This time of year, as the sun sets earlier and earlier in time for us to leave the office, you are probably noticing a bright spot to either side of the sun shortly before the sun drops below the horizon.
The scientific term for the bright spots is parhelia, but they are more commonly known as sundogs.
How sundogs form
Sundogs are a form of atmospheric optics that occur when the sun’s rays pass through ice crystals high in the atmosphere. The ice crystals indicate that moisture is present in the atmosphere, well above the freezing layer.
In the case of sundogs, the ice crystals are oriented in such a way that their large faces are parallel with the ground. This allows sunlight to pass through one of the edges, when it then refracts (or bends). The refraction is occurs at 60 degrees from the edge of the crystal, or 20 degrees from the original path of the sun’s ray if it hadn’t entered the crystal.
The sunlight travels through the crystal and exits out of a different edge, refracting once again. Red light refracts a little more than the blue light, so you see the reddish hue closer towards the sun.
It takes more than one ice crystal to produce a sundog. In fact, the phenomenon that you see occurs because many crystals are present and their sunlight is being directed back to your eyes.
A lot of geometry and optical physics are involved with sundogs. For more information, be sure to check out the excellent Atmospheric Optics site.