How does the "mythical" Viking Sunstone work?

 

    “What is this thing Sunstone and why was it mythical? The Sun Stone is also written in Icelandic sagas (myths). It roughly explains how this object works: it shows the position of the Sun in the sky, even if the sky is cloudy, there is fog, or even if the Sun is below the horizon. Sounds magical, and for a long time it was thought that these descriptions really didn’t really correspond to reality. But did the Vikings somehow orient themselves in the vast seas? An archeological find helped to unravel the mystery of Viking navigation - but not from Viking times.

 

   At the end of the   16th century, an English naval ship sank in the English Channel, off the island of Alderney. In 1977, its remains were explored, and a set of navigational devices was discovered in the last decade. A palm-sized crystal was found among them, which was found to be Iceland spar. A couple of years earlier, studies have been published showing that Icelandic feldspar can be used to determine the position of the Sun. So, it seems, everything is clear - the Vikings took Iceland spar and saw where the Sun is.

 

    But how does it really work? Iceland spar is a crystal of calcium carbonate. It occurs in large readily cleavable crystals, is easily divisible into rhombuses, and is remarkable for its birefringence. This means that the index of refraction of the crystal is different for light of different polarization. A ray of unpolarized light passing through the crystal divides into two rays of perpendicular polarization directed at different angles, called double refraction.

 

That is, any beam entering the crystal is divided into two, so when looking through the crystal, we see a double image. As the crystal rotates, the two images also move, and the brightness of the images differs depending on the angle between the direction of the light source and the position of the crystal. When you rotate the crystal at a certain angle, the brightness of the images becomes the same. By discovering this angle and looking in which direction the Sun is from the crystal, the measurements can be calibrated: then even on a cloudy day or the Sun is slightly below the horizon, its position in the sky can be determined by tilting the crystal. "Equally bright" is not an accurate measurement, but our eyes notice the differences very well, so using a crystal from the eye makes it possible to determine the position of the Sun with one degree of accuracy.

 

    Why Iceland spar? There are more crystals with similar optical properties, but this is abundant in Iceland and elsewhere in Northern Europe, where the Vikings were able to collect and use them on ships. Even much later, with the advent and spread of magnetic compasses, the Sun's stones did not lose their importance, as Alderney's 16th century findings prove.  By the end of the 1990s, magnetic compasses were already on board of ships, but they were unreliable and inaccurate due to poor manufacturing quality, misunderstanding of magnetic anomalies and even iron cannon tubes causing distortion."