Wednesday, December 17, 2008

The Magic Sunstone

A thousand years ago the Vikings were the master sailors of the world. Their "longships," up to 100 feet long and carrying two or three dozen men, raided and traded several thousand miles from their homeland southward along the coast of Europe and then eastward through the full length of the Mediterranean to Constantinople; farther north, up the Volga River into Russia; and even westward across the stormy and uncharted, iceberg-laden North Atlantic Ocean.

At the turn of the millennium, Leif Eriksen (that is, "Leif, the son of Erik"--his father was Erik the Red), at the mature, experienced age of 25 (a boy became a man at age 12; a man was old if he lived to be 40) set out with a few dozen hearty men (to row if there was no wind to fill the square sail--to fight if unknown peoples were encountered) and even with a few women and some cattle (to forge a settlement if hospitable lands were found). His hope was to find the lands that had been seen on the far distant horizon by earlier intrepid, ocean-faring Norse explorers before they turned back homeward.

Leif's ship was not deep-keeled--it drafted less than three feet--it could be run up on a sandy beech. It was steered by a wooden board lashed on the right side at the rear (a "steering board"; the "star-board" side is still the right side of a ship; the other side of the ship was always the side tied to the dock in port, hence the "port side"). It had a square sail, small by any future maritime standards, but when the wind was fair and in the right direction (the ship could not tack or turn away from the wind), it helped the rowers propel the ship.

Across the sometimes stormy and often cloudy--even foggy--ocean they ventured. They had no magnetic compass--it would not be invented (or imported from China) for hundreds of years--and they often could not see the Sun or stars for days and nights on end. How did they navigate? A ship at sea without any shoreline, Sun, stars, or compass to guide it, will tend to travel in a circle; the smallest deviation from a straight course turns into a long, slow arc until the helmsman finds, to his consternation, that he is following in his own wake.

But Viking navigators had discovered a magic stone--they called it a "sunstone." It was a large crystal of rare, Icelandic feldspar. This mineral, technically a form of double-refracting cordierite, refracts, or bends, light that passes through it with the peculiar property that if that light is polarized (that is, filtered so that the waves of light are aligned), it "doubly refracts" this light along two distinct paths. Very few materials are known, even in our wonderful modern age of materials science, that have this double-refracting property.

The light from the Sun is partially polarized by the atmosphere. Although this polarization is not detectable by the unaided human eye (although some birds and insects are able to use it to navigate), the Norsemen discovered they could hold their magic sunstone toward the clouded or foggy horizon and then turn until the two refracted images were aligned, and that was the direction of the Sun--the Sun they needed to steer by but could not see.

The magic sunstone was often written of in the Norselanders' sagas, but its true nature and use have only been uncovered in the past few years.