The Viking age (roughly 793–1066 CE) saw Norse seafarers navigate vast stretches of the North Atlantic Ocean, reaching Iceland, Greenland, and even North America long before the invention of the magnetic compass. While they relied heavily on coastal landmarks, wildlife, ocean swells, and celestial navigation (using the sun and stars), the notoriously cloudy and foggy weather of the North Atlantic posed a severe threat.
To overcome this, historical texts and modern science suggest the Vikings used a navigational tool known as a "sunstone" (sólarsteinn)—a calcite crystal that allowed them to locate the sun on heavily overcast days using the physics of polarized light.
Here is a detailed explanation of the science, history, and practical application of this remarkable navigational technique.
1. The Navigational Challenge
A primary tool for Viking navigation was the wooden sun compass, which used the shadow cast by a central pin (gnomon) to determine true north. However, this required direct sunlight. In the extreme northern latitudes, fog, dense cloud cover, and long periods of twilight (when the sun is just below the horizon) frequently rendered the sun compass useless. To maintain a straight heading across open ocean for days or weeks, the Vikings needed a way to locate the sun when it was invisible.
2. The Science of Polarized Light
To understand how the sunstone works, one must understand how light behaves in our atmosphere. * Unpolarized Light: Light emitted by the sun travels in unpolarized waves, meaning the light waves vibrate in all possible directions. * Atmospheric Scattering: When sunlight enters the Earth's atmosphere, it collides with air molecules and scatters (Rayleigh scattering). * Polarization: This scattering process causes the light waves to become polarized, meaning they are forced to vibrate in a single, predictable plane. This polarized light forms concentric rings in the sky centered around the sun.
Even when thick clouds obscure the sun itself, the polarization pattern remains intact in the sky above. Human eyes cannot detect polarized light, but certain crystals can.
3. The Calcite Crystal (Iceland Spar)
The specific stone believed to be the Viking sunstone is Iceland spar, a transparent variety of calcite crystal found abundantly in the Nordic regions.
Iceland spar possesses a unique optical property called birefringence, or double refraction. When light passes through the crystal, the internal atomic structure of the calcite splits the light beam in two. If you look at a dot on a piece of paper through Iceland spar, you will see two dots.
Crucially, the crystal acts as a polarizing filter. The brightness of the two split beams changes depending on how the crystal is oriented relative to the polarized light in the sky.
4. How the Vikings Used the Crystal
Navigating with a sunstone was a precise, methodical process: 1. Scanning the Sky: The navigator would look through the calcite crystal at a patch of sky, either a break in the clouds or directly at the cloud cover. 2. Observing the Double Image: Looking through the crystal, the navigator would see two images or spots of light. 3. Rotating the Crystal: The navigator would slowly rotate the crystal. As it turned, one of the images would grow brighter while the other grew dimmer. 4. Finding the Balance: When the crystal was rotated to the exact point where both images were of equal brightness, it meant the crystal was perfectly aligned with the polarization rings in the sky. 5. Pinpointing the Sun: Because the polarization rings are concentric to the sun, this alignment provided a precise line pointing directly toward the hidden sun.
By taking two readings from different patches of the sky, the navigator could triangulate the exact position of the sun behind the clouds, or even up to 40 minutes after the sun had set below the horizon. Once the sun's position was known, the navigator could align their wooden sun compass and maintain their bearing.
5. Historical and Archaeological Evidence
For a long time, the sunstone was considered a myth. However, evidence has steadily mounted to support its historical reality: * The Sagas: The Rauðúlfs þáttr, an Icelandic saga concerning King Olaf II, explicitly describes the king using a sólarsteinn during a heavy snowstorm to pinpoint the sun's location and verify his navigator's guess. * The Alderney Shipwreck: In 2013, maritime archaeologists discovered an Iceland spar crystal next to navigational dividers in the wreckage of an Elizabethan ship that sank off the coast of Alderney in 1592. Because early magnetic compasses were easily thrown off by the iron cannons on ships, navigators continued to use sunstones as a backup centuries after the Viking Age. * Modern Computer Simulations: In recent years, researchers (such as those at Eötvös Loránd University in Hungary) ran computer simulations of thousands of Viking voyages. They found that navigating with a sunstone every three hours was highly accurate, resulting in a 92% to 100% success rate of reaching Greenland from Norway in cloudy conditions.
Conclusion
The Viking use of the sunstone represents a profound, intuitive grasp of physics centuries before the scientific method was formally established. By utilizing the birefringent properties of Iceland spar to "read" the invisible polarized light of the sky, Viking navigators transformed an impassable, stormy ocean into a navigable highway, enabling some of the most daring explorations in human history.