A “sundog” (parhelion) is a concentrated patch of sunlight occasionally seen about 22° to the left or right, or on both sides, of the Sun. Sundogs are often white but sometimes quite colorful, looking like detached pieces of rainbow.
Sundogs are caused by the refraction and scattering of light from plate-shaped hexagonal ice crystals either suspended in high, cold cirrus or cirrostratus clouds, or drifting in freezing moist air at low levels as diamond dust. They disappear as the Sun rises.
A Sun halo (also known as icebow, nimbus, or gloriole), a circle of light that creates a circle 22° wide around the Sun, is a related phenomenon. Unlike sundogs, which are generally only seen when the Sun is near the horizon, the halo is visible even when the Sun is high.
“Parhelion” comes from Ancient Greek, meaning ‘beside the sun.’ As for “sundog,” The Oxford English Dictionary says it is “of obscure origin.” It has been in use since the early 1600s. However, there are some interesting suggestions.
In Abram Palmer’s 1882 book Folk-etymology: A Dictionary of Verbal Corruptions Or Words Perverted in Form Or Meaning, by False Derivation Or Mistaken Analogy, sundogs are defined: “The phenomena of false suns which sometimes attend or dog the true [sun] when seen through the mist.” The same source says, “dog is no doubt the same word as dag, dew or mist as “a little dag of rain,” ignoring the fact that “dogging” is a slang term for “following.”
One suggestion is that the term arose from Norse mythology and archaic names — Danish: solhunde (sun dog), Norwegian: solhund (sun dog), Swedish: solvarg (sun wolf). In the Scandinavian mythology, constellations of two wolves hunting the Sun and the Moon, one after and one before, may also be a possible origin for the term.
In the Anglo-Cornish dialect of Cornwall, United Kingdom, sundogs are known as weather dogs. Aristotle referred to them as “mock” suns.
My own preference is for the Norse mythology, perhaps because the imagery of a wolf hunting the Sun sounds like the beginning of a good story.