1999 – Gnarr Borgrum – The Viking Sun Compass

The Gnarr was the trading vessel of the Viking era. More heavily built than a raiding galley, they could be rowed but were usually sailed. The Borgrum is owned by the Bergan Museum and is a replica of an 11th Century vessel found near Roskilde in Denmark. Measuring 60 feet, double ended, with a single mast and square sail, and requiring some 8 tons of stone ballast, she proved to be very sea kindly. The navigation experiment commenced at the islands of and finished at Sound in the Shetlands. Navigation was entirely by the sun compass, and when the sun was hidden by clouds, the boat was steered by keeping it aligned with the waves. The sun compass can only give direction, but the approximate error at the destination was 3 miles. Further experiments have shown that with the right clear sky conditions, a mile error in 50 miles is quite achievable. See “Viking Navigation” by Soren Thirslund. The Viking Sun Compass The first disc was discovered in Greenland in 1947, but in 2000 another was found in a Viking site in Poland. Both date to about 1000AD and have gnomon curves scratched on their surfaces. The arrival of the magnetic compass made the sun compass redundant, and being of wood, few will have survived. But the simple technology works just as well to-day and is easy to use and remarkably accurate. It is simple to create. Make a disc of about 10 centimetres diameter and place a small pin in the middle about 1 centimetre high. This pin is called a gnomon. Lay the disc absolutely horizontal and then spend a day marking the limit of the shadow of the sun cast by this pin on the surface of the disc. If you cannot be certain that the disc is horizontal and live close to the sea, line the disc up with the sailor’s horizontal datum, the horizon. After the sun has set, draw a line through the marks to create a curve across the disc, called a gnomon curve. In practice a perfectly satisfactory curve can be drawn from marks made at hourly intervals. You have now created a simple sun compass. The first thing you will notice is that the curve slowly approaches the Gnomon until the sun is at its zenith and then it moves away to the edge of the disc as the day advances. Thus where the curve is shortest is pointing towards the North. (The reverse is true in the Southern hemisphere) It can be made easier to use for steering if the points of the compass are marked on it once north has been established. The next day, at any time during the day, place the disc so that it is horizontal and twist it round until the shadow cast by the sun coincides with the gnomon curve you drew. When this occurs you will find that the position where the curve is closest to the pin is always pointing to the north. The time does not matter, the only decision required is to ensure that you are using the morning part of the curve in the morning and the afternoon part after noon. It might require two people to read the sun compass accurately, one to sight along the surface at the horizon to ensure the disc is level and the other to do the twisting to align the shadow with the curve. To see the importance of having the disc level, drop one side slightly. It is immediately obvious that the disc must be twisted the bring the shadow back to the curve which creates an error. Because the sun moves relative to the earth, or, in navigational terms, its declination changes, the curve you have drawn will only be accurate for a short time. The curve will last longest at midsummer solstice and mid winter when the sun’s declination is changing slowly, but will have only a short accurate life at the equinoxes (March 21st and September 23rd). Thus a compass drawn in mid June can be relied upon for at least a fortnight either side, but in March and September its accuracy will be limited to only a few days. In practice this does not matter as much as might appear as any error in bearing in the morning is automatically compensated by an equal and opposite error in the afternoon. The other reason why a new gnomon curve might have to be drawn is if the Latitude is changing. Each curve is set for the Latitude in which it is drawn. If the observer moves north or south, the shadow will lengthen or shorten respectively.