Sun Dog

Thousands of men are waiting to do battle. Armor glints dully in the winter sunlight, beneath a deadly thicket of spears and halberds, axes and swords. Horses stamp nervously, their breath steaming on the bitterly cold air. Banners stir restlessly above ranks of armored knights, and archers count the arrows in their quivers, waiting for the signal to advance. Suddenly, a ripple of fear passes through the ranks, and men are seen gazing upwards at a strange sight. Not one, but three, suns are in the sky. The outermost are connected by a thin, curving gleam. The warriors begin to mutter in dread at this bizarre sight, wondering what ill fortune will result from this strange omen.

However, before their fear can take hold, a tall, blond-haired man in armor rides out along the front of the gathered army. He raises his visor, revealing a handsome, if rather fleshy, face with a cheerful expression. Raising a hand to point at the three suns, he shouts, "Fear nothing -- it is a sign that the Holy Trinity is on our side this day!" A fierce roar of approval greets his words, and the men's dread melts away into a mood of eagerness and exultation. They begin to move forward, the ground shaking under the impact of thousands of feet, and within a few short hours, a massive victory has been won for King Edward IV of England -- who adopts the sun as his emblem after this fight, the battle of Mortimer's Cross.

In this way, the meteorological phenomenon called the "sun dog" had one of its few recorded effects on human affairs. Sun dogs are reflections of the sun, giving the illusion of one or two smaller suns flanking the actual sun, often with a halo effect connecting them. These harmless weather effects are generally no more than an interesting sight with a pleasantly quaint name. They are caused by much the same processes that cause rainbows and haloes -- the reflection of light from atmospheric moisture.

What causes sun dogs

Sun dogs, known officially as parhelia by meteorologists, are bright, sun-like patches of light, flanking the sun symmetrically at exactly 22 degrees distance and the same height above the horizon. They are brightest when the sun is close to the horizon but can occur at any time of the day when conditions are right. They can be fairly dim, displaying a rainbow-like range of colors, or they can be extremely bright and 'sun-like,' which is apparently the type that Edward IV and his army saw on that fateful 15th century day.

Sun dogs are caused by the sunlight refracting through airborne ice crystals, either in cirrus clouds and other high cloud types, or through diamond dust (low level ice crystals) in extremely cold weather. The ice crystals must be of a specific type -- flat and hexagonal -- and lie flat in the air rather than randomly, which is usually the result of their sinking. If the plate crystals aren't aligned properly, no sun dogs will be visible, and instead, there will be a large halo around the sun at around 22 degrees. If the ice crystals are present on one side of the sun and not the other from the observer's viewpoint, then only a single sun dog will be seen.

Since the plate crystals seldom all lie perfectly flat, sun dogs are usually more elongated than the sun, with a slightly to markedly oval shape. However, if they are very bright, this may be difficult to see.

Sun dogs are actually fairly common weather phenomena but generally pass unnoticed. Some scientists have estimated that if any person spent the whole day watching for sun dogs, they would see the phenomenon at least twice a week, year-round.

The same phenomenon can produce 'moon dogs,' which are sun dogs flanking the moon. Since the moon is a reflector rather than a direct light source, it isn't bright enough to produce full-sized images of itself, and moon dogs are usually much smaller than sun dogs and embedded in a halo. Whether they are produced by the sun or moon, however, parhelia are interesting sights for those who like the weather or are simply interested in Nature's beauty -- and, at least once in human history, managed to influence the outcome of an important battle.