In some Icelandic sagas—embellished stories of Viking life—sailors relied on so-called sunstones to locate the sun’s position and steer their ships on cloudy days.
The stone would’ve worked by detecting a property of sunlight called polarization.
Polarization is when light—which normally radiates randomly from its source—encounters something, such as a shiny surface or fog, that causes the rays to assume a particular orientation.
Due to this property, as sunlight moves through the atmosphere, the resulting polarization gives away the direction of the original source of the light.
Detecting light’s polarization is a natural ability of some animals, such as bees.
In 1969, a Danish archaeologist suggested real-life Vikings might have used sunstones to detect polarized light, using the stones to supplement sundials, stars, and other navigational aids.
Since then, researchers have been probing how such a sunstone might have worked. On that point, though, the sagas were silent.
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