THE Icelandic sagas of the 10th century tell the tales of their remarkable voyages. Among the poetic recitations of battles, politics and adventure-filled quests is mention of a mysterious “solar stone” guiding their way.

We know the Vikings used the Sun and stars in the same way as other ancient mariners. The North Star was a reliable compass point. And where the Sun was at any given point of the day could also tell an experienced practitioner where they were.

But the Vikings had a particular challenge.

Atrocious weather.

Fog and cloud would often obscure the skies for days on end. The stars would be obscured. Even the light of the Sun would be randomly scattered — if not totally blotted out.

It was a recipe for disaster.

But through the gloom, the sagas say, the solar stones guided Viking hands at the helms.

But what were they?

Figments of mythological imagination? Or a real piece of ancient technology?

GUIDING LIGHT

No solar stone has ever been found. And only one etched timber fragment has been identified as being potentially part of a solar compass.

But the myths are persistent. Even insistent.

The weather was thick and snowy as Sigurður had predicted. Then the king summoned Sigurður and Dagur (Rauðúlfur’s sons) to him. The king made people look out and they could nowhere see a clear sky. Then he asked Sigurður to tell where the sun was at that time. He gave a clear assertion. Then the king made them fetch the solar stone and held it up and saw where light radiated from the stone and thus directly verified Sigurður’s prediction

— Rauðúlfs þáttr

And there is evidence they are real.

While it dated several centuries later than the Vikings, a piece of Icelandic Spar crystal was found aboard the wreck of a 1592 Elizabethan ship.

In effect, it’s a crystal Sun-finder.

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And if you know where the Sun is at dawn and dusk, you know where you are.

All it would take to turn this into a true solar compass would have been a little knowledge of maths. And.a sundial-like set of reference points — etched in stone and metal.

One such fragment was found in the ruins of a Benedictine convent in Greenland. It’s just half of an etched and scratched timber disk. When combined with a solar crystal, it could have provided a point for north.

PROOF OF CONCEPT

A research team Eötvös Loránd University in Hungary has set out to determine if there was any substance behind the myths.

They identified several different types of crystal available to Viking craftsmen.

And they tested their ability to discern a sun amid the clouds.

To make the test truly scientific, a variety of polarising crystals were used under a variety of weather conditions at different times of the year.

Exactly how effectively they defined the location of the Sun was measured.

Cordierite and Tourmaline crystals were tested. So was crystallised calcium carbonate, otherwise known as Iceland spar. This is the same stone as found in the Elizabethan wreck.

Calcite was found to be generally the most accurate. But tourmaline and cordierite were better at particular times of the year, or at particular places.

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The crystal works in a similar way to the polarisation in a pair of sunglasses.

It’s all about the structure of the calcite crystal itself. It’s assymetric structure bends light by different amounts at different angles.

Light scattered by the clouds behaves differently to that which comes directly from the Sun itself. The polarised crystals filter out these errant rays

But they also split the direct-incoming light in two.

Looking through its opaque surface on a heavily clouded day produced a bright, double-dot. As the polarising crystal was pointed closer to the Sun, the distance between the dots closed — finally fusing when lined up.

An experienced practitioner can use this to good effect.

And once they knew where the Sun was, a calculating tool — such as the engraved timber disc — could point the way to safe harbour.