Experiments : trying to locate the position of the sun - Sunstone - Iceland Spar - Optical Calcite -

How to find the sun with a viking sunstone

We find various claims asserting the sun can be located, when not visible because of the clouds, with an Iceland Spar crystal. Some show experimental videos and conclude that their experiment proves that. I couldn't replicate the experiments. More precisely, I could replicate the experiment, but i couldn't see how it proved that the sun was actually located. 

Conclusions of my first experiments 

  • so far, no obvious way to tell which side is the sun ...
    • the sun is on the left or on the right, when "maximum" light polarization is detected
  • brainstorming for further tries [apparently, almost all nonsense : see "More information", lower]: 
    • let light enter from different faces of the crystal;
    • really go do the experiment where the Vikings were said to use those crystals for navigation : North Sea, high latitude, on a boat ;
    • use refraction ; shape differently the hole that lets the light in ; use 2 crystals ; 
    • use reflection on the inside of the faces of the crystal;
    • use crystals with various kind of  inhomogeneities, giving complementary information ;
      • for example, explore the changes in colors and patterns in some crystals ;
    • detect the polarization from under the surface of the water;
    • ignore light polarization and the "Raleigh sky model", find some other model... (because the Raleigh sky model seems to have a symmetry that wouldn't allow to decide which side is the sun); 

Videos of the my experiments :







More information

Not Calcite, but Cordierite, tourmaline, ... ?


"the Viking sunstones described in the old sagas could have been dichroic cordierite, andalusite and tourmaline or birefringent calcite (Iceland spar) crystals that could serve as linear polarization analysers."



"real calcite crystals also have disadvantages, and thus cordierite and tourmaline can also be at least as good sunstones."

Prerequisite step : calibrating and marking the crystal ?

[meaning not immediately clear for the lay person !]

"— Calibration step: In cloudless weather [...] rotate (adjust) the crystal until its well-determined orientation (e.g. minimal or maximal intensity of skylight transmitted through a dichroic sunstone, or minimal or maximal intensity difference between the two slots/spots of a birefringent sunstone), where it was fixed, and thereafter he calibrated the crystal by engraving the direction pointing towards the sun on the crystal surface. 

— Navigation step 1: Applying this sunstone rotational adjustment under a cloudy or foggy sky at two different celestial points, the navigator could determine the directions perpendicular to the local E-vectors of skylight shown by the engraved straight markings of the sunstones, pointing towards the sun.  [?? how many sunstones are needed ??]

— Navigation step 2: The intersection of the two celestial great circles crossing the sunstones parallel to their engravings gives the position of the invisible sun." [?? celestial circles crossing the sunstones ??]

Process for calibrating an Iceland spar (calcite)

"the calcite is rotated until the intensity difference between the two light spots is maximal. This occurs four times with 90° periodicity during a full 360° rotation of the crystal. [...] the Viking navigator [...] has to scratch only one straight mark (pointing towards the sun) onto the sunstone, and this sun mark can be used under all weather conditions to determine the position of the invisible sun."

You need two sun stones, not only one !?

[another description, where we see that 2 sunstones are involved. 
And we still have to find a proposal about how you perform that on a narrow crowded ship... maybe here, if you want to pay for access : https://www.osapublishing.org/ao/abstract.cfm?uri=ao-52-25-6185 ]

"Step 1 (Fig. 1A): Viking navigators are assumed to have determined the direction of skylight polarization in at least two celestial points with the use of two sunstones to estimate the position of the sun occluded by cloud/fog or being below the horizon. [...] 
Fig. 1A

Step 2 (Fig. 1B): A short scratch on each sunstone could help the navigator to set two celestial great circles across the two investigated sky points parallel to the scratches being perpendicular to the local direction of skylight polarization. Then the navigator determined the above-horizon intersection of these celestial circles. According to the Rayleigh theory of sky polarization [9], this intersection coincides with the position of the invisible sun.
Fig. 1B

Step 3 (Fig. 1C): [... hypothesis for finding the geographical north ...]  "
[better explained here, maybe, for who accepts to pay for science  : https://www.osapublishing.org/ao/abstract.cfm?uri=ao-52-25-6185 ]

Other hints to evaluate 

using 2 sunstones

Haidinger's brush : the easy / feasible method to test ?

"if you look through the crystal in its depolarizing position and then pull it away suddenly from your line of sight, you can catch a glimpse of a faint, elongate yellowish pattern known as a Haidinger's Brush. The key here is that the ends of that yellow shape point directly toward the sun."
[ The thing is not self-explanatory... there are two opposite "ends" in this elongate pattern... and they can't both point directly toward the sun... ]

The study from where this assertion comes from is here:
https://royalsocietypublishing.org/doi/10.1098/rspa.2011.0369
[i'll need to try harder to understand the paper, because at first look, i can't see why we should find the direction of the sun with the Haidinger's brush... which is not directional... we find in the study this beautiful illustration, that is not as convincing as it seems to be
if you manage to see that magnificent Haidinger's brush, you can still rotate 180° and do the experiment again, then... would you not assert that the sun is in the opposite direction ? ]

Navigation by lead and line, and other means

Was it even necessary to locate the sun, the north or any direction ?

Read here how navigation by lead and line was used to determine where the boat was
https://www.cambridge.org/core/journals/journal-of-navigation/article/early-navigation-in-the-north-sea-the-use-of-the-lead-and-line-and-other-navigation-methods/EDA8012AE267C583E8F2EA14EE36E145/core-reader
[ok, not doable when crossing or exploring unknown deep seas]
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