To locate the Sun after Sunset, the Vikings used a pair of crystals known as Sunstones, which are calcite stones that produce patterns when they're exposed to the polarization of UV rays within Sunlight. When the crystals are held up to the sky, the orientation of these patterns cast within the stone can help pinpoint the position of the Sun below the horizon.
Once they had determined the position of the hidden Sun, they used a specially designed wooden slab called a shadow stick to simulate the shadow of the gnomon, based on the angle at which the hidden Sun would hit it. The location of the outer edge of that imaginary shadow could then have been used to determine their cardinal direction. A wide hole within the center of the disk served as a holding spot for this so-called central gnomon.
Researchers have estimated the plausible accuracy of this solar compass and found that it would have worked with only 4 degrees of error, which is better than other forms of celestial navigation, and comparable to modern magnetic pocket compasses. The compass would have functioned for as long as 50 minutes after Sunset around the spring equinox.
The findings are a testament to the sophistication of this group of people often remembered as heathens, and shed further light on the Vikings’ connection to, and observation of, the Sun.
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