Sunstone (medieval)

[2] One medieval source in Iceland, Rauðúlfs þáttr,[3][4] mentions the sunstone as a mineral by means of which the sun could be located in an overcast and snowy sky by holding it up and noting where it emitted, reflected or transmitted light (hvar geislaði úr honum).

"Rauðúlfs þáttr", a tale of Saint Olav, and the only medieval source mentioning how the sunstone was used, is a thoroughly allegorical work.

[11] A round and rotating house visited by Olav has been interpreted as a model of the cosmos and the human soul,[12] as well as a prefiguration of the Church.

[16] The presence of the sunstone in "Rauðúlfs þáttr" may be entirely symbolic[17] but its use is described in sufficient detail to show that the idea of using a stone to find the sun's position in overcast conditions was commonplace.

Research in 2011[25] confirms that one can identify the direction of the sun to within a few degrees in both cloudy and twilight conditions using Iceland spar and the naked eye.

The process involves moving the stone across the visual field to reveal a yellow entoptic pattern on the fovea of the eye.

Attempts to replicate this work in both Scotland and off the coast of Turkey by science journalist Matt Kaplan and mineralogists at the British Geological Survey in 2014 failed.

Kaplan communicated with Ropars, and neither could understand why the samples of Iceland spar that were being used during the trials did not reveal the sun's direction, with the author hypothesizing that the stones require some experience to be handled effectively.

[26] The recovery of a piece of Iceland spar from an Elizabethan ship that sank near Alderney in 1592 suggests the possibility that this navigational technology may have persisted after the invention of the magnetic compass.

[28] Beyond nautical navigation, a polarizing crystal would have been useful as a sundial, especially at high latitudes with extended hours of twilight, in mountainous areas, or in partly overcast conditions.

[10] A Hungarian team proposed that a sun compass artifact with crystals might also have allowed Vikings to guide their boats at night.

Iceland spar , possibly the medieval sunstone used to locate the Sun in the sky when clouds obstruct it from view
Demonstration of a sunstone used to locate the sun's position by comparing the intensity of the ordinary and extraordinary images of a black dot marked on the crystal's surface opposite the observer.