A sun dog appears on the horizon in Yellowstone.

This is an atmospheric phenomenon known as sundogs. It’s also known as a phantom sun or mock sun. The technical name is a parhelion, which means “beside the sun.”  During very cold weather (like this zero degree day in Yellowstone) sundogs are formed by ice crystals called diamond dust which are drifting randomly in the air. The ice crystals act as prisms forming two rainbow colored bright spots of light (the sundogs) which anchor a halo around the sun.