Vikings were skilled seafarers spreading from Sweden and Norway. They initiated settlements in Northern England, Iceland, Greenland, and may have been the first Europeans to arrive in North America.
The Vikings did not have compasses and used the sun and stars for navigation (in addition to coast lines and bird migration patterns). In the far north, they faced the problem of foggy weather that made it often difficult to see the sun.
Danish archaeologist Thorkild Ramskou suggested in 1967 that Vikings used so-called “sunstones”. A sunstone is a crystal that polarizes light passing through the clouds and makes it possible to determine the position of the sun. [See this video by NASA to see sunstones in action.]
Read more on Viking Sunstone Navigation here.