http://www.guardian.co.uk/science/story/0,,2007399,00.html

Crystals may have aided Viking sailors
Ian Sample, science correspondent
Guardian

Vikings may have used special crystals called sunstones to ensure gloomy
weather did not come between them and an outing to sack British villages
or discover new lands to the west, scientists have discovered. Tests
aboard a research vessel in the Arctic ocean found that certain crystals
can be used to reveal the position of the sun, a trick that would have
allowed early explorers to ascertain their position and navigate, even
if the sky was obscured by cloud or fog.

The finding adds weight to the controversial suggestion that the Vikings
exploited an unusual optical property of crystals known as birefringence
to stay on course during bad weather.

Researchers led by Gabor Horvath at Eotvos University in Budapest spent
a month recording the polarisation of sunlight while at sea. Although
polarisation is not apparent to the naked eye, it can be seen using
birefringent crystals. The researchers found that the crystals were
likely to work in all but the worst weather conditions, they write in
the journal Proceedings of the Royal Society today.

Professor Michael Berry, a physicist at Bristol University, said:
"They've shown that even if the sky is full of clouds and moisture, the
polarisation of the sunlight doesn't change very much, and that's a real
surprise. If you know the time already, then once you know the position
of the sun you know what direction you're sailing in."