The Mystery of the Viking Sunstone
The Vikings were legendary seafarers, navigating vast stretches of the North Atlantic centuries before modern instruments. But how did they manage to stay on course in a time before compasses, especially in cloudy or foggy conditions? That’s where the mysterious Viking sunstone comes in—a tool believed to help them locate the sun even when it wasn’t visible.
What was the sunstone? How did it work? And why was it so important for Viking navigation? Let's find out.
What Was the Viking Sunstone?
The Viking sunstone was a type of crystal believed to have been used as a navigational aid. According to medieval Icelandic sagas, it was a tool that allowed sailors to locate the sun’s position even when the sky was overcast or foggy.
While no physical sunstones have been definitively linked to Viking ships, researchers have tested certain minerals and found it's viable that such a tool did exist.
Why Did the Vikings Need a Sunstone?
Navigating the North Atlantic wasn’t easy. Heavy cloud cover, dense fog, and the long summer days where the sun never fully set made traditional navigation methods unreliable. The Vikings, however, were masters of the sea, and a tool like the sunstone would have given them a huge advantage.
With a sunstone, they could stay on course even when the sky was grey. This allowed them to reach distant lands like Iceland, Greenland, and even North America with confidence.
How Did the Sunstone Work?
Medieval Icelandic texts mention a “sunstone” that helped Viking sailors determine the sun’s position, even on overcast days. While these texts don’t go into great detail, modern scientists believe the sunstone relied on the principles of light polarization.
When sunlight scatters through the atmosphere, it becomes polarized. Certain crystals can filter this light in a way that reveals the sun’s position. By holding up a sunstone and rotating it until the light changed, Viking navigators could have pinpointed the direction of the sun with surprising accuracy.
Even when the sun was completely obscured by clouds, a sunstone would have revealed the direction of the strongest polarized light, i.e. the sun's location. This effect is particularly useful at sea, where there are no fixed landmarks for reference. In twilight hours or even just after sunset, a sunstone could still help determine the last position of the sun, allowing the sailors to adjust their course accordingly.
Possible Sunstone Candidates
Several minerals have been proposed as the real Viking sunstone and below are the leading two candidates.
Iceland Spar (Clear Calcite)
This transparent form of calcite has a special optical property called birefringence—it splits light into two rays. This effect makes it particularly useful for detecting polarized light and locating the sun’s position.
If a Viking navigator looked through an Iceland spar crystal, they would see a double image due to its birefringence. By rotating the crystal, they could identify the point where the light was most evenly split or where the brightness changed in a predictable way, revealing the sun’s hidden position.
While many gemstones exhibit birefringence, not all of them are effective for sun navigation. Iceland spar is unique because of its extreme clarity and strong birefringence, which produces a clear double-image effect. This allows sailors to detect subtle changes in polarized light, whereas most other birefringent gemstones would distort or scatter light in a way that makes precise navigation impractical.
Where the Vikings could have found it: Iceland spar is naturally abundant in Iceland and parts of Scandinavia, making it easily accessible to Viking explorers.
Iolite (Cordierite)
Iolite is a pleochroic gemstone, meaning it changes colour depending on the angle you view it from. This property allows it to act as a natural polarizing filter, helping sailors determine the sun’s location.
When viewed through an iolite crystal, the light would shift in hue depending on the viewing angle. The optimal hue would indicate the direction of maximum polarization, giving sailors a clue as to where the sun was located. Without such a tool, they would have been left guessing, risking dangerous deviations from their intended route.
Iolite was once the most widely accepted candidate for the Viking sunstone, which led to its nickname, the "Viking compass."
However, later discoveries and research have led scholars to favour Iceland spar instead. While iolite can act as a polarizing filter, its colour shifts can be more subtle and less precise compared to the birefringence of Iceland spar. Additionally, iolite is not as transparent as Iceland spar, which could make it harder to use in low-light conditions. Despite this, iolite remains a strong possibility, as its filtering effect could still assist in locating the sun.
Where the Vikings could have found it: Iolite deposits are found in Norway and Greenland, regions well within Viking travel routes.
The exact identity of the Viking sunstone is still a mystery, but it’s fascinating to think that ancient sailors may have used gemstones not just for decoration, but as cutting-edge navigation tools! Whether it was Iceland spar, iolite, or another mineral, the sunstone would have been an invaluable.