What is a Sundog?

Sundogs are bright flares of light that can appear on either side of the sun in the sky. They form when incoming sunlight bends, or refracts off tiny, prism-shaped ice crystals in the atmosphere. Sundogs appear only when the sun is fairly low in the sky. When the sun climbs too high, sunlight doesn’t hit the ice crystals at the right angles. Light can’t refract in a way we can see, so sundogs disappear. Sundogs are most common at cold temperatures, but they can occur when it isn’t freezing out. They can appear anywhere on Earth, as long as there are ice crystals in the atmosphere.

What is a Moon Dog?

Moon dogs appear as part of the 22° halo, roughly 10 Moon diameters outside the Moon. They are exactly analogous to sun dogs, but are rarer because the Moon must be bright, about quarter moon or more, for the moon dogs to be observed. Moon dogs show little color to the unaided human eye because their light is not bright enough to activate the cone cells.