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A sun dog (or 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.
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, angular distance 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.
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 —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".
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:
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 — (sun dog), (sun dog), (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 (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 sign of bad weather.
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.p. 125 Artemidorus – The Interpretation Of Dreams Oneirocritica by Artemidorus Translation and Commentary by Robert J. White c. 1975 1990 Original Books, Inc. 2nd Edition
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:
Seneca makes an incidental reference to sun dogs in the first book of his Naturales Quaestiones.Seneca, Ricerche sulla Natura, P. Parroni editor, Mondadori, 2010
Pliny the Elder references accounts of sun dogs in the 31st chapter of the 2nd book of his Historia Naturalis:
The 2nd-century Roman writer and philosopher Apuleius in his Apologia says "What is the cause of the prismatic colours of the rainbow, or of the appearance in heaven of two rival images of the sun, with sundry other phenomena treated in a monumental volume by Archimedes of Syracuse."
Fulcher of Chartres, writing in Jerusalem in the early twelfth century, notes in his Historia Hierosolymitana (1127) that on February 23, 1106
The observation most likely occurred in Auspitz (Hustopeče), Moravia on 31 October 1533. The original was written in German and is from a letter originally sent in November 1533 from Auspitz in Moravia to the Adige Valley in South Tyrol. The Kuntz Maurer and Michel Schuster mentioned in the letter left Hutter on the Thursday after the feast day of Simon and Jude, which is 28 October. The Thursday after was 30 October. It is likely that the "two rainbows with their backs turned toward each other, almost touching" involved two further halo phenomena, possibly a circumzenithal arc (prone to co-occur with sun dogs) together with a partial 46° halo or supralateral arc.
While mostly known and often quoted for being the oldest color depiction of the city of Stockholm, Vädersolstavlan (Swedish language; "The Sundog Painting", literally "The Weather Sun Painting") is arguably also one of the oldest known depictions of a halo display, including a pair of sun dogs. For two hours in the morning of 20 April 1535, the skies over the city were filled with white circles and arcs crossing the sky, while additional suns (i.e., sun dogs) appeared around the sun. The phenomenon quickly resulted in rumours of an omen of God's forthcoming revenge on King Gustav Vasa (1496–1560) for having introduced Protestantism during the 1520s and for being heavy-handed with his enemies allied with the Danish king.
Hoping to end speculations, the Chancellor Olaus Petri (1493–1552), a Lutheranism scholar, ordered a painting to be produced documenting the event. When confronted with the painting, the King, however, interpreted it as a conspiracy — the real sun, of course, being himself —threatened by competing fake suns, one being Olaus Petri and the other the clergyman and scholar Laurentius Andreae (1470–1552). Both were thus accused of treachery, but eventually escaped capital punishment. The original painting is lost, but a copy from the 1630s survives and can still be seen in the church Storkyrkan in central Stockholm.
/ref>. A series of complex parhelia displays in Rome in 1629, and again in 1630, were described by Christoph Scheiner in his book Parhelia, one of the earliest works on the subject. It had a profound effect, causing René Descartes to interrupt his metaphysical studies and led to his work of natural philosophy called The World. René Descartes – Metaphysical turn, Stanford Encyclopaedia of Philosophy
On 20 February 1661 the people of Gdańsk witnessed a complex halo display, described by Georg Fehlau in a pamphlet, the Sevenfold Sun Miracle, and again the following year by Johannes Hevelius in his book, Mercurius in Sole visus Gedani.
On 18 June 1790 Johann Tobias Lowitz, in Saint Petersburg, Russia, observed a complex display of haloes and parhelia which included his .
"Part of the time we marched in the teeth of a biting storm of snow, and at every hour of the day the sun could be discerned sulking behind soft grey mists in company with rivals, known in the language of the plains as 'Sun-dogs', whose parahelic splendors warned the traveler of the approach of the ever-to-be-dreaded 'blizzard'."
On 14 February 2020, people in Inner Mongolia witnessed a different complex halo display called the Five-fold sun miracle, in which all five sun halos were linked to each other by rays, forming a circle among them.
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