Viking sunstone

The “sólarsteinn” or “sunstone” is a crystal described in the sagas as showing the position of the sun even in overcast weather.

The “sunstone” is mentioned several times in medieval Icelandic texts. These mentions appear notably in inventories of churches and monasteries dating from the 14th and 15th centuries, so the existence of such an object is hard to deny.

[With the green fluorite octahedron placed behind it, the birefringence of optical calcite, or Iceland spar, is clearly visible. © www.carionmineraux.com]

The first text is the saga of Hrafn (Hrafns saga Sveinbjarnarsonar). There the sunstone is described as a precious gift from Bishop Guðmund which, when stolen by thieves, is miraculously turned into a worthless object. The same account is found in parallel in the narratives of the bishop’s life.

The second text, the tale of Rauðúlf and his sons (Rauðúlfs þáttur), is the only one that explicitly mentions the use of the sunstone.

“The weather was overcast and snowy, as Sigurður had predicted. Then the king summoned Sigurður and Dagur. He asked his men to look around them; no one could find the slightest patch of blue sky. Then he ordered Sigurður to point out the sun, and Sigurður gave a firm answer. Then the king sent for the sunstone and, holding it above him, saw the light shine out from it, and so was able to verify directly that Sigurður’s prediction was right.”

This calcite has the property of “depolarising” sunlight, that is, of filtering it differently depending on how the stone is turned. Optical calcite gives the direction of the sun: looking at the light through it, one obtains two different “beams”, an “ordinary” one (partially polarised, that is, oriented in one direction) and a “depolarised” one. An accuracy of a few degrees can be reached even in twilight conditions, notes a study published in the British scientific journal Proceedings of the Royal Society.