Sun stones, also known as solar compasses or sun compasses, were a type of crystal believed to have been used by ancient seafarers during the Viking Age and medieval times, particularly in the Northern Hemisphere.

These transparent crystals or minerals were used as navigational aids to determine the position of the sun even when it was obscured by clouds or below the horizon.

While the exact historical evidence for the use of sun stones is limited, there are references to their potential use in ancient texts and sagas.



Rauðúlfs þáttr, also known as “The Tale of Rauðúlfr,” is an allegorical short story or þáttr from the medieval Icelandic sagas. It is a part of the larger collection known as the Þættir, which are short narrative episodes typically found within the sagas. Rauðúlfs þáttr is believed to have been written in the 13th century.

The Viking long ship ‘Lofotr’. Image by Geir Are Johansen CC BY-SA 2.0

The saga recounts the story of a Viking named Rauðúlfr, who possessed a magical crystal that allowed him to navigate at sea even when the sun was hidden.

Read More The PT Boat Turret With a Deadly Bite – The Elco Thunderbolt

According to the saga, Rauðúlfr was sailing with King Harald Hardrada when they encountered a thick fog that obscured their visibility. Rauðúlfr then pulled out his sun stone, a magical device given to him by his father, and used it to locate the position of the hidden sun. With the aid of the sun stone, Rauðúlfr was able to navigate through the fog and guide the ship safely to its destination.

This tale is one of the earliest written accounts that explicitly mentions the use of a sun stone for navigation. It highlights the belief in the magical properties of such stones and their ability to aid seafarers in challenging conditions.

So, was there any truth behind this?

The First Crystal Experiments

Several experiments have been conducted to investigate the potential use of sun stones in navigation, inspired by the Viking legends and historical accounts. These experiments aim to test the feasibility and accuracy of using specific crystals or stones to locate the position of the sun even when it is obscured.

One notable experiment was conducted by a Danish archaeologist and sailor named Thorkild Ramskou. In the late 1960s, Ramskou performed a series of tests using a polarizing crystal called Iceland spar (a form of calcite) similar to ones found in Viking burial sites. He attempted to determine whether the crystal could help him find the direction of the hidden sun. By rotating the crystal and aligning the double images, he was able to estimate the position of the sun with some success.

Guy Ropars Crystal Experiment

Later studies conducted by physicist Guy Ropars explored the potential of using Iceland spar, in combination with the naked eye, as a means to determine the direction of the sun. The researchers found that this technique could provide reasonably accurate results, with an accuracy of a few degrees, even in challenging weather conditions such as cloudy or twilight situations.

One method described in the study involved moving the Iceland spar stone across the visual field. As it is moved, a distinct yellow pattern becomes visible on the fovea of the eye. This pattern serves as an indication of the direction of the sun.

A painting of Norsemen landing in Iceland.

Another method discussed in the study involved placing a dot on top of the Iceland spar crystal. When the crystal is observed from below, the light passing through the crystal becomes depolarized and fractured, resulting in the appearance of two dots instead of one. By carefully rotating the crystal until the two dots have the same luminosity, the angle of the top face of the crystal can be used to determine the direction of the sun.

Recent Experiment

In 2014, science journalist Matt Kaplan and mineralogists from the British Geological Survey attempted to replicate the findings of the study conducted by Ropars in both Scotland and off the coast of Turkey. However, their attempts were unsuccessful in determining the direction of the sun using Iceland spar.

Kaplan reached out to Ropars to discuss their puzzling results, and both were unable to comprehend why the samples of Iceland spar used in the trials did not provide accurate indications of the sun’s direction. As a possible explanation, the author hypothesized that the effective handling of the stones may require some level of experience or expertise.

Read More What IS the Difference Between a Dreadnought and a Battleship?

This suggests that the successful utilization of Iceland spar as a sunstone might involve a certain level of skill or understanding that was not fully grasped during the replication attempts.

The Alderney Crystal Shipwreck

The discovery of a piece of Iceland spar from a sunken Elizabethan ship near Alderney in 1592 is an intriguing historical finding. The shipwreck, known as the “Alderney Crystal Shipwreck,” was located in the waters surrounding the Channel Islands. The ship is believed to have belonged to the fleet of Sir Walter Raleigh, an English explorer and courtier.

The exact purpose of the Iceland spar found on the Alderney Crystal Shipwreck remains uncertain. It is speculated that it could have been part of a navigational instrument or device used by the crew for determining the position of celestial bodies, such as the sun or stars, and aiding in navigation. However, due to the limited information available about the specific context and arrangement of the crystal on the ship, its precise function and usage are still subjects of speculation and ongoing research.

An example of an Iceland Spar crystal. Image by ArniEin CC BY-SA 3.0

Further Experiments With The Crystals

In recent years, further experiments have been carried out using different types of crystals and stones. Researchers have explored the use of polarizing filters, similar to those used in photography, to simulate the properties of Viking sun stones. These experiments involved manipulating the filters to detect the polarization angle of sunlight and determine the position of the hidden sun.

Read More The Battle of Jutland: A Naval Clash of Titans

Furthermore, computer simulations have been used to model the behavior of light passing through various crystals and atmospheric conditions. These simulations aim to understand how different crystals might affect the visibility of the sun and provide insights into the accuracy and limitations of using sun stones for navigation.

It is worth noting that while these experiments have shown some promise in recreating the principles behind the Viking sun stones, their practicality and reliability in real-world navigation situations are still subject to debate. The experiments serve as valuable explorations into the possibilities, but they do not provide conclusive evidence of the widespread use of sun stones by Vikings.