I took this picture of a sundog this morning at work (Champaign, IL, on the campus of the University of Illinois). Several Facebook friends have posted similar pictures.
The scientific name for the sundog is parhelion. They are bright spots of light on either side of the sun and are caused by ice crystals, acting as tiny prisms bending (refracting) light from the Sun. Because they act like prisms, most of the time you can see several colors in the bright spots. It is fairly common to see one or both bright spots on either side of the Sun but I have rarely seen the full halo effect like this one.You can read all about sundogs on Wikipedia.
Sun dog taken this morning in Champaign, IL at work. The air temperature was -12. Click to enlarge for the full 2 Mb picture.
Illinois State Climatologist 
I live in Norwood Ontario Canada, 2 hours east of Toronto, just south of highway 7 in the country and I viewed the most beautiful sundog on both sides of the sun. It looked like miniture rainbows. But, I know it isn’t. I researched, and found out that it shines through ice crystals 22 degrees out from the sun. I am an older mother and showed my son the beauty of nature and what these mean. They mean a change in weather. I find them so beautiful and fascinating. I am trying to show my son the beauty and the power of nature and to admire and respect it as a wonderful creation. He is almost 11 years old. I love the country and what it has to offer, including quietness and hard work. My son is slowly learning these values in this electronic world. Thank for your info on sundogs. Please continue to inform us on your weather knowledge. I find weather and nature exciting and adventuresome, as you are always learning something new. All the best in the new year Yours truly, Catherine Salmon
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