The Telegraph finds the truth behind a navigational legend – a stone that Vikings used locate the sun on cloudy days:
Now experiments have shown that a crystal, called an Iceland spar, could detect the sun with an accuracy within a degree – allowing the legendary seafarers to navigate thousands of miles on cloudy days and during short Nordic nights.
Dr Guy Ropars, of the University of Rennes, and colleagues said “a precision of a few degrees could be reached” even when the sun was below the horizon.
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One Icelandic saga describes how, during cloudy, snowy weather, King Olaf consulted Sigurd on the location of the Sun. To check Sigurd’s answer, Olaf “grabbed a sunstone, looked at the sky and saw from where the light came, from which he guessed the position of the invisible Sun.”
Using the polarisation of the skylight, as many animals like bees do, the Vikings could have used to give them true bearings.
The Viking routes in the North Atlantic were often subject to dense fog and the stone could also be used to locate the sun on very cloudy days.
The researchers said such sunstones could have helped the Vikings in their navigation from Norway to America before the discovery of the magnetic compass in Europe.
They would have relied upon the sun’s piercing rays reflected through a piece of the calcite. The trick is that light coming from 90 degrees opposite the sun will be polarised so even when the sun is below the horizon it is possible to tell where it is.