Often regarded as ruthless robbers, the Vikings were also impressive mariners capable of traversing the North Atlantic along a nearly straight line. Now, new interpretations of a medieval compass suggest the sea robbers may have skillfully used the sun to operate the compass even when the sun had set below the horizon.

The remains of the supposed compass — known as the Uunartoq disc— were found in Greenland in 1948 in an 11th-century convent. Though some researchers originally 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. [Images: Viking Twilight Compass Helps Navigate North Atlantic]...