Its always been a bit of a mystery how the Viking sailed such long voyages without navigational equipment. They no doubt used the stars, but in a region where cloud cover is common, clouds, sun and moon prove less reliable. They no doubt used course corrections, sailing blind at times, but during any break in cloud cover, setting their course to right by the stars they knew so well.
But rumor trickled down through the ages of a sun-stone. A stone that guided them in night or day. a magnetic? Perhaps, but some new evidence MAY have come to light in the shape of an Icelandic Spar; crystal that captures faint light and casts it back.
With the discovery of such a stone upon the wreck Alderney that sank in 1592 researchers, and Vikings, think they might be a little closer to the sun stone.