A sun dog or sundog (scientific name parhelion, plural parhelia, for "beside the sun") is a common bright circular spot on a solar halo. It is an atmospheric optical phenomenon primarily associated with the reflection or refraction of sunlight by small ice crystals making up cirrus or cirrostratus clouds. Often, two sun dogs can be seen (one on each side of the sun) simultaneously.
Sundogs typically, but not exclusively, appear when the sun is low, e.g. at sunrise and sunset, and the atmosphere is filled with ice crystal forming cirrus clouds, but diamond dust and ice fog can also produce them. They are often bright white patches of light looking much like the sun or a comet, and occasionally are confused with those phenomena. Sometimes they exhibit a spectrum of colours, ranging from red closest to the sun to a pale bluish tail stretching away from the sun. White sundogs are caused by light reflected off of atmospheric ice crystals, while colored sundogs are caused by light refracted through them. White sundogs are also thought to be caused by the light from the sun reflecting off of water on the ground and focusing the reflected light on the clouds above.
The ice crystals causing atmospheric phenomena are shaped as hexagonal prisms (ice Ih, e.g. with a hexagonal top and bottom and six rectangular sides). Some of these crystals are elongated, some are flat; the latter causing crisp and bright sundogs if evenly oriented with their hexagonal ends aligned horizontally, while the former produces other atmospheric phenomena, such as parhelic circles, 22° halos, circumzenithal arcs, upper tangent arcs, and lower tangent arcs. A mixture of various crystals with different alignments produces several of these phenomena at the same time.
When sunlight passes through the sides of a flat crystal, both the angle of the sun rays and the orientation of the crystals affects the shape and colour of the sundogs. Misaligned or wobbling crystals produce colourful and elongated sundogs, while light passing through the crystal in non-optimal deviation angles (up to 50°) produces the "tail" of the sundog stretching away from the sun. As refraction is dependent on wavelength, the sundogs tend to have red inner edges while the colours farther from the sun tend to be more bluish-white as colours increasingly overlap.
When the sun is low, the two sundogs are located on the circle of the 22° halo. As the sun rises, the sundogs slowly move along the parhelic circle away from the sun, finally to vanish as the sun reaches 61° over the horizon (e.g. the sundogs move from the 22° halo to the circumscribed halo).
On Earth, the first planet (counting from the sun) with significant amounts of ice crystal-carrying clouds, the pair of sundogs flanking the sun are aligned with the horizon. On other planets and moons where water and ice are less prevalent, however, various crystal structures produce different halos. On the giant gas planets—Jupiter, Saturn, Uranus and Neptune—other crystals form the clouds of ammonia, methane, and other substances that can produce halos with four or more sundogs.
Be it so, said Tubero; and since you invite me to discussion, and present the opportunity, let us first examine, before any one else arrives, 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.
My beloved children, I want to tell you that on the day after the departure of our brothers Kuntz and Michel, on a Friday, we saw three suns in the sky for a good long time, about an hour, as well as two rainbows. These had their backs turned toward each other, almost touching in the middle, and their ends pointed away from each other. And this I, Jakob, saw with my own eyes, and many brothers and sisters saw it with me. After a while the two suns and rainbows disappeared, and only the one sun remained. Even though the other two suns were not as bright as the one, they were clearly visible. I feel this was no small miracle …The observation most likely occurred in Auspitz (Hustopeče), Moravia in very late October or very early November of 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 Tirol. The Kuntz Maurer and Michel Schuster mentioned in the letter left Jakob Hutter on the Thursday after the feast day of Simon and Jude, which is October 28. (This quote is also referenced by Fred Schaaf on page 94 of the November 1997 and December 1997 issues of Sky and Telescope.)
While mostly known and often quoted for being the oldest colour 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 still can be seen in the church Storkyrkan in central Stockholm.
In her history Shipwreck at the Bottom of the World: The Extraordinary True Story of Shackleton and the Endurance, telling the story of Endurance's ill-fated polar expedition in 1912, Jennifer Armstrong writes:
… All around them, too, were signs that the Antarctic winter was fast approaching: there were now twelve hours of darkness, and during the daylight hours petrels and terns fled toward the north. Skuas kept up a screeching clamor, and penguins on the move honked and brayed from the ice for miles around. Killer whales cruised the open leads, blowing spouts of icy spray. The tricks of the Antarctic atmosphere brought mock suns and green sunsets, and showers of jewel-colored ice crystals.
The short (166) Canto One, with all those amusing birds and parhelia, occupies thirteen cards.
A visit to the cabin showed him the glass lower still: sickeningly low. And back on the poop he saw that he was by no means the only one to have noticed the mounting sea – an oddly disturbed sea, as if moved by some not very distant force; white water too, and a strange green colour in the curl of the waves and in the water slipping by. He glanced north-west, and there the sun, though shining still, had a halo, with sun-dogs on either side. Ahead, the aurora had gained in strength: streamers of an unearthly splendour.
Hastings laughed, too, and shook his head. 'Men do make their luck, Lady Margaret, and never have I seen that better proven than at Mortimer's Cross. For ere the battle, there appeared a most fearsome and strange sight in the sky.' He paused. 'Three suns did we see over us, shining full clear.'
In a footnote it is clarified: "Phenomenon known as a parhelion, generally caused by the formation of ice crystals in the upper air."
Two pages later, again mentioning the English king Edward IV, she adds: "Many, she saw, flaunted streaming sun emblems to denote her son's triumph under the triple suns at Mortimer's Cross."
Shakespeare also appears to mention the phenomenon in his Henry VI, Part III when he has Edward say, "Dazzle mine eyes, or do I see three suns?"
Sundogs appear in the film The Deer Hunter. At the beginning of the film, as the men are leaving work, they see the phenomenon. Robert De Niro's character describes it as an 'old Indian thing' and "A blessing on the hunter sent by the great wolf to his children".
I've played the unraveler, the parhelion But even Apocalypse is fleeting There's no death, no ugly world
Jack London has a short story called The Sun Dog Trail.
One of Robert Rauschenberg's early experiments in 1962 employing the silk-screen process to reuse previously published images in "Combine Paintings" is titled "Sun Dog".
The poem Die Nebensonnen ("The Parhelia"), by Wilhelm Müller from his 1823-24 cycle Die Winterreise, was set to music by Franz Schubert: it begins "Drei Sonnen sah ich am Himmel stehn..." ("Three Suns I saw in the sky").