Check out this photo. It's from Adrienne Gauthier. It has a small rainbow in it. But, that's not what's most interesting about this shot.
It's the dark shadows appearing to converge in the sky on the OPPOSITE (eastern) horizon from the sunset. Take a look at the next three figures and then read on...
Figure 1: Photo of anticrepuscular ray (see the dark area?)
Figure 2: Photo of anticrepuscular ray with illustration.
Figure 3: A more obvious anticrepuscular ray.
When observed, the viewer's back has to be turned toward the sunset. This means they are looking opposite the sunset.
The fascinating thing about these rays is that they appear to converge. But, that's just an optical illusion!
First, what causes them?
Broken, spotty clouds on the horizon during sunset, or even mountains, create areas of shadows streaming across the sky.
Why do they appear to converge?
This is the optical illusion! Check out the photo below. See how the road seems to go from normal width at the foreground of the picture and end as a single point deep in the distance? This is the optical illusion you experience when you observe anticrepuscular rays!
Figure 4: A road going into the picture appears to get more and more narrow the farther away it is.
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