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- Bernath, Balazs, et al.
(författare)
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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
- 2014
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Ingår i: Royal Society of London. Proceedings A. Mathematical, Physical and Engineering Sciences. - : The Royal Society. - 1364-5021. ; 470:2166
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Tidskriftsartikel (refereegranskat)abstract
- Vikings routinely crossed the North Atlantic without amagnetic compass and left their mark on lands as far away as Greenland, Newfoundland and Baffin Island. Based on an eleventh-century dial fragment artefact, found at Uunartoq in Greenland, it is widely accepted that they sailed along chosen latitudes using primitive Sun compasses. Such instruments were tested on sea and proved to be efficient hand-held navigation tools, but the dimensions and incisions of the Uunartoq find are far from optimal in this role. On the basis of the sagasmentioning sunstones, incompatible hypotheses were formed for Viking solar navigation procedures and primitive skylight polarimetry with dichroic or birefringent crystals. We describe here a previously unconceived method of navigation based on the Uunartoq artefact functioning as a 'twilight board', which is a combination of a horizon board and a Sun compass optimized for use when the Sun is close to the horizon. We deduced an appropriate solar navigation procedure using a twilight board, a shadow-stick and birefringent crystals, which bring together earlier suggested methods in harmony and provide a true skylight compass function. This could have allowed Vikings to navigate around the clock, to use the artefact dial as a Sun compass during long parts of the day and to use skylight polarization patterns in the twilight period. In field tests, we found that true north could be appointed with such a medieval skylight compass with an error of about +/- 4 degrees when the artificially occluded Sun had elevation angles between +10 degrees and -8 degrees relative to the horizon. Our interpretation allows us to assign exact dates to the gnomonic lines on the artefact and outlines the schedule of the merchant ships that sustained the Viking colony in Greenland a millennium ago.
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| 2. |
- Horvath, Gabor, et al.
(författare)
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Imaging polarimetry of the fogbow: polarization characteristics of white rainbows measured in the high Arctic
- 2011
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Ingår i: Applied Optics. - 2155-3165. ; 50:28, s. 64-71
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Tidskriftsartikel (refereegranskat)abstract
- The knowledge on the optics of fogbows is scarce, and their polarization characteristics have never been measured to our knowledge. To fill this gap we measured the polarization features of 16 fogbows during the Beringia 2005 Arctic polar research expedition by imaging polarimetry in the red, green and blue spectral ranges. We present here the first polarization patterns of the fogbow. In the patterns of the degree of linear polarization alpha fogbows and their supernumerary bows are best visible in the red spectral range due to the least dilution of fogbow light by light scattered in air. In the patterns of the angle of polarization alpha fogbows are practically not discernible because their alpha-pattern is the same as that of the sky: the direction of polarization is perpendicular to the plane of scattering and is parallel to the arc of the bow, independently of the wavelength. Fogbows and their supernumeraries were best seen in the patterns of the polarized radiance. In these patterns the angular distance delta between the peaks of the primary and the first supernumerary and the angular width sigma of the primary bow were determined along different radii from the center of the bow. delta ranged between 6.08 degrees and 13.41 degrees, while sigma changed from 5.25 degrees to 19.47 degrees. Certain fogbows were relatively homogeneous, meaning small variations of delta and sigma along their bows. Other fogbows were heterogeneous, possessing quite variable delta- and sigma-values along their bows. This variability could be a consequence of the characteristics of the high Arctic with open waters within the ice shield resulting in the spatiotemporal change of the droplet size within the fog. (C) 2011 Optical Society of America
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