Here is a picture sent to me by new AAVSO member, Dr. Douglas Allen of an atmospheric effect called sun dogs. You can see a portion of a circle, the halo, against the sky, and two bright spots either side of the sun. The bright spots are sun dogs.
Sun dogs are created by hexagonal atmospheric ice crystals refracting sunlight. Sun dogs are visible when the sun is near the horizon and on the same horizontal plane as the observer and the ice crystals. As sunlight passes through the ice crystals, it is bent by 22 degrees before reaching our eyes, so sun dogs always appear the same distance and angle from the sun.
The movement and orientation of the ice crystals determine a sun dog's shape, sharpness, and color. Mottled, wobbling, or tall crystals, generally result in more diffuse or colorful displays. An excellent web page describing sun dogs can be found at the Atmospheric Optics website.
Despite the fact this picture was taken on a frigid, 4 degrees F, Iowa morning yesterday, sun dogs don't require cold ground temperatures. The atmospheric phenomena can be seen around the world in any season—probably even on other worlds. Octagonal ammonia crystals in the atmospheres of Jupiter and Saturn, may spawn quadruple sun dogs!
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