Vikings may have used a special crystal called a sunstone to help navigate the seas even when the sun was obscured by fog or cloud, a study has suggested.
The sun was not necessary for Vikings to navigate, say researchers
Researchers from Hungary ran a test with sunstones in the Arctic ocean, and found that the crystals can reveal the sun's position even in bad weather.
This would have allowed the Vikings to navigate successfully, they say.
The sunstone theory has been around for 40 years, but some academics have treated it with extreme scepticism.
Researcher Gabor Horvath from Eotvos University in Budapest led a team that spent a month recording polarisation - how rays of light display different properties in different directions - in the Arctic.
Polarisation cannot be seen with the naked eye, but it can be viewed with what are known as birefringent crystals, or sunstones.
Birefringence, or double refraction, is the splitting of a light wave into two different components - an ordinary and an extraordinary ray.
The researchers found that the crystals could be used to find out where the sun was in the sky in certain foggy or cloudy conditions.
It is already thought that Vikings used sundials aboard ships to navigate.
Vikings were a seafaring race from Scandinavia who used their longboats to explore and conquer parts of Europe, Greenland, Iceland and Russia.