(Wikimedia Commons)
A transparent calcite crystal found
30 years ago on a ship that sank
in the English Channel in 1592
could help explain how Vikings were
able to sail from Norway to North
America 1,000 years ago without magnetic
compasses.
The sailors likely relied on the sun
and the stars as their guides. Now
researchers from France's University
of Rennes have demonstrated how a
crystal called an Iceland spar (found in
Iceland and Scandinavia, among other
places), which was recovered from the
shipwreck, could be used—even on a
cloudy day—to ascertain the sun's position
to within a few degrees.
When light passes through the crystal,
it is double refracted—the light is split in
two, creating an effect similar to a 3-D
movie viewed without light-polarizing
glasses. According to the authors, as a
person holds up the crystal to the sun
and rotates it, there's a particular angle
at which the two beams of light appear
equally bright. By holding the spar at the
same orientation and scanning a cloudy
sky for a point where the beams line up,
Vikings could locate the sun through
cloud cover. "Vikings could have exploited
the high sensitivity of the human eye
to small contrasts," the authors write in
Proceedings of the Royal Society A.