A “sun dog” is a common name for an optical effect known as a “parhelion.”
It is also sometimes called a “mock sun” because the bright spot in the sky resembles a small sun to the right and/or left of the actual sun.
A parhelion is a type of halo, usually found in pairs, on the edges of a 22 degree halo on either side of the sun.
Sun dogs are formed by plate-shaped hexagonal ice crystals in high and cold cirrus clouds or during very cold weather where ice crystals drift in the air at low levels, also known as diamond dust.
The sunlight refracts or bends as it shines through the ice crystals, which act as tiny prisms, sometimes separating the light into colors.
They can be seen anywhere in the world during any season, but they are not always obvious or bright.
Sun dogs are best seen and are most conspicuous when the sun is low.
Sun dogs are red-colored at the side nearest to the sun and farther out the colors grade through oranges to blue.
However, the colors overlap considerably and so are muted that they are never a pure color or saturated.
Finally, the colors of the sun dog merge into the white of the parhelic circle.