A sun dog is an atmospheric phenomenon where you can see additional bright patches in the sky on either side of the Sun. Sometimes you just see bright spots, and sometimes you can actually see an arc or even a halo around the Sun. These are all related to sun dogs, and have to do with very specific atmospheric conditions. If you’ve ever seen a sun dog, you were very lucky, and they only occur rarely.
Sun dogs occur because of sunlight refracting through ice crystals in the atmosphere. The crystals cause the sunlight to bend at a minimum angle of 22°. All of the crystals are refracting the Sun’s rays, but we only see the ones which are bent towards our eyes. Because this is the minimum, the light looks more concentrated starting at 22° away from the Sun; about 40 times the size of the Sun in the sky. At this 22° point you can get arcs, a halo, or just bright spots in the sky.
They can occur at any time of the year and from any place on Earth; although, they’re easiest to see when the Sun is lower on the horizon. As the Sun rises, the sun dog can actually drift away from the 22° point. Eventually the Sun gets so high that the sun dog disappears entirely.
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There are no set colors with sun dogs. The light from the Sun is being refracted equally by the ice crystals and so we don’t see the colors broken up as we do with a rainbow.
If you’d like more info on sun dogs, check out this site.
We’ve recorded several episodes of Astronomy Cast about the Sun. Listen here, Episode 30: The Sun, Spots and All.