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.
abstract: Vikings routinely crossed the North Atlantic without a magnetic 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 sagas mentioning 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° when the artificially occluded Sun had elevation angles between +10° and -8° 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.
authors.affiliation: Environmental Optics Laboratory, Department of Biological Physics , Physical Institute, Eötvös University , Pázmány sétány 1, 1117 Budapest, Hungary.
authors.forename: Balázs
authors.initials: B
authors.lastname: Bernáth
authors.affiliation: Environmental Optics Laboratory, Department of Biological Physics , Physical Institute, Eötvös University , Pázmány sétány 1, 1117 Budapest, Hungary.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Farkas
authors.affiliation: Environmental Optics Laboratory, Department of Biological Physics , Physical Institute, Eötvös University , Pázmány sétány 1, 1117 Budapest, Hungary.
authors.forename: Dénes
authors.initials: D
authors.lastname: Száz
authors.affiliation: Environmental Optics Laboratory, Department of Biological Physics , Physical Institute, Eötvös University , Pázmány sétány 1, 1117 Budapest, Hungary.
authors.forename: Miklós
authors.initials: M
authors.lastname: Blahó
authors.affiliation: Environmental Optics Laboratory, Department of Biological Physics , Physical Institute, Eötvös University , Pázmány sétány 1, 1117 Budapest, Hungary ; Estrato Research and Development Ltd, Mártonlak utca 13, 1121 Budapest, Hungary.
authors.forename: Adám
authors.initials: A
authors.lastname: Egri
authors.affiliation: Environmental Optics Laboratory, Department of Biological Physics , Physical Institute, Eötvös University , Pázmány sétány 1, 1117 Budapest, Hungary ; Estrato Research and Development Ltd, Mártonlak utca 13, 1121 Budapest, Hungary.
authors.forename: András
authors.initials: A
authors.lastname: Barta
authors.affiliation: Department of Biology, Centre for Animal Movement Research , Lund University , Ecology Building, 223 62 Lund, Sweden.
authors.forename: Susanne
authors.initials: S
authors.lastname: Akesson
authors.affiliation: Environmental Optics Laboratory, Department of Biological Physics , Physical Institute, Eötvös University , Pázmány sétány 1, 1117 Budapest, Hungary.
authors.forename: Gábor
authors.initials: G
authors.lastname: Horváth
keywords: Viking navigation; horizon board; sky polarization; skylight compass; sunstone; twilight board