The World of Vikings
To understand how this might have worked, we need to understand some things about the way light, and particularly sunlight, can be affected. Light coming from the sun is scattered and polarized by the atmosphere. This occurs when light is absorbed and re-emitted with the same energy by air molecules and by different amounts depending on the light’s wavelength. The blue end of the light spectrum is scattered more than the red, as explained in theory developed by the British physicist Lord Rayleigh in the 19th century. Scattering by particles in the atmosphere explains why the sky appears blue. More importantly, scattered light waves are also polarized to a certain extent. That means they vibrate in one plane rather than in all directions at once. The amount of polarization a beam of sunlight undergoes depends on its angle to the viewer and whether the light has been further scattered by cloud and other particles that cause depolarization. Around the coastline of Norway and Iceland are found crystalline chunks of calcium carbonate known as calcite or Iceland spar. When polarised sunlight enters a calcite crystal, something very interesting happens. Calcite is strongly birefringent, meaning that it splits light passing through it into two separate waves that are bent or refracted in different directions and with different intensities, although the total intensity will be constant. This means that objects viewed through a calcite crystal appear in double. More importantly for our purposes, the different intensities of the two light waves depends on how the original light is polarized and the position and orientation of the crystal compared to the light source. Crystal clear double vision. Tourmaline and cordierite are crystals with similar properties, except instead of splitting light like calcite they are strongly dichroic. This means they absorb one component of polarisation more strongly than the other. Again, the dichroic properties depend on how the original light is polarized and the position and orientation of the crystal compared to the light source. So, in theory at least, examining how sunlight passes through one of these crystals – and appropriately calibrated – could be used as a guide for sailors to estimate the position of the sun. This could then allow them to determine the direction of geographic north – even without understanding the scientific principles behind these phenomena. If we make the huge assumption that the Vikings had these sunstone crystals on board their ships and, more importantly, knew what they were doing with them, the question this is whether the difference in the light would be detectable to their eyes? And would it be detectable with enough accuracy (after errors because of imperfections in the crystals and depolarization), to be used as a navigation aid even in overcast conditions.
Testing the theory
The latest in an impressive roster of publications on the subject recently appeared in Royal Society Open Science , seeking to address this precise question. Gabor Horvath and his colleagues looked at whether the optical signals from these three types of crystal would be strong enough to be detected and with enough accuracy to predict the position of the sun under a cloudy sky.
Source: Ancient Origins