sundog

Sundog — also sun dog or mock sun — is an English word of uncertain origin for a parhelion: a bright spot that appears on one or both sides of the sun, caused by the refraction of sunlight through hexagonal ice crystals in cirrus clouds. The 'dog' element is unexplained. One theory connects it to the idea of the bright spots 'dogging' or following the sun. Another links it to Norse mythology, where the sun was chased by wolves. No theory is conclusive.

The phenomenon is common in cold climates. Sundogs appear when the sun is low on the horizon and the atmosphere contains flat, hexagonal ice crystals oriented horizontally. The crystals act as prisms, refracting light at a consistent 22-degree angle from the sun. The result is two bright spots — sometimes with rainbow coloring — flanking the sun at equal distances. In the Arctic and subarctic, sundogs are a regular winter occurrence.

The Greek word parhelion (para + helios = beside + sun) is the scientific term. It appeared in Aristotle's Meteorologica (~340 BCE), making it one of the earliest documented atmospheric optical phenomena. Medieval Europeans interpreted parhelia as omens. Three suns in the sky — the real one and two false ones — suggested divine warning. The Battle of Mortimer's Cross in 1461 was preceded by a parhelion that Edward IV interpreted as the Trinity blessing his cause.

Sundogs are photographed frequently in the American and Canadian Great Plains, where cold, dry winter air and flat horizons combine to produce spectacular displays. The word 'sundog' is primarily North American. British English prefers 'mock sun' or 'parhelion.' The American term has the advantage of being memorable, even if no one can explain it.