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Journal of the Optical Society of America A

Journal of the Optical Society of America A

| OPTICS, IMAGE SCIENCE, AND VISION

  • Editor: Franco Gori
  • Vol. 31, Iss. 7 — Jul. 1, 2014
  • pp: 1645–1656

Accuracy of sun localization in the second step of sky-polarimetric Viking navigation for north determination: a planetarium experiment

Alexandra Farkas, Dénes Száz, Ádám Egri, Miklós Blahó, András Barta, Dóra Nehéz, Balázs Bernáth, and Gábor Horváth  »View Author Affiliations


JOSA A, Vol. 31, Issue 7, pp. 1645-1656 (2014)
http://dx.doi.org/10.1364/JOSAA.31.001645


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Abstract

It is a widely discussed hypothesis that Viking seafarers might have been able to locate the position of the occluded sun by means of dichroic or birefringent crystals, the mysterious sunstones, with which they could analyze skylight polarization. Although the atmospheric optical prerequisites and certain aspects of the efficiency of this sky-polarimetric Viking navigation have been investigated, the accuracy of the main steps of this method has not been quantitatively examined. To fill in this gap, we present here the results of a planetarium experiment in which we measured the azimuth and elevation errors of localization of the invisible sun. In the planetarium sun localization was performed in two selected celestial points on the basis of the alignments of two small sections of two celestial great circles passing through the sun. In the second step of sky-polarimetric Viking navigation the navigator needed to determine the intersection of two such celestial circles. We found that the position of the sun (solar elevation θS, solar azimuth φS) was estimated with an average error of +0.6°Δθ+8.8° and 3.9°Δφ+2.0°. We also calculated the compass direction error when the estimated sun position is used for orienting with a Viking sun-compass. The northern direction (ωNorth) was determined with an error of 3.34°ΔωNorth+6.29°. The inaccuracy of the second step of this navigation method was high (ΔωNorth=16.3°) when the solar elevation was 5°θS25°, and the two selected celestial points were far from the sun (at angular distances 95°γ1, γ2115°) and each other (125°δ145°). Considering only this second step, the sky-polarimetric navigation could be more accurate in the mid-summer period (June and July), when in the daytime the sun is high above the horizon for long periods. In the spring (and autumn) equinoctial period, alternative methods (using a twilight board, for example) might be more appropriate. Since Viking navigators surely also committed further errors in the first and third steps, the orientation errors presented here underestimate the net error of the whole sky-polarimetric navigation.

© 2014 Optical Society of America

OCIS Codes
(000.4930) General : Other topics of general interest
(010.1290) Atmospheric and oceanic optics : Atmospheric optics
(260.1180) Physical optics : Crystal optics
(260.5430) Physical optics : Polarization
(330.5510) Vision, color, and visual optics : Psychophysics
(330.7321) Vision, color, and visual optics : Vision coupled optical systems

ToC Category:
Vision, Color, and Visual Optics

History
Original Manuscript: April 17, 2014
Manuscript Accepted: May 13, 2014
Published: June 30, 2014

Citation
Alexandra Farkas, Dénes Száz, Ádám Egri, Miklós Blahó, András Barta, Dóra Nehéz, Balázs Bernáth, and Gábor Horváth, "Accuracy of sun localization in the second step of sky-polarimetric Viking navigation for north determination: a planetarium experiment," J. Opt. Soc. Am. A 31, 1645-1656 (2014)
http://www.opticsinfobase.org/josaa/abstract.cfm?URI=josaa-31-7-1645

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