The fabled ‘Viking sunstone' thought to be used by ancient Norse mariners as a navigation aid, may have been found in a 16th Century shipwreck off the coast of Alderney, an island between Britain and France.
The ‘sunstone’ found in the shipwreck is about the size of a bar of soap and made from Iceland spar, a type of calcite crystal. It should be transparent but the shipwrecked stone is a milky white after centuries immersed in water and abrasion by sand.
The crystal allows navigation by the sun when it’s obscured by clouds, or dipped below the horizon, by observing a dot made on the top of the crystal. When observed from below the crystal, the dot refracts into ‘two’ dots. By rotating the crystal until the dots are the same intensity, the direction of the sun can be found.
"Using a transparent crystal similar to the original, the scientists were able to follow the track of the setting Sun in poor light, with an accuracy of one degree. In a second experiment, they were able to locate the Sun for 40 minutes after sunset.’