DID THE VIKINGS USE ICELAND SPAR TO NAVIGATE?
Iceland spar is a clear, transparent, colorless crystallized variety of calcite (calcium carbonate, CaCO3). Large pieces are split along natural cleavage planes to form natural rhombs. Iceland spar is probably best known for exhibiting the optical property of double refraction - so, anything viewed through the crystal appears double. It has many uses, in everything from precision optical instruments to LCD screens, and was even used during WWII to make bombsights. The perfect, flawless, colorless Iceland spar that is used in optical instruments sells for more than $1000 per kg. Historically, the first, best quality, and most abundant source of this clear calcite was in Iceland, which is where it got its name. Recently, Iceland spar has been in the news because of new research that shows that this mineral was almost certainly used by the Vikings for navigation.
Over 1,000 years ago, the Vikings sailed from Scandinavia to America without the use of magnetic compasses, astrolabes, maps or any other known device. Yet they still managed to find their way in spite of the clouds, fog, and long summer twilights characteristic of near Polar Regions. Centuries old Viking legends attribute their navigational success to the use of mysterious, glowing “sunstones” to find the position of the sun and set the ship’s course even on cloudy days. Although still controversial, many researchers now believe that these “magical sunstones” were in fact Iceland spar.
When Iceland spar splits light into two polarized rays, the two different images will have different brightness depending on the polarization. Because sunlight is polarized when it enters the Earth’s atmosphere, it is possible to change the orientation of a piece of Iceland spar to determine the direction of the sun. This is done by moving the crystal until the relative brightness of the two images are equal, which only happens when the crystal is aligned to the sun. This is possible even when the Sun is hiding behind clouds or just below the horizon.