22-degree Halo

Light from the sun or moon, bent through hexagonal ice crystals in cirrus clouds, often forms a ring removed at a 22-degree angle from the position of its source.  The coloring looks similar to a primary rainbow, but in reverse, with reds on the inside grading through yellow and green to blue and purple hues on the outside.  However, the process is not the same, since the water state involved is solid instead of liquid, and since the sun is in front of the observer and not behind.  Because the colors in the spectrum have different wavelengths, they bend at very slightly different angles through the ice crystals:  a fraction of a degree under 22 for red, a fraction more for blue.  Note the darker moat of sky inside the halo, related to the absence of refraction there.  For this shot, I used a physical neutral-density filter to lessen glare and better distinguish these features.

Norman OK (19 Oct 17) Looking S

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