Viking Medieval Compass

For hundreds of years historians have seen the Vikings through the eyes of the medieval Irish christians priests who met these people and suffered the terror they brought to their lands. But the first 10 years of the XXI century brought new interesting discoveries from the Viking period, and what was once myth, is now reality. The Vikings have been often regarded as ruthless pillagers, but they are also known as outstanding mariners.
New discoveries and interpretations of a medieval compass, suggest that the Vikings may have skillfully used the sun to operate this object even when the sun had set, and even before rising again.

The remains of the supposed compass known as the Uunartoq disc, found in Greenland in 1948 in an 11th century convent, led some researchers to argued it was simply a decorative object. Other researchers have suggested the disc was an important navigational tool that the Vikings would have used in their roughly 1,600-mile-long (2,500 kilometers) trek from Norway to Greenland. Though only half of the wooden disc remains, it is estimated to have been roughly 2.8 inches (7 centimeters) in diameter with a now-lost central pin that would have cast a shadow from the sun indicative of a cardinal direction.

Researchers from a University in Hungary have studied the fragment in detail. It was concluded that although the disc could have functioned as a single entity, it was more likely used in conjunction with other tools, including a pair of crystals and a flat, wooden slab to help navigate when the sun was low in the sky or even below the horizon.
When the sun is low above the horizon, the shadow of a small item can fall off the board, and such situations are frequent in the northern seas, as Balázs Bernáth says.
To help solve this shadow problem, the Vikings may have used an object in the middle of the compass to create a wider, shorter shadow. A wide hole within the center of the disk, which was previously thought to be the place to grip the compass, could have served as a holding sport for the object that would solve the problem.

The researchers think that, to locate the sun after sunset, the Vikings could have used a pair of crystals known as sunstones, which are calcite stones that produce patterns when they're exposed to the polarization of UV rays within sunlight. When the crystals are held up to the sky, the orientation of these patterns cast within the stone can help pinpoint the position of the sun below the horizon.
Once the Vikings had determined the position of the hidden sun, they could have used a specially designed wooden slab called a shadow stick to simulate the shadow of the gnomon based on the angle at which the hidden sun would hit it. The location of the outer edge of that imaginary shadow could then have been used to determine their cardinal direction.

Field tests were held by the researchers, to estimate the plausible accuracy of this twilight compass. They have found that this would have worked with only 4 degrees of error, which is way better than other forms of celestial navigation. It is estimated that this compass would have functioned for as long as 50 minutes after the sun was down the horizontal line, around the spring equinox, when the Vikings are thought to have used this compass based on etchings in the wood.

No shadow sticks or sunstones have been found in conjunction with the disc, but evidence of both exist in medieval written records, suggesting they would have been available to the Vikings.