Hello brave explorers! Recently we dived into the past and discovered Vikings’ tales and magical powers, then we confronted the laws of nature, unveiling how the ancient artifact polarize light. Now it’s time to use our artifacts, polarizing crystals, and find the sun when it cannot be seen, covered by clouds or beyond the horizon.
Previously we described a Vikings’ legend, that described the king Olaf who asked Sigurd to find the sun while the snow was falling, and there was not a patch of clear sky to be seen. Sigurd claimed to have a special power that helps him to detect the sun in every weather. He showed where the sun is, and the king verified that by using the sunstone, an artifact long believed to be legendary, not real, but recently (re)discovered to be a polarized crystal, that shows signs of surrounding polarized light, when available.
The Sky
The light coming from the sun is unpolarized. If you will take a single ray of light and measure it, you will find that it is polarized, but, all of the rays in the same point taken together have polarizations in every possible direction, making the total unpolarized. Nevertheless, the sky is partially polarized, thanks to, among others, water particles in the atmosphere.
The availability of the polarization pattern is complicated and depends on the location of the sun, humidity, the amount of clouds or fog in the sky, and more. But in general, the sky is polarized perpendicular, 90 degrees, to the sun.
The Sunstone acts, in a way, as polarity detection. Thanks to birefringence, one of its output rays may be stronger depending on the input’s ray polarity. By noting the polarity of the sky, we can determine where the sun is, according to the sky polarization pattern presented above. Sunstones’ are not so easy to use and there are many possible ways to use them, but the only method that worked for me is presented below.
Wrap the Sunstone with some non-transparent material, like paper. Leave one side open, and create a small hole on the opposite side. If you look from the open side, you will see two dots of light – these are the two rays of light with different polarization, and therefore, different refraction angles. Since my stone has some breaks in it, the holes of light were mirrored and I normally saw four dots instead of two, but the idea is the same.
Since the measurement of the polarization of the sky requires a fair amount of skill, and they are only partially polarized, I used my laptop with its strongly polarized LCD screen instead (Works well with a smartphone too. The tests with the sky on a cloudy day were not that conclusive, probably because they are only partially polarized. The breaks in my stone didn’t help either, but maybe it’s just because I wasn’t skilled enough). Looking through the stone onto the screen I saw four dots of light (normally, should be two) when the polarization was the highest. Rotating the stone clockwise and counterclockwise, I had only two (normally: one) dots visible, a different pair of dots in every direction. That allowed me to detect the polarization of my screen, similar to how I previously tested with the 3D glasses. Doing the same with the sky, and practicing it, will allow detecting the sun and navigating in cloudy or foggy days.
It is interesting to note that independently, a similar sun-detecting sky compass was developed in the 40s, based on the same principle. It could locate the sun “during twilight, and when the sun is several degrees below the horizon, as well as when the region of the sky containing the sun is overcast, so long there is a clear patch of sky overhead. The sky compass is thus of particular value when the sun compass and the sextant are not usable. Since the extent of polarization of the sky’s light is greatest at right angles to the incident beam of sunlight, the compass is most accurate in the polar regions, where it is also most useful, because of the long duration of twilight.” [1] [2] Pay attention that a patch of clear sky is normally available close to the horizon, even on cloudy days.
Birefringence was officially discovered and documented only in the 17th century by Erasmus Bartholinus and the phenomenon wasn’t explained in terms of polarization of light until the 19th century, centuries after the Vikings, but it does not mean that people could not use it before.
The Sixth Sense – Polarization
While even the concept of polarization of light is challenging for us, some animals use it on a daily basis. For example, honeybees, but also ants, crickets, mayflies, many other insects, and even bats.
But that’s not the whole story, because it appears that not only some animals can see or detect polarization, but people too! In some sense, we have a sixth sense of polarization! This special skill was first described by Austrian physicist Wilhelm Karl von Haidinger in 1844, it’s named after him, The Haidinger’s brush and it is little known to most people to this day.
Why we can see it? The research is still active [3], but for that, we probably should thank a combination of multiple mechanisms in our eye. These are related to macula lutea, an oval-shaped pigmented area near the center of the retina, the innermost, light-sensitive layer of tissue of the eye, and the arrangement of its pigments, that has the dichroism property, according to which light in different polarization angles has a different absorption.
The angle of polarization is visible as a blur and faint pattern, that cannot be photographed directly as it is visible in the eye itself. It appears as a yellow hourglass, with blue dots next to it, and its angle indicates the light polarization angle.
It is quite challenging to see the brush and requires some practice, but it’s totally worth it. I used an LCD screen on my laptop, staring at a white screen and turning it 90 degrees every time until I finally saw it. Smartphones can be used as well, but pay attention that they are normally at 45 degrees, unlike the laptops. Also, it is important to note that it requires some time and patience, as the brush is hardly seen [4].
We will probably never know, but maybe Sigurd’s superpower, or sixth sense, that allowed him to determine how far the sun had travelled when there was not a patch of clear sky to be seen and to give a precise answer that was later confirmed using a sunstone, was the ability to see the Haidinger’s brush and detect polarization. So, next time you read a legend, think about and wonder, what in it could be real, even if we don’t yet know how it’s possible.
Do you believe in magic? Well, now, I certainly do.
[1] National Bureau of Standards. Technical News Bulletin. May 1949 – Sky Compass. https://babel.hathitrust.org/cgi/pt?id=uiug.30112007624197&view=1up&seq=357
[2] Polarization.com http://www.polarization.com/viking/viking.html, http://www.polarization.com/haidinger/haidinger.html.
[3] The polarization sense in human vision. Albert Le Floch, Guy Ropars, Jay Enoch, Vasudevan Lakshminarayanan. Vision Res. 50 (20): 2048–2054. 2010. https://doi.org/10.1016/j.visres.2010.07.007
[4] Celestial polarization patterns sufficient for Viking navigation with the naked eye: detectability of Haidinger’s brushes on the sky versus meteorological conditions Gábor Horváth, Péter Takács, Balázs Kretzer, Szilvia Szilasi, Dénes Száz, Alexandra Farkas and András Barta. https://royalsocietypublishing.org/doi/full/10.1098/rsos.160688
Featured image by Al Seeger from Pixabay.
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