Anti-Crepuscular Rays

Anticrepuscular Rays Over Colorado
Photographer Katharine Allen

The sun was setting yesterday in the Sierra Nevada mountains of central California when a fan of shadowy rays sprang up from the horizon. They looked like ordinary sunset rays except for one thing–“the sun was on the opposite side of the sky,” says Katharine Allen, who witnessed the display and photographed it while she was walking her dogs.

“What was it?” 

Anti-solar (Anti-crepuscular) ray
Rays in the east opposite the setting sun the foreground and clouds are still sunlit.
Crepuscular rays appear to converge on the sun, anticrepuscular or antisolar rays converge in the opposite direction and you must have your back to the sun or sunset point to see them. They appear to converge towards the antisolar point, the point on the sky sphere directly opposite the sun. Like crepuscular rays they are parallel shafts of sunlight from holes in the clouds and their apparently odd directions are a perspective effect. Think of a long straight road, it converges towards the horizon but turn around and it also converges to the opposite horizon.   Crepuscular and anticrepuscular rays behave in the same way. 
Anticrepuscular rays are not rare but they must be sought carefully. When ordinary crepuscular rays are visible, turn around and search for their opposite numbers.   More rarely, sun rays pass right across the sky.

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