Q&A: Sun Dogs

Question: Last night around sunset I saw these two rainbow-like things on both sides of the Sun. But the colors didn’t seem quite right. What were they? — IN, Cambridge, England

Answer: What you saw was most likely a “sun dog,” scientifically known as a parhelion. The plural is parhelia, which is good to know since they often appear in pairs. They’re called “sun dogs” because they “follow” the Sun in the sky, just like real dogs will follow their master.

You’ll never see them in a totally clear sky, because they’re caused by ice crystals in clouds that refract and reflect sunlight. Naturally, sun dogs are more common in winter when the upper air is cooler. Even when they are present, you might not notice them, since they’re faint compared to the Sun. And you need to look toward the brightest part of the sky to see them, since they’re always 22° from each side of the Sun, and at an equal elevation. Rainbows, in comparison, are always on the opposite side of the sky from the Sun.

When you do see sun dogs, the red color is almost always the most distinct, with fainter orange and yellow regions flanking it. Green, blue, and violet are there too, but rarely visible. The red part is sometimes stretched into an upward arc that can meet up with the arc from the other parhelion and form a circle. Compared to a rainbow, the colors seem washed out and much less vivid. Those simulated in the graphic are exaggerated to emphasize that the entire spectrum is present.

Next time you see one, stand where something blocks the Sun (power pole, cactus, etc.) and the sun dogs will be much more vivid. And you can photograph them. Look for my attempts in future columns.

Hercules - A Closer Look
Light Pollution Test