Sundogs (and Sun)
Astronomy has always been my first love, so people sometimes ask me what causes sundogs, rings around the sun or moon, light pillars, etc. I used to say, “Uh, it’s ice crystals.” That seemed to satisfy most people. But when I read the book “Rainbows, Halos, and Glories,” I learned what it really meant — how, exactly, ice crystals cause the various spots, arcs and rings you see in the sky from time to time. Suddenly the whole sky really came alive for me all over again.
Sundogs are the colorful spots you sometimes see on either side of the sun:
Sundog Sun Sundog
* O *
______________________________________ horizon
They’re caused by ice crystals shaped like hexagonal plates — like thick-ish stop signs, miniaturized. These little hex plates fall through the air with their faces parallel to the ground. So, picture billions of tiny quarters made of ice, all falling either “heads” or “tails”, not standing on the edge.
As they fall, the sun reflects off their faces and edges, and also passes through them making prismic colors (or not), depending on the angle of the sun and the angle from which you view them. There’s one particular angle that’s really sweet — an angle at which the sunlight passes horizontally right into the edge, along the plate’s face, and with the plate rotated “just so”. At that angle, the ice crystal passes a nice rainbow through itself.
Towards a certain direction in the sky, all the crystals that happen to be in this orientation “light up” that part of the sky. The direction works out to be about 22 degrees to the left and right of the sun:
* 22deg O 22deg *
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It turns out that various other angles are also “sweet” for different reasons. And ice crystals can have different shapes — for example, hexagonal cylinders, like pencils — which, in turn, creates a huge variety of wild arcs and rings and spots, which you can see if you are both lucky and alert.
Visit Les Crowley’s beautiful site on Atmospheric Optics or read “Rainbows, Halos, and Glories.”