When Iceland spar splits light into two polarized rays, the two different images will have different brightness depending on the polarization.

Because sunlight is polarized when it enters the Earth's atmosphere, it is possible to change the orientation of a piece of Iceland spar to determine the direction of the sun. This is done by moving the crystal until the relative brightness of the two images are equal, which only happens when the crystal is aligned to the sun. This is possible even when the Sun is hiding behind clouds or just below the horizon.

However, it can be very difficult to determine when the two images are of equal brightness.
A recent study suggests that the Vikings may have built a simple device to better use the sunstone.

The crystal is covered with a solid screen with a small hole in its center, and a pointer.
When light passes through the hole onto the crystal, a dark surface below it receives the projection of the double image for comparison.
Experiments showed that by rotating the device until the two images were equally bright, the sun's position on a cloudy day could be determined to within 1º of accuracy, and that it is also very accurate even when the sun is below the horizon.

A modern version of a simple device the Vikings may have used to determine
where the sun was hidden on cloudy days over 1,000 years ago.

Another version of a sunstone holding device.

Our Stone
Size: L 2.0079" x W 1.456" x H 0.905
Weight: 111g
Material: Calcite
Origin: CHINA

Secrets of the Viking Navigators
by Leif K. Karlsen

Most navigators have heard of Viking sunstones, but few realize they are more than legend. Leif Karlsen has brought them to life. He shows us how they work—now, and a thousand years ago—and what it was that led the Viking navigators to develop this unique tool for finding the direction to the sun, even when the sun is obscured by clouds or fog.

Also includes other techniques including explanation of how techiques from our Emergency Navigation book might likely have been used in Viking times.