A sun dog, sundog or mock sun, also called a parhelion (plural parhelia) in atmospheric science, is an atmospheric optical phenomenon that consists of a bright spot to one or both sides of the Sun. Two sun dogs often flank the Sun within a 22° halo, parallel to the horizon. The sun dog is a member of the family of halos caused by the refraction of sunlight by ice crystals in the atmosphere. Sun dogs typically appear as a pair of subtly colored patches of light, around 22° to the left and right of the Sun, and at the same altitude above the horizon as the Sun. They can be seen anywhere in the world during any season, but are not always obvious or bright. Sun dogs are best seen and most conspicuous when the Sun is near the horizon.
Formation and characteristics
Sun dogs are commonly caused by the refraction and scattering of light from horizontally oriented plate-shaped hexagonal ice crystals either suspended in high and cold cirrus or cirrostratus clouds, or drifting in freezing moist air at low levels as diamond dust. The crystals act as prisms, bending the light rays passing through them with a minimum deflection of 22°. As the crystals gently float downwards with their large hexagonal faces almost horizontal, sunlight is refracted horizontally, and sun dogs are seen to the left and right of the Sun. Larger plates wobble more, and thus produce taller sun dogs. Sun dogs are red-colored at the side nearest the Sun; farther out the colors grade through oranges to blue. The colors overlap considerably and are muted, never pure or saturated. The colors of the sun dog finally merge into the white of the parhelic circle (if the latter is visible). The same plate-shaped ice crystals that cause sun dogs are also responsible for the colorful circumzenithal arc, meaning that these two types of halo tend to co-occur. The latter is often missed by viewers, since it is located more or less directly overhead. Another halo variety often seen together with sun dogs is the 22° halo, which forms a ring at roughly the same angular distance from the sun as the sun dogs, thus appearing to interconnect them. As the Sun rises higher, the rays passing through the plate crystals are increasingly skewed from the horizontal plane, causing their angle of deviation to increase and the sun dogs to move farther from the 22° halo, while staying at the same elevation. It is possible to predict the forms of sun dogs as would be seen on other planets and moons. Mars might have sun dogs formed by both water-ice and CO2-ice. On the giant planets—Jupiter, Saturn, Uranus, and Neptune—other crystals form clouds of ammonia, methane, and other substances that can produce halos with four or more sun dogs. A related phenomenon, the Crown flash is also known as a "leaping Sundog".
Terminology A somewhat common misconception among the general public is to refer to any member of the ice halo family as a "sun dog" (especially the 22° halo, being one of the most common varieties). However, sun dogs represent just one of many different types of halos. For referring to the atmospheric phenomenon in general, the term (ice crystal) halo(s) is more appropriate.
Etymology The exact etymology of sun dog largely remains a mystery. The Oxford English Dictionary says it is "of obscure origin". 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, sun-dogs are defined:
The phenomena [sic] of false suns which sometimes attend or dog the true when seen through the mist (parhelions). In Norfolk a sun-dog is a light spot near the sun, and water-dogs are the light watery clouds; dog here is no doubt the same word as dag, dew or mist as "a little dag of rain" (Philolog. Soc. Trans. 1855, p. 80). Cf. Icel. dogg, Dan. and Swed. dug = Eng. "dew". Other sources observe that Dog in English as a verb can mean "hunt, track, or follow", so Dog the true [sun] has meant track the true [sun] since the 1510s. Alternatively, Jonas Persson suggested that out of Norse mythology and archaic names — Danish: solhunde (sun dog), Norwegian: solhund (sun dog), Swedish: solvarg (sun wolf) — in the Scandinavian languages, constellations of two wolves hunting the Sun and the Moon, one after and one before, may be a possible origin for the term. Parhelion (plural parhelia) comes from Ancient Greek: παρήλιον (parēlion, 'beside the sun'; from παρά (para, 'beside') and ἥλιος (helios, 'sun')). In the Anglo-Cornish dialect of Cornwall, United Kingdom, sun dogs are known as weather dogs (described as "a short segment of a rainbow seen on the horizon, foreshowing foul weather"). It is also known as a lagas in the sky which comes from the Cornish language term for the sun dog lagas awel meaning 'weather's eye' (lagas, 'eye' and awel, 'weather/wind'). This is in turn related to the Anglo-Cornish term cock's eye for a halo round the Sun or the Moon, also a portent of bad weather.
History
Antiquity Aristotle (Meteorology III.2, 372a14) notes that "two mock suns rose with the sun and followed it all through the day until sunset." He says that "mock suns" are always to the side, never above or below, most commonly at sunrise or sunset, more rarely in the middle of the day. The poet Aratus (Phaenomena, lines 880–891) mentions parhelia as part of his catalogue of Weather Signs; according to him, they can indicate rain, wind, or an approaching storm. Artemidorus in his Oneirocritica ('On the Interpretation of Dreams') included the mock suns amongst a list of celestial deities. A passage in Cicero's On the Republic (54–51 BC) is one of many Roman authors who refer to sun dogs and similar phenomena:
... what can be the nature of the parhelion, or double sun, which was mentioned in the senate. Those that affirm they witnessed this prodigy are neither few nor unworthy of credit, so that there is more reason for investigation than incredulity. Seneca makes an incidental reference to sun dogs in the first book of his Naturales Quaestiones. Pliny the Elder references accounts of sun dogs in the 31st chapter of the 2nd book of his Historia Naturalis:
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