song at any moment in celebration of the glorious display in the heavens above you.

A wonderful crepuscular display in the heart of the Mournes.

As I've written before, they are formed as the light passes through tiny droplets of water and dust in the sky, causing the light to be refracted into the distinctive columnar shapes, similar to the illuminated shards that form when light shines into a dusty room. The regions where the cloud is blocking the light appear as the darker bands between the rays themselves. And, amazingly, they are not actually fan shaped, but more or less parallel. It's perspective that makes them take on their converging appearance - rather like the lines of a long, straight road appear to converge to a perpsective point in the far distance.

Crepuscular rays - and their reflection - at Portrush.

That can be a bit hard to visualise sometimes. But this photo might help. Taken from the International Space Station above the Indian Ocean with the sun setting to the west, it shows how the cloud creates rays of light travelling out to the east - but more or less parallel to one another. The vantage point of the ISS is far enough away and looking down from above that the perspective effect is not apparent. If, however, you were on a ship to the east of the cloud, you would see that classic crepuscular fan shape.

Source:  NASA

Source: NASA

What are anticrepuscular rays?

But, this blog is not primarily about crepuscular rays, exciting as they are to see and photograph. Rather, it's about the crepuscular rays' more elusive cousin. Fainter and more subdued, these other rays tend to hide in plain sight, coming out when the more effusive crepscular rays are putting on their showy display over by the sun.

And their name? Anticrepuscular rays, of course. While the c-rays are loudly doing their thing, the ac-rays can sometimes be found at the exact opposite point in the sky. The anti-solar point, in fact (hence the name).

How anticrepuscular rays are formed is all to do with that counterintuitive fact that the rays are nearly parallel. As the rays extend out past the cloud that is interrupting the sunlight, they in fact continue on their parallel journey above our heads into the sky above. As they pass overhead, they continue to travel in their parallel lines. But again, due to the effect of perspective, from our point of view they appear to converge on a point on the exact opposite side of the sky to the sun (our friend the antisolar point). That means that, if you want to see them, you'll have to tear your eyes away from the glories of the setting sun, with those dramatic God's Rays, and look to the east, for their fainter cousin to see if he is making one of his rare appearances.

Spotting the elusive antis

I've known about anticrepuscular rays for quite some time, but I've struggled to capture an image of them I like. Maybe its because I've been too distracted by the more dazzling crepuscular display going on by the sun. Maybe because they can really be quite faint and can often go unnoticed.

You will have to look carefully, but to the very right of the picture above Cove Mountain, you may just be able to make out the very faint rays fanning upwards, and continuing at an angle over Donard and Commedagh.

But recently on top of Slieve Bearnagh in the Mournes, I managed to grab a shot of them. Only I didn't realise it at the time! It was only when I got home and processed this panoramic shot that I noticed something towards the east.

At first, I thought it was Photoshop struggling to stitch the images together, producing a banding in the sky. But, on closer inspection, I saw it - that telltale pattern of convergence. I had got some anticrepuscular rays! Okay, so they are faint, and not as dramatic as the God Rays. But maybe their very elusivness made them all the more appealing to me.

So next time you see the heavens put on one of their grandest and most beautiful shows, stop and enjoy the display. But don't forget to take a look over your shoulder to see if the sunlight and perspective is putting on that quiet little display behind you, hiding in plain sight, just waiting for those in the know to notice. Happy hunting!