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.
One medieval source in Iceland, "Rauðúlfs þáttr", 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). Sunstones are also mentioned in Hrafns saga Sveinbjarnarsonar (13th century) and in church and monastic inventories (14th–15th century) without discussing their attributes. The sunstone texts of Hrafns saga Sveinbjarnarsonar were copied to all four versions of the medieval hagiography Guðmundar saga góða.
The description in "Rauðúlfs þáttr" of the use of the sunstone is as follows:
- Thorsteinn Vilhjalmsson translation:
- 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.
- In Icelandic:
- "Veður var þykkt og drífanda sem Sigurður hafði sagt. Þá lét konungur kalla til sín Sigurð og Dag. Síðan lét konungur sjá út og sá hvergi himin skýlausan. Þá bað hann Sigurð segja hvar sól mundi þá komin. Hann kvað glöggt á. Þá lét konungur taka sólarstein og hélt upp og sá hann hvar geislaði úr steininum og markaði svo beint til sem Sigurður hafði sagt".
Allegorical nature of the medieval texts
Two of the original medieval texts on the sunstone are allegorical. Hrafns saga Sveinbjarnarsonar contains a burst of purely allegorical material associated with Hrafn’s slaying. This involves a celestial vision with three highly cosmological knights, recalling the horsemen of the Apocalypse. It has been suggested that the horsemen of Hrafns saga contain allegorical allusions to the winter solstice and the four elements as an omen of Hrafn’s death, where the sunstone also appears.
"Rauðúlfs þáttr", a tale of Saint Olav, and the only medieval source mentioning how the sunstone was used, is a thoroughly allegorical work. A round and rotating house visited by Olav has been interpreted as a model of the cosmos and the human soul, as well as a prefiguration of the Church. The intention of the author was to achieve an apotheosis of St. Olav, through placing him in the symbolic seat of Christ. The house belongs to the genre of "abodes of the sun," which seemed widespread in medieval literature. St. Olav used the sunstone to confirm the time reckoning skill of his host right after leaving this allegorical house. He held the sunstone up against the snowy and completely overcast sky and noted where light was emitted from it (the Icelandic words used do not make it clear whether the light was reflected by the stone, emitted by it or transmitted through it). It has been suggested that in Rauðúlfs þáttr the sunstone was used as a symbol of the Virgin, following a widespread tradition in which the virgin birth of Christ is compared with glass letting a ray of the sun through.
The allegories of the above mentioned texts exploit the symbolic value of the sunstone, but the church and monastic inventories, however, show that something called sunstones did exist as physical objects in Iceland. The presence of the sunstone in "Rauðúlfs þáttr" may be entirely symbolic 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.
Danish archaeologist Thorkild Ramskou posited that the sunstone could have been one of the minerals (cordierite or Iceland spar) that polarize light and by which the azimuth of the sun can be determined in a partly overcast sky or when the sun is just below the horizon. The principle is used by many animals and polar flights applied the idea before more advanced techniques became available. Ramskou further conjectured that the sunstone could have aided navigation in the open sea in the Viking period. This idea has become very popular, and research as to how a sunstone could be used in nautical navigation continues.
Research in 2011 by Ropars et al., confirms that 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.
The recovery of an Iceland spar sunstone from an Elizabethan ship which sank near Alderney in 1592 suggests the possibility that the navigational technology may have persisted after the invention of the magnetic compass. Although the stone was found near a navigational instrument, its use remains uncertain.
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. This use would require the polarizing crystal to be used in conjunction with known landmarks; churches and monasteries would have valued such an object as an aid to keep track of the canonical hours.
A Hungarian team proposed that a sun compass artifact might with crystals have allowed Vikings to guide their boats at night too. A type of crystal they called a sunstone can use scattered sun light from below the horizon as a guide. What they suggest is that calcite stone crystals similar to one found amongst navigational tools on a sixteenth century sunken ship was used in combination with Haidinger's brush. If so, the Vikings could have used them in the northern latitudes where during the summer it never goes completely dark.
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