A variety of the sundog is the
120° parhelion. These are caused by the same
hexagonal, plate-shaped ice crystals in the same orientation, but the plates must be sufficiently thick to allow sunlight to
reflect off of two adjacent internal faces before exiting a base face.
They appear as glowing white spots at the same elevation as the sun, 120° to the left and/or right. If needle-shaped ice crystals are also present, you may see a parhelic arc that you can follow across the sky to see where the 120° parhelia should be. They are rarely as bright as primary parhelia, but as they are visually isolated from the sun and its major halos, they can be quite arresting, floating there like an eye of the unblinking sky.
Sources:
- http://www.sundog.clara.co.uk/halo/120pars.htm
- http://www.meteoros.de/arten/ee18e.htm