To avoid getting lost on their voyages across the North Atlantic 1000 years ago, Vikings relied on the sun to determine their heading. (This was long before magnetic compasses were available in Europe.) But cloudy days could have sent their ships dangerously off course, especially during the all-day summer sun at those far-north latitudes. The Norse sagas mention a mysterious "sunstone" used for navigation. Now a team of scientists claims that the sunstones could have been calcite crystals and that Vikings could have used them to get highly accurate compass readings even when the sun was hidden.
The trick for locating the position of the hidden sun is to detect polarization, the orientation of light waves along their path. Even on a cloudy day, the sky still forms a pattern of concentric rings of polarized light with the sun at its center. If you have a crystal that depolarizes light, you can determine the location of the rings around the hidden sun.If you are interested in how it read in the sagas, try this:
The weather was thick and snowy as Sigurður had predicted. Then the king summoned Sigurður and Dagur (Rauðúlfur's sons) to him. The king made people look out and they could nowhere see a clear sky. Then he asked Sigurður to tell where the sun was at that time. He gave a clear assertion. Then the king made them fetch the solar stone and held it up and saw where light radiated from the stone and thus directly verified Sigurður’s prediction.