On 20 April, 1535, Sweden's capital city Stockholm experienced a natural spectacle so unique it was believed to be a warning directly from God. For two hours the morning skies over the city were filled with brilliant white circles and arcs. The sun was surrounded by a number of iridescent, almost ghostly companion suns.
Residents of the city believed God was indicating his displeasure with the actions of the country's ruler King Gustav Vasa. Not only had the king recently introduced the protestant faith to Sweden, but he was renowned for his intolerance of those allied with King of Denmark - Gustav Vasa had led a successful rebellion against Denmark before he became king.
The atmospheric phenomenon responsible for causing such thinking is known as a sundog. Similar in shape to the natural sun, a sundog can also look like a comet, and always appears beside the sun. It's not unusual to see two sundogs, one on each side of the sun. The scientific name for them is parhelia, from the Greek words "para" (meaning "next to"} and "helios" (meaning "sun"). Greek philosopher Aristotle called them "mock suns".
Sundogs are usually seen at sunrise and sunset, when the sun is close to the horizon. They are created when sunlight reflects off or refracts through the tiny ice crystals in cirrus or cirrostratus clouds the clouds that look like white bands or patches of fleece. Each piece of cloud ice is hexagonally shaped and no bigger than the smallest grain of sand. When sunlight refracts or passes through horizontally-shaped ice crystals it bends the light the same way a prism does, producing a spectrum of colours which fan out to create a crisp, bright sundog. The part of the sundog closest to the sun is red, gradually fading to pale blue on the side furthest from the sun. Sunlight reflecting off an ice crystal will produce a white sundog.
When the light refracts through an elongated ice crystal an arc or a halo appears in the sky. Obviously many different shapes made up the ice crystals in the clouds that day in Stockholm. When one considers how differently the light reacts in each crystal shape it's not surprising people viewed the ethereal images in the sky as an omen. In an attempt to put a stop to the rumours about the divine warning to the king, a painting was commissioned to commemorate the occasion.
When King Gustav saw the painting he believed it to be a conspiracy, seeing himself as the real sun surrounded by treacherous "mock suns". All those involved in the commission and production of the painting were sentenced to death, although the sentences were eventually lifted and the men freed after paying a fine. The whereabouts of the original painting is unknown, but a copy from the 1630s is displayed in Sweden and is considered the oldest depiction of sundogs.
Ice fog and ground level cloud known as diamond dust will also cause sundogs to appear. A similar effect is caused by a bright moon, earning the resultant less bright image the obvious name "moon dog". While sundogs are not an unusual event, they can occur about ten times a year in any given area. Naturally the earth's colder regions are the best places to see them, although they have been seen on crisp winter mornings in the warmer regions on our planet. The appearance of sundogs and other atmospheric phenomena like arcs, haloes and rings often means that rain or snow will fall within the next 18 to 36 hours.
Another pictorial reference to sundogs appears on the woodcut "1561 Nuremburg event" by German artist Hans Glaser. On 14 April that year residents in the city were treated to a morning sky filled with sundogs, arcs and haloes, and some of the images on the woodcut resemble the atmospheric phenomenon experienced by the German city.
It's easy to miss the sight of a sundog, because their proximity to the sun means one has to look almost directly into the sun to see them. It is suggested to look slightly to the left or right of the sun for smaller, bright objects that often have long tails facing away from the sun. Putting your fist in front of the moon or sun will help you protect your eyes and aid in catching sight of these interesting and unique natural phenomena.