[MR] Fwd: [Bryn-gwlad] In Nature today - Did Vikings navigate by polarized light?

[Stefan li Rous]

There was some recent discussion on this list about medieval navigation and plotting boards, so I thought this article might be interesting to those here studying medieval navigation, even if this is several centuries earlier than the timeframe of the earlier discussion.

Stefan

Begin forwarded message:

> From: Jeremy Johnson <phlagm at gmail.com>
> Date: January 31, 2011 8:11:16 AM CST
> To: Barony of Bryn Gwlad <bryn-gwlad at lists.ansteorra.org>
> Subject: [Bryn-gwlad] In Nature today - Did Vikings navigate by polarized light?
> Reply-To: Barony of Bryn Gwlad <bryn-gwlad at lists.ansteorra.org>
> 
> Hey all.  I know I've been absent for a while, but I saw this first
> thing this morning and thought I would pass it along.  But since I
> don't know how many of you have access to nature articles, I decided
> to just go ahead and  copy, paste, and post the whole thing to the
> list:  Having spent some winters in Germany lately, I can attest to
> having experienced periods of several days where the entire sky is
> dull grey and gives absolutely no indication of the direction of the
> sun.
> 
> Enjoy
> 
> -Sigthorn
> 
> --------------------------------------------------------------------------------------------------------
> Did Vikings navigate by polarized light?
> 
> 'Sunstone' crystals may have helped seafarers to find the Sun on cloudy days.
> 
> Jo Marchant
> 
> A Viking legend tells of a glowing 'sunstone' that, when held up to
> the sky, revealed the position of the Sun even on a cloudy day. It
> sounds like magic, but scientists measuring the properties of light in
> the sky say that polarizing crystals — which function in the same way
> as the mythical sunstone — could have helped ancient sailors to cross
> the northern Atlantic. A review of their evidence is published today
> in Philosophical Transactions of the Royal Society B1.
> 
> The Vikings, seafarers from Scandinavia who travelled widely and
> settled in swathes of Northern Europe, the British Isles and the
> northern Atlantic from around 750 to 1050 AD, were skilled navigators,
> able to cross thousands of kilometres of open sea between Norway,
> Iceland and Greenland. Perpetual daylight during the summer sailing
> season in the far north would have prevented them from using the stars
> as a guide to their positions, and the magnetic compass had yet to be
> introduced in Europe — in any case, it would have been of limited use
> so close to the North Pole.
> 
> But Viking legends, including an Icelandic saga centring on the hero
> Sigurd, hint that these sailors had another navigational aid at their
> disposal: a sólarsteinn, or sunstone.
> 
> The saga describes how, during cloudy, snowy weather, King Olaf
> consulted Sigurd on the location of the Sun. To check Sigurd's answer,
> Olaf "grabbed a sunstone, looked at the sky and saw from where the
> light came, from which he guessed the position of the invisible Sun"2.
> In 1967, Thorkild Ramskou, a Danish archaeologist, suggested that this
> stone could have been a polarizing crystal such as Icelandic spar, a
> transparent form of calcite, which is common in Scandinavia2.
> 
> Light consists of electromagnetic waves that oscillate perpendicular
> to the direction of the light's travel. When the oscillations all
> point in the same direction, the light is polarized. A polarizing
> crystal such as calcite allows only light polarized in certain
> directions to pass through it, and can appear bright or dark depending
> on how it is oriented with respect to the light.
> 
> Centred on the light
> 
> Scattering by air molecules in the atmosphere causes sunlight to
> become polarized, with the line of polarization tangential to circles
> centred on the Sun. So Ramskou argued that by holding a crystal such
> as calcite up to the sky and rotating it to check the direction of
> polarization of the light passing through it, the Vikings could have
> deduced the position of the Sun, even when it was hidden behind clouds
> or fog, or was just beneath the horizon.
> 
> Historians have debated the possibility ever since, with some arguing
> that the technique would have been pointless, because it would only
> work if the crystal was pointed at patches of clear sky, and in such
> conditions it would be possible to estimate the position of the Sun
> with the naked eye, for example from the bright lining of cloud tops3.
> 
> Gábor Horváth, an optics researcher at Eötvös University in Budapest,
> and Susanne Åkesson, a migration ecologist from Lund University,
> Sweden, have been testing these assumptions since 2005. The special
> issue of Philosophical Transactions of the Royal Society B in which
> their review appears is dedicated to biological research on polarized
> light1.
> 
> In one study, the researchers took photographs of partly cloudy or
> twilight skies in northern Finland through a 180° fisheye lens, and
> asked test subjects to estimate the position of the Sun4. Errors of up
> to 99° led the researchers to conclude that the Vikings could not have
> relied on naked-eye guesses of the Sun's position.
> 
> To check whether sunstones would work better, in 2005 they measured
> the polarization pattern of the entire sky under a range of weather
> conditions during a crossing of the Arctic Ocean on the Swedish
> icebreaker Oden5,6.
> 
> Through the clouds
> 
> The researchers were surprised to find that in foggy or totally
> overcast conditions the pattern of light polarization was similar to
> that of clear skies. The polarization was not as strong, but Åkesson
> believes that it could still have provided Viking navigators with
> useful information.
> 
> "I tried such a crystal on a rainy overcast day in Sweden," she says.
> "The light pattern varied depending on the orientation of the stone."
> 
> She and Horváth are now planning further experiments to determine
> whether volunteers can accurately work out the Sun's position using
> crystals in various weather conditions.
> 
> Sean McGrail, who studied ancient seafaring at the University of
> Oxford, UK, before retiring, says that the studies are interesting but
> there is no real evidence to indicate that the Vikings actually used
> such crystals. "You can show how they could be used, but that isn't
> proof," he says. "People were navigating long before this without any
> instruments."
> 
> Surviving written records indicate that Viking and early medieval
> sailors crossed the north Atlantic using the Sun's position on clear
> days as a guide, in combination with the positions of coastlines,
> flight patterns of birds, migration paths of whales and distant clouds
> over islands, says Christian Keller, a specialist in North Atlantic
> archaeology at the University of Oslo. "You don't need to be a
> wizard," he says. "But you do need to combine a lot of different sorts
> of observations."
> 
> Keller says he is "totally open" to the idea that the Vikings also
> used sunstones, but is waiting for archaeological evidence. "If we
> find a shipwreck with a crystal on board, then I would be happy," he
> says.
> 
> References
> 
> Horváth, G. et al. Phil. Trans. R. Soc. B 366, 772-782 (2011).
> Ramskou, T. Skalk 2, 16-17 (1967).
> Roslund, C. & Beckman, C. Appl. Opt. 33, 4754-4755 (1994).
> Barta, A. , Horváth, G. & Meyer-Rochow, V. B. J. Opt. Soc. Am. A22,
> 1023-1034 (2005).
> Hegedüs, R. , Åkesson, S. , Wehner, R. & Horváth, G. Proc. R. Soc.
> A463, 1081-1095 (2007).
> Hegedüs, R. , Åkesson, S. & Horváth, G. J. Opt. Soc. Am. A 24,2347-2356 (2007).
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--------
THLord Stefan li Rous    Barony of Bryn Gwlad    Kingdom of Ansteorra
   Mark S. Harris           Austin, Texas          StefanliRous at austin.rr.com
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