Sir Arthur C. Clarke, a famous Science Fiction author, and inventor, once said that “Any sufficiently advanced technology is indistinguishable from magic”. The amazing story of the sunstone and the Vikings, that we’ve recently started covering in the first part of this series, “When a Legend Becomes Reality. Part I. Vikings’ Magical Powers”, proves exactly that.
In the first part, we described an ancient Vikings’ saga (Rauðúlfs þáttr), in which during overcast weather, while the snow was falling and there was not a patch of clear sky to be seen, the “King asked Sigurd to determine how far the sun had travelled. He gave a precise answer. So the King had the sunstone held aloft, and observed where it cast out a beam; the altitude it showed was exactly as Sigurd had said.”
That part of the story, as the whole saga, sounds absolutely fantastical and was considered so until the 21st century. That perception had been changed thanks to a stone that was found in the wreck of a 16th-century ship, next to other navigational instruments. The stone was an Iceland spar (a calcite crystal), and multiple studies, in addition to some others that are performed up to these days, prove that this stone was indeed the mythical and magical sunstone. The Vikings had to wait almost one-thousand years until their secrets were finally unveiled and scientifically explained.
These secrets lie in two phenomena: the polarization of light, and birefringence.
Polarization
Polarization is a property that applies to some types of waves, for example, guitar strings. Depending on how the string is plucked, the vibrations can be in a vertical or horizontal direction, or at any angle perpendicular to the string. Light has this property too, and although it’s less intuitive, it’s not less physical and we not only know how to detect it, but also make proper use of it.
Take for example 3D movies. To see the 3D effect, we wear a special type of glasses. Although in modern cinema-glasses both lenses seem to be transparent to us (unlike the previously used red-blue glasses) and through them we can see all colors, but we can’t see all polarization angles. Normally, the light comes unpolarized, which means that it has all the polarization angles and vibrates in every direction. What these glasses do is filter the coming light in such a way that each lens passes only a specific polarization angle.
In the 3D cinema, if we don’t use the glasses, we see a blurred image. That is because there are actually two similar images with a different perspective and a small distance between them, and each one is transmitted with different polarization angles. Each lens passes only one of the images, and by that allows each eye to see only a single image – only a single perspective. The distance between the images and their perspectives creates the 3D effect.
Knowing that we can start playing with the glasses, and detect polarized light! One example would be LCD screens, that are used on most of the laptops. These screens emit polarized light, which means that our 3D glasses can filter that light.
Water is polarized too. The light that is reflected in water is partially or fully polarized in a direction parallel to the surface of the water. Water absorbs most of the light that is not parallel to the surface and reflects the rays that are polarized in parallel to the surface (this effect is used in polarized sunglasses). We can easily check that with 3D glasses:
Water can be found not only on the ground but also in the sky in forms of humidity and clouds, which that is exactly what helped the Vikings to navigate, by using Sunstones and polarization.
Birefringence
We are used to the fact that light, when it passes through a material, like glass or water, changes its direction. This phenomenon is called refraction, and it is responsible for things to seem to be in a different position than they are when we put them in water, like a visual deviation of a spoon in a glass of water.
Sunstones, or Iceland spar (a calcite crystal), has a more complicated property, birefringence, which is responsible for double refraction. This phenomenon causes a ray of light when it meets a birefringent material like the sunstone, split into two rays taking slightly different paths. The paths they take depends on the rays’ polarization.
Since the rays of light are moving in different paths, the resulted image is seen twice. Rotating the sunstone causes one of the images to rotate around the other, without changing its orientation.
Now we have all we need to create our ancient GPS and find the sun on a cloudy day or when it is beyond the horizon! How it can be done and more, we will describe in our third and final part of that series, that will be published in the very near future!
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Featured image: A sunstone (Iceland spar). Own work.