Sunstone (medieval)

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 {{ill|Hrafns saga Sveinbjarnarsonar|qid=Q5903775}} (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.

Thorsteinn Vilhjalmsson translates the Icelandic description in Rauðúlfs þáttr of the use of the sunstone as follows:

{{Verse translation|lang=is|

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.

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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.

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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.

File:Sunstone in practice.png

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 some insects. Polarization is a phenomenon that occurs when light encounters an obstacle, such as a shiny surface or a fog bank, deviating to a particular orientation and bees, for example, are known to be able to detect the polarization of the sunlight.{{Cite web |title=The Viking Sun Stones: navigational marvels of ancient seafarers! • Neperos |url=https://www.neperos.com/article/samv7jf524a4e388 |website=Neperos.com|date=20 March 2024 }} Polar flights applied the idea before more advanced techniques became available. Ramskou further conjectured that Iceland spar 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,Horvàth, G. et al. (2011). 'On the trail of Vikings with polarized skylight: experimental study of the atmospheric optical prerequisites allowing polarimetric navigation by Viking seafarers' Phil. Trans. R. Soc. B (2011) 366, 772–782 {{doi|10.1098/rstb.2010.0194}} often in the context of the Uunartoq disc.

Research in 2011 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. Alternatively, a dot can be placed on top of the 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. 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.{{Cite book|title=Science of the magical : from the Holy Grail to love potions to superpowers|author=Kaplan, Matt|isbn=9781476777108|edition=First Scribner hardcover|location=New York|oclc=904813040|date = 2015-10-27}}

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.{{cite news |title=Shipwreck may contain near-mythical Viking navigation aid |work=The Raw Story |agency=Agence France-Presse |date=March 5, 2013 |url=http://www.rawstory.com/rs/2013/03/05/shipwreck-may-contain-near-mythical-viking-navigation-aid/ |access-date=March 6, 2013 |archive-date=March 7, 2013 |archive-url=https://web.archive.org/web/20130307070957/http://www.rawstory.com/rs/2013/03/05/shipwreck-may-contain-near-mythical-viking-navigation-aid/ |url-status=dead }} Although the stone was found near a navigational instrument, its use remains uncertain.{{cite news |url=https://www.cbsnews.com/news/researchers-may-have-found-a-viking-sunstone/ |title=Researchers may have found a Viking sunstone |work=CBS News |date=March 8, 2013}}

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 would have required 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 with crystals might also have allowed Vikings to guide their boats at night. A type of crystal they called sunstone can use scattered sunlight from below the horizon as a guide. What they suggest is that Iceland spar crystals were used in combination with Haidinger's brush. If so, Vikings could have used them in the northern latitudes where it never becomes completely dark in summer.{{cite journal |first1=Balázs |last1=Bernáth |first2=Alexandra |last2=Farkas |first3=Dénes |last3=Száz |first4=Miklós |last4=Blahó |first5=Ádám |last5=Egri |first6=András |last6=Barta |first7=Susanne |last7=Åkesson |first8=Gábor |last8=Horváth |title=How could the Viking Sun compass be used with sunstones before and after sunset? Twilight board as a new interpretation of the Uunartoq artefact fragment |date=26 March 2014 |doi=10.1098/rspa.2013.0787 |pmid=24910520 |journal=Proceedings of the Royal Society A |volume=470 |pages=20130787 |number=2166 20130787 |bibcode = 2014RSPSA.47030787B |pmc=4042717 }} In areas of confused magnetic deviation (such as the Labrador coast), a sunstone could have been a more reliable guide than a magnetic compass.

• Allegory in the Middle Ages
• Pfund sky compass
• Solar compass

{{cite journal |last=Ramskou |first=Thorkild |author-link=Thorkild Ramskou |year=1967 |title=Solstenen |journal=Skalk |volume=2 |pages=16–17 |language=da}}

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{{cite book|author1=Gábor Horváth|authorlink1=Gábor Horváth (biophysicist)|author2=Dezsö Varjú|title=Polarized Light in Animal Vision: Polarization Patterns in Nature|url=https://books.google.com/books?id=jkwvub-1zy8C&pg=PA447|date=12 January 2004|publisher=Springer|isbn=978-3-540-40457-6|page=447}}

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Ropars, G. et al., 2011. A depolarizer as a possible precise sunstone for Viking navigation by polarized skylight. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science. Available at: http://rspa.royalsocietypublishing.org/content/early/2011/10/28/rspa.2011.0369.abstract Accessed December 5, 2011.{{doi|10.1098/rspa.2011.0369}}

• [http://www.nordskip.com/vikingcompass.html#sun The Fabled Viking Sunstone]
• [http://www.polarization.com/viking/viking.html The Viking Sunstone Is the legend of the Sun-Stone true ?]

Category:Culture of Iceland

Category:History of navigation

Category:Navigational equipment

Category:Polarization (waves)

Category:Gemstones in culture

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