Researchers believe they have solved the mystery behind how Viking mariners were able to navigate the northern seas centuries before the invention of the compass.

While experts long argued that Vikings knew how to use blocks of light-fracturing crystal, referred to as sunstone, to locate the Sun through dense clouds, archeologists had never found hard proof, and doubts remained as to exactly what kind of material it might be.

However, an international team of researchers led by Guy Ropars of the University of Rennes in Brittany, marshaling experimental and theoretical evidence, has come up with an answer.

Vikings, they argue, used transparent calcite crystal – also known as Iceland spar – to fix the true bearing of the Sun to within a single degree of accuracy, according to Agence France-Presse.

This naturally occurring stone has the capacity to “depolarise” light, filtering and fracturing it along different axes, the researchers explained.

This enabled the Norse mariners to travel long distances across seas at northern latitudes often socked in by light-obscuring fog and clouds, allowing them to venture thousands of miles from home toward Iceland and Greenland, and most likely as far as North America.

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