Amazing Ancient Viking Sun Compass Even Worked After Sunset

Even the best Eagle Scout prefers to navigate during the day. The Vikings, apparently, would have laughed at such a preference—according to new research, the North Atlantic seafarers' sun compass was so advanced it even worked after dark, thanks to clever engineering and mystical crystals.

In a Royal Society paper published this week, a team of researchers examined the Uunartoq disc, a fragment of a Viking sun compass discovered in a medieval convent in Greenland nearly 70 years ago. First thought to be a decorative object, the 2.8-inch disc was later assumed to be part of a compass that used a missing central piece to cast a shadow showing cardinal direction.

The Hungarian research team didn't buy that hypothesis, for two reasons. First, they say the Uunartoq disc's dimensions and markings make it "far from optimal" as a solar compass. Second, and perhaps more importantly, a simple solar compass wouldn't explain the mythical "sunstone," a seemingly magical crystal mentioned in Norse folklore that could locate the sun's position at night or on a cloudy day that scientists recently discovered actually exists.

Let's dig into those crystals for a second. Made of a type of calcite known as Iceland spar, the birefringent mineral creates a unique pattern when exposed to the sun's UV rays, even if those rays are coming from a sun that's dipped below the horizon.

The Hungarian team theorizes that a pair of sunstones would help a Viking navigator locate the position of the hidden sun. Then, a specially-designed shadow stick would be placed on the disc to approximate where an actual shadow would fall if the sun was visible. The outer edge of the imaginary shadow would then be used for navigation.

In testing, the team was able to locate true north within a 4-degree margin of error using the twilight compass, comparable to the magnetic needle compass you might have in your pocket. What's more, the team estimates the method would work for as long as 50 minutes after sunset, making the Uunartoq ring useful in sunlight and near-darkness.

Makes you feel a little bit spoiled using your smartphone to navigate your own town, doesn't it? [Proceedings of the Royal Society A via LiveScience]

Image: Proceedings of the Royal Society A; Balazs Bernath; Alexandra Farkas; Denes Szaz; Miklos Blaho; Adam Egri; Andras Barta; Susanne Akesson; and Gabor Horvath