Archaeologists have discovered what appears to be a Viking navigational sundial. It is clear that Vikings would have used the position of the stars at night, and the sun during the day. The Nordic sagas mention a sunstone. The idea is that the Vikings may have used the interaction of sunlight with particular types of crystal to create a navigational aid that may even have worked in overcast conditions has been held for sometime, with the popular TV series Vikings using a similar device in one episode.
for an explanation of how this may have worked click here.