Orientation with a Viking sun-compass, a shadow-stick, and two calcite sunstones under various weather conditions

Abstract

It is widely accepted that Vikings used sun-compasses to derive true directions from the cast shadow of a gnomon. It has been hypothesized that when a cast shadow was not formed, Viking navigators relied on crude skylight polarimetry with the aid of dichroic or birefringent crystals, called "sunstones. " We demonstrate here that a simple tool, that we call "shadow-stick," could have allowed orientation by a sun-compass with satisfying accuracy when shadows were not formed, but the sun position could have reliably been estimated. In field tests, we performed orientation trials with a set composed of a sun-compass, two calcite sunstones, and a shadow-stick. We show here that such a set could have been an effective orientation tool for Vikings only when clear, blue patches of the sky were visible.

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

“…However, both the accuracy and the extra information provided by such measurements in the field were queried [5,10,13]. Such sunstones are more practical for estimating the position of the solar meridian at low solar elevations when the horizon is covered by fog or clouds, but the zenith is clear ( figure 6) [2,8,16,19].…”

“…An efficient method is covering all faces of this crystal in such a way that only slits perpendicular to the crystallographic c-axis of the calcite are left clear on the two greatest faces (see fig. 2 in Bernáth et al [8]). Partially linearly polarized light passing through the entrance slit is divided into totally linearly polarized ordinary and extraordinary rays that diverge while crossing the crystal, and that form two parallel images of the entrance slit on the exit face.…”

“…A pair of nonoptical quality but polished calcite rhombohedra (5 cm × 5 cm × 2.5 cm) were used as sunstones (see the electronic supplementary material, figure S2a, and see fig. 2 in Bernáth et al [8]). …”

“…(e) Estimation of the solar azimuth and elevation angles with sunstones Several atmospheric optical phenomena can be used to estimate the position of the occluded Sun, but the Vikings have been hypothesized to rely on the polarization pattern of the sky [1,2,4,5,[8][9][10][11]. Skylight is formed predominantly by first-order Rayleigh scattering of sunlight on atmospheric molecules and aerosols, and is partially linearly polarized (figure 6).…”

“…It has also been hypothesized that this hole might have housed a lost central part with a pin-like gnomon and a complement of the gnomonic lines [1]. The artefact can also be associated with different navigation procedures [7,8].…”