A sun dog (or sundog), mock sun or phantom sun, scientific name parhelion, is an atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun.
Sundogs may appear as a colored patch of light to the left or right of the sun, 22° distant and at the same distance above the horizon as the sun, and in ice halos. They can be seen anywhere in the world during any season, but they are not always obvious or bright. Sundogs are best seen and are most conspicuous when the sun is low.
The exact etymology of sun dog largely remains a mystery. The Oxford English Dictionary states it as being “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 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.”
Alternatively, Jonas Persson suggested that out of Norse mythology, constellations of two wolves hunting the sun and the moon, one after and one before, may be a possible origin for the term.
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 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.
While mostly known and often quoted for being the oldest color depiction of the city of Stockholm, Vädersolstavlan (Swedish; “The Sundog Painting”, literally “The Weather Sun Painting”) is arguably also one of the oldest known depictions of a sun dog. For two hours in the morning of April 20, 1535, the skies over the city were filled with white circles and arcs crossing the sky, while additional suns 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 and Lutheran scholar Olaus Petri (1493–1552) 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 thus accused of treachery, but eventually escaping 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.