Three Suns in the Sky? Halo Phenomena and the Sun Dog

6 min read

Two bright, colorful patches glow on either side of the Sun, as if three suns were shining at once, or a huge ring of light surrounds the Sun or the Moon. The phenomenon has frightened and fascinated people for centuries, and it is still sometimes reported as a UFO. Yet it is caused by sunlight refracted by ice crystals high in the atmosphere.

What do people see in these cases?

The most common sight is one or two bright, sometimes rainbow-colored patches next to the Sun, at the same height as the Sun. At other times a large, faint ring of light surrounds the Sun or the Moon, or a vertical column of light extends upward from the setting Sun. In cloudy weather the sun dog can appear as a blurred, glowing patch, and because it moves together with the Sun, it can seem as if it were following the observer.

What is a halo phenomenon?

A halo phenomenon forms when the light of the Sun or the Moon is refracted or reflected by ice crystals floating in the atmosphere. The crystals are usually in thin veil clouds (cirrostratus) at an altitude of about 5–10 kilometers, where the temperature is below freezing. In severe cold, tiny ice crystals can also float near the ground; this is called diamond dust.

The ice crystals are hexagonal and behave like small prisms. Which phenomenon forms depends on the shape and orientation of the crystals: whether they are flat plates or elongated columns, and whether they float randomly or horizontally.

The most common phenomena

22-degree halo: the most frequently seen halo. According to German halo observers, it appears on as many as 80–120 days a year. A large, faint ring of light around the Sun or the Moon, with a radius of about 22 degrees. Its inner edge is reddish, shifting outward to whitish and bluish, meaning the color order is the reverse of a rainbow's. Inside the ring the sky looks darker, because the crystals do not refract light there.
Sun dog (parhelion): a bright, often colorful patch on one or both sides of the Sun, at roughly 22 degrees, at the same height as the Sun. It forms when flat, hexagonal ice plates floating horizontally refract the light. Sometimes it is colorless, but when it is colored, the side facing the Sun is reddish, and outward it is greenish, bluish. According to Atmospheric Optics, in Europe it can be seen on average twice a week if you look for it. If the Moon is bright enough, a similar "moon dog" can rarely form as well.
Parhelic circle: a white, horizontal band of light that runs across the sky at the height of the Sun and passes through the sun dogs.
Tangent arcs: V-shaped or gull-shaped arcs at the top of the 22-degree halo, whose shape changes with the Sun's elevation.
Circumzenithal arc: a colorful arc resembling a smile, seen high overhead, which is also called an "upside-down rainbow."
Light pillar: a vertical band of light that extends upward, or more rarely downward, from the rising or setting Sun or from the Moon. It is reflected by horizontally floating ice plates.

How does a sun dog form?

Two side faces of a hexagonal ice plate, set at a 60-degree angle to each other, act as a prism. Sunlight enters through one side face, exits through the other, and is deflected by at least about 22 degrees in the process. Because the plates float horizontally, the refracted light reaches our eyes at the same height as the Sun, to its right and left: this is how the two sun dogs are born. Red light is refracted less than blue, so the side of the sun dog facing the Sun is reddish.

How does the sun dog change?

At sunrise and sunset the sun dog sits exactly on the 22-degree halo. As the Sun climbs higher, the sun dogs move farther and farther from the halo, but they always stay at the same height as the Sun. According to Atmospheric Optics, above a solar elevation of about 40 degrees they are hard to make out. That is why the brightest sun dogs are seen in the morning and afternoon, when the Sun is low.

Old "miracles" in the sky

Stockholm, 1535: on April 20, a rich halo display was visible over the city: a complete parhelic circle with the Sun and five "sun dogs" on it, as well as a 22-degree halo and several arcs. Bishop Olaus Petri commissioned a painting of the phenomenon, the Vädersolstavlan ("the Sun Dog Painting"). The original was lost; its 1636 copy can be seen today in Stockholm's Storkyrkan (Great Church). The interpretation of the signs caused a dispute between the church and King Gustav Vasa. According to an analysis by Atmospheric Optics, the painting renders the halo phenomena surprisingly accurately.
Nuremberg, 1561: Hans Glaser's woodcut broadsheet reported a celestial battle with spheres, crosses and tubes. Carl Jung republished the image in 1958 in his book about flying saucers, and since then it has often been cited in UFO literature. According to Frank Johnson and Jason Colavito, it may be an after-the-fact depiction, based on hearsay, of an unusually spectacular sun dog display.

A modern UFO case

On the afternoon of April 23, 2007, Ray Bowyer, a pilot for the airline Aurigny, saw two bright, yellow objects over the English Channel near the island of Alderney; a pilot from another airline also reported something similar. Two astronomers suggested they may have been sun dogs. According to an analysis by the British UFO research association (BUFORA), however, the accounts do not support this, because the objects were below the aircraft, and the two pilots saw them from almost opposite directions. A sun dog, by contrast, always appears at the same height as the Sun, about 22 degrees from it. The case shows well how such an explanation can be checked.

How can it be recognized?

It moves together with the Sun (or the Moon): it always stays in the same direction and at the same distance from it.
It is at the same height as the Sun, and about 22 degrees to the side. This is easy to check by hand: if you cover the Sun with your thumb at arm's length and spread your fingers, the tip of your little finger roughly reaches the halo.
It is often accompanied by a halo or veil clouds: the phenomenon is visible only with thin, high cloud cover or cold air containing ice crystals.
The side facing the Sun is reddish, if it is colored at all.
It fades when the cloud moves away, or when the Sun rises high.

Halos and veil clouds are often signs of an approaching front. According to a researcher at the Norwegian Meteorological Institute, the old saying that a ring around the Sun is followed by precipitation within 12 hours is roughly accurate, although reality is more complex than that.

The sober take

What is it? Halo phenomena are caused by hexagonal ice crystals floating high up, which refract the light of the Sun or the Moon like prisms. A sun dog is a bright, often colorful patch appearing on both sides of the Sun at about 22 degrees, and the 22-degree halo is a large ring of light around the Sun or the Moon.

The most important signs for recognizing it:

It is seen next to the Sun (or the Moon) and moves together with it.
The sun dog is at the same height as the Sun, and about one outstretched hand's width to the side.
If it is colored, the side facing the Sun is reddish.
It is accompanied by veil clouds or cold, crystalline air, and fades along with them.

How does it differ from a truly unidentified light? A halo phenomenon never separates from the Sun or the Moon and does not move independently. If a light travels independently of the Sun's direction, changes direction, or appears near the Sun in a clear, cloudless sky, its cause may be something else.