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The Wndsn Legend

Science and mythology of the Norseman.

The Depolarizing Sunstone

Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefringence is often quantified as the maximum difference between refractive indices exhibited by the material. Crystals with asymmetric crystal structures are often birefringent, as are plastics under mechanical stress. Birefringence is responsible for the phenomenon of double refraction whereby a ray of light, when incident upon a birefringent material, is split by polarization into two rays taking slightly different paths.[1][2]

Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3).[3]

Using Iceland spar to determine the azimuth of the sun in a partly overcast sky or when the sun is just below the horizon

The sunstone (Icelandic: sólarsteinn) is a type of mineral attested in several 13th–14th century written sources in Iceland, one of which describes its use to locate the sun in a completely overcast sky. Sunstones are also mentioned in the inventories of several churches and one monastery in 14th–15th century Iceland. A theory exists that the sunstone had polarizing attributes and was used as a navigation instrument by seafarers in the Viking Age.[4]

Wndsn: Turquoise, Sunstone, and Vegvisir

[...] one can identify the direction of the sun to within a few degrees in both cloudy and twilight conditions using the sunstone 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. Alternatively a dot can be placed on top of crystal so that when you look at it from below, two dots appear, because the light is “depolarised” and fractured along different axes. The crystal can then be rotated until the two points have the same luminosity. The angle of the top face now gives the direction of the sun.[5]

See also:


  1. Birefringence (Doppelbrechung) 
  2. Picture: Two rays with perpendicular polarization passing through a positively birefringent material. 
  3. Calcite (Kalkspat / Doppelspat) 
  4. Sunstone (medieval) 
  5. Picture: Silfurberg