Sun dogs are an atmospheric phenomenon caused by the refraction of sunlight through ice crystals such as those hosted in cirrus clouds. A number of specific conditions must prevail for this phenomenon to form. Many observers miss them because they form close to the sun, requiring the viewer to look almost directly into the sun to see them. However, when conditions are right, sun dogs can be clearly viewed, and are quite striking, looking like smaller false suns next to the real sun.
In order for sun dogs to form, the sun must be in the sky, usually less than 45 degrees from the horizon, and in the same horizontal plane as the viewer. Small hexagonal ice crystals must also be in roughly the same plane, and be oriented parallel to the ground. If all of the ice crystals are relatively flat, sun dogs will form approximately 22 degrees away from the sun. Usually, they come in pairs, one on either side of the sun, and they are sometimes accompanied by a halo, caused by the refraction of sunlight through ice crystals oriented in multiple directions.
Parhelion is the other name for a sun dog, a reference to the fact they are found next to, or para the sun, helios. This phenomenon has been observed and written about for centuries, and sometimes was viewed as an evil omen during certain periods of history. In other instances, sun dogs were considered to be augurs of good luck for the observer. Several pieces of classical art depict what appear to be sun dogs, suggesting that they were a recognized phenomenon. On rare occasions, they are quite vivid and brilliant, making it seem like there are multiple suns in the sky.
Colder regions are excellent places to observe sun dogs, although many temperate winters form enough ice crystals to create them. Patient observers can also spot moon dogs when the moon is bright, although they can be very difficult to photograph well without an excellent camera and a tripod. In both cases, look slightly to the left or right of the major light emitting body in the sky for smaller, bright objects that often have long tails facing away from the true moon or sun. Putting your fist in front of the moon or sun will help you protect your eyes and spot these interesting phenomena, and if you live in a cold region, you may see them quite frequently.