|A Viking ship is late in its return home from the newly discovered
lands far west. Winter is around the corner and the weather will soon turn
ugly. It's imperative that the helmsman maintains the course due East. But
where exactly is Home? The sky is becoming more cloudy every passing day.
Most nights the stars are not visible and even during the day the sun has
difficulty breaking through. Daylight is short and during good part the sun
illuminates the sky from below the horizon . . . somewhere. Hanging from
the top of the knorrship mast a sailor squints his eyes looking for clues
in the brightness of the clouds . . . to no avail. Then Leif the Lucky spots
an opening in the clouds. He reaches for the pouch hanging from his waist
and takes out his Sunstone. Through the crystal he looks at the small patch
of blue sky. He turns the rock until it becomes yellow. Next he shouts
to the helmsman with his stretched arm pointing starboard . . . towards
Bees do it. Ants do it. Did the Vikings do it? Can it be that the Vikings used the polarization of skylight as a navigation compass? Did the Vikings find their way to America by looking at the sky through a crystal, the proverbial sunstone?
The Icelandic sagas tell the story of how the Vikings sailed from Bergen on the coast of Norway to Iceland, continuing to Greenland and, likely, Newfoundland in the American continent. This remarkable sailing achievement was realized circa 700 -1100 AC, before the magnetic compass reached Europe from China (it wouldn't have helped much, anyway, so close to the Magnetic Pole). How did they steer true course in the long voyages out of land sight, especially in the common bad weather and low visibility of those high latitudes?
In 1967, a Danish archaeologist, Thorkild Ramskou, suggested that the Vikings might have used the polarization of the skylight for orientation when clouds hid the sun position. They would have used as polarizers natural crystals available to them, the famous sunstones described in the sagas. To find the location of the sun they only needed a clear patch of sky close to the zenith to determine the great circle passing through the sun. The pros and the cons of this theory are the following.
|Interestingly, in the late 40's the US National Bureau of Standards
(now NIST) developed a Sky Compass based on the same principle. It was
inspired by a previous "twilight compass" developed by Dr. A. H. Pfund of
Johns Hopkins University. From a NBS 1949 paper: "The principal advantage
of the sky compass . . . is during twilight, and when the sun is several
degrees below the horizon, as well as when the region of the sky containing
the sun is overcast, so long there is a clear patch of sky overhead. The
sky compass is thus of particular value when the sun compass and the sextant
are not usable. Since the extent of polarization of the sky's light is greatest
at right angles to the incident beam of sunlight, the compass is most accurate
in the polar regions, where it is also most useful, because of the long duration
of twilight . . ." The US Navy and Air Force experimented with
the sky compass in the 1950's and Scandinavian Airlines (SAS) used
it for several years on its polar flights. Polarization.com has recently
developed an inexpensive educational Skylight
When Ramskou originally proposed this theory, it was well received and widely accepted by the general public and also by the scientific community, and remained so for more than two decades. The Viking navigational triumphs became very fashionable, especially the exploits of Eirik the Red and his son Leif (Eiricksson) the "Lucky" circa 1000 AC, and the "discovery" of America centuries before Columbus. Both, Scientific American and National Geographic magazines carried the story of skylight navigation. However, in the 90's the theory was disputed on the basis that no real material proof exists and that the advantage provided to navigation would have been marginal. My personal take is that polarized skylight could have been of real use to the Vikings but, until direct evidence is found, one should be skeptic and stick to the simplest explanation: that the Norsemen where damn good sailors!
However, the image of the Vikings in a quest of faith into mysterious and dangerous seas, following west the light from the sky viewed through a magic crystal, has its obvious romantic appeal . . .
T. Ramskou, "Solstenen," Skalk, No. 2, p.16, 1967 (Ramskou's original publication, but I haven't read it)
"Sky Compass," Review of Scientific Instruments, Vol. 20, p.460, June 1949
H. LaFey, "The Vikings," National Geographic, Vol. 137, p.528, 1970
R. Wehner, "Polarized-light navigation by insects," Scientific American, Vol. 31(1), p.106, 1976
Curt Roslund and Claes Beckman, "Disputing Viking navigation by polarized skylight", Applied Optics, Vol. 33, No. 21, p.4754, July 1994.
Bradley E. Schaefer, "Vikings and Polarization Sundials", Sky & Telescope, May 1997, p. 91