Anticrepuscular Rays
This recent Astronomy Picture of the Day deserves a look, and some thoughtful consideration.
This image contains one of my favorite optical phenomena — anticrepuscular rays. If you’ve ever seen a sunset with searchlight straight beams of sunlight seeming to radiate outward from the position of the sun (just below the horizon or behind some clouds), then you’ve seen crepuscular rays. Many vernacular terms exist for these sunbeams; god rays, fingers of god, etc. What you’re seeing in these cases are interrupted rays of sunlight, obstructed by mountains, hills, clouds, etc. The rays look like they’re radiating out from a central source, but they’re actually parallel — perspective makes them appear to diverge from a point as they pass over your head.
Sometimes, when conditions are right, those rays will (usually invisibly) pass over your head and re-converge at the opposite horizon. Whenever you see crespecular rays at sunset, it always pays to turn around and look behind you, toward the east. When those rays reconverage in the east, they’re called anticrepuscular rays. Below is a shot from the first time I ever saw this effect, near Taos, New Mexico in 2005.
The convergence point of anticrepuscular rays is always directly opposite the sun’s position. If the sun is slightly above the horizon, the rays will appear to converge slightly below the horizon. If the sun is below the horizon, the convergent point will be just above.
In the APOD image above, the photographer has captured this beautiful phenomena from an airplane, with the sun above the horizon. So in this case, the photographer is looking down at the reconvegence, and the converging rays are coming from both above and below the convergence point. This is absolutely, breathtakingly beautiful and a great opportunity to both marvel at nature’s beauty and learn something from it!
A few years ago, I was lucky enough to capture a singularly rare example of anticrepuscular rays paired with a rainbow:
As the APOD description explains, my image captures two phenomenon that can only appear at the anti-solar point, anticrepuscular rays and a rainbow. My image was taken at sunset at Bryce Canyon, Utah. I owe this shot to my daughter, who spotted it before I did as we hoofed it up the hill to Inspiration Point, trying not to miss the setting sun. Because of this show to the east, we barely even glanced to the west!
Further reading on anticrepuscular rays:
