The image above shows a pair of bright sundogs as seen from the city of Fargo in North Dakota. Also visible here are sections of the arcs passing through each sundog. These are known as sun pillars, which are the vertical bars of light extending vertically upwards from the Sun and the parhelic circle, or the bright bars of light extending horizontally from the Sun towards each sundog.
What causes Sundogs?
Sundogs (‘parhelion’ or plural ‘parhelia’) are members of a larger family of atmospheric phenomena known as halos. Sundogs in particular are formed when sunlight is refracted in the horizontal plane through six-sided, plate-like ice crystals that float in the atmosphere, or are suspended in high and cold cirrus or cirrostratus clouds.
Although sundogs can appear singly, it is more common to see one on either side of the Sun, within a halo of about 22 degrees, but always at the same elevation as the Sun. In practice, the ice crystals act as prisms, and since the prisms are largely identical, the combined effect of light being refracted through them concentrates the bent light rays in two spots on either side of the Sun.
However, in some cases, the hexagonal ice crystals grow too large to fall evenly. Put in another way, the crystals fall in much the same way as a large oak leaf would when it falls – wobbling from side to side. When flat ice crystals fall in this manner, sunlight is scattered to a greater degree, which results in sundogs that are distinctly taller than the roughly spherical sundogs that are formed by smaller crystals.
The same crystals that cause sundogs to form also cause another phenomenon, known as the circumzenithal arc, an often-colorful arc that few observers ever see, since it passes more or less directly overhead, hence the name, circumzenithal arc. Yet another phenomenon created by the same crystals is the 22° halo, which appears at the same angular distance from the Sun as the sundogs, and therefore appears to connect the sundogs to each other.
However, as our Sun rises above the horizon, the angle of incidence of its light progressively deviates from the horizontal plane, meaning that the sunlight striking the ice crystals strike the crystals at an ever-increasing angle. The practical effect of this is to increase the angle of refraction, so the sundogs appear to move further away from the Sun than 22 degrees, while maintaining the same elevation as the Sun.
While many observers have reported sundogs that are brightly colored, the general rule is that colors in sundogs range from red closest to the Sun, to muted orange and blue furthest away from the Sun. There may be exceptions to this under favorable conditions, but in general, sundog colors are never pure or saturated, and usually blend into the visible white of the parhelic circle.
Sundogs on other Worlds
Since it is now known what causes sundogs to form, it is possible to predict where in the solar system they are also likely to form. For instance, since the atmosphere of Mars contains both water vapor and carbon dioxide ice, sundogs are almost certain to form there as well, although no reports of actual sundogs occurring on the red planet are known to exist. Similarly, the various types of ice crystals in the outer atmospheres of Jupiter, Saturn, Uranus, and Neptune will also almost certainly form up to four sundogs on each planet, although none have been observed to date.
Why are they called Sundogs?
The origin of the word “sundog” is largely a mystery, with the Oxford English Dictionary describing its origin as“obscure.” However, Abraham Palmer defines the word in his book with the tongue-twisting title, Folk-etymology: A Dictionary of Verbal Corruptions Or Words Perverted in Form Or Meaning, by False Derivation Or Mistaken Analogy, as follows:
“The phenomena [sic] of false suns which sometimes attend or dog the true when seen through the mist (parhelions). ”
The above explanation may or may not be accurate, but there is an alternative possible origin for the word. In English, the word “dog” can also be used as a verb, and in this case, “dog” would mean [to] “hunt,” “track,” or “follow.” So in this context, “sundog” can refer to the illusion of the spots of light “following,” “hunting,” or “tracking” the true Sun.
Sundogs in History
In Ancient Greece, Aristotle (384-322 BC) noted in his treatise entitled ‘Meteorology’ that “two mock suns rose with the sun and followed it all through the day until sunset.” Similarly, the ancient Roman author Cicero (54–51 BC) might have been referring to a scene much like the one described in the opening passage when he penned the following description in his book called ‘On the Republic’:
“Be it so, said Tubero; and since you invite me to discussion, and present the opportunity, let us first examine, before anyone 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”.
Nevertheless, the phenomenon has been mentioned numerous times throughout history, including at the Battle of Mortimer’s Cross during the Wars of the Roses (1455-1485), with the event later dramatized by William Shakespeare in King Henry VI, Part 3:
Three glorious suns, each one a perfect sun;
Not separated with the racking clouds,
But sever’d in a pale clear-shining sky.
See, see! they join, embrace, and seem to kiss,
As if they vow’d some league inviolable:
Now are they but one lamp, one light, one sun.
In this the heaven figures some event.