Sunstone (medieval)
Navigation aids
However, the orientation only by solar position and no exact time has reached its limits in the polar summer, when the sun is always visible. While the astronomical twilight is an orientation to the stars largely quite possible (not in the polar summer ), but because knowledge of astronomy and the calendar date and the determination of the nocturnal time are crucial much more difficult here also.
There are several approaches to assign this story a real mineral. Two theories of crystalline material and operation of a sun stone face each other:
Use of polarization filters
One interpretation is that it is the sun stone concerned a crystal, which acts as a polarizing filter.
Polarization pattern
The sun is a light source which produces unpolarized light. Rayleigh scattering occurs due to the polarized light in the atmosphere. This applies to an observer at a right angle to the incident sunlight, from a very wide semicircle ( a part of the circle segment is below the horizon ) around the Sun.
Perception as Haidinger Clump
The human eye can not differentiate in direct consideration different levels of vibration of polarized light.
Looks an observer but a few seconds to a rigid polarization pattern and then changes the position of the head a little, without changing its direction of view can be perceived with some practice a diffuse yellow or blue appearance.
Detection of the polarization pattern by polarization filter
Looking through a polarizing filter, polarized light by rotating be identified on the basis of reduced light transmission and thus allows to detect a darker large semi-circle around the sun. Convincing representations of the polarization pattern of a semicircle are expensive because they are overlaid by the direct incident sunlight and other scattering. Already partially cloud-free sky games could allow to perceive the polarization pattern and thus to determine the position of the sun. This method would encounter in dense fog to their limits.
Natural polarization filter
Cordierite, the Thorkild Ramskou designated as the sólsteinn, offers the possibility to detect the sky polarization pattern, as long as a larger uncovered blue sky area is, if possible at its zenith, while the ( low-lying ) sun can be covered.
Current state of knowledge
When uncovered sky the trained observer can detect a sky polarization pattern even without aid during sunrise or sunset, but opens up the no additional useful information for navigation. It has, however, to practice the complicated use of a polarization filter for this observation.
Some insects can use the polarization pattern of the partially covered the daytime sky of the sun determine (eg bees, according to studies by Karl von Frisch).
Terms of birefringence
Due to the refraction of light by a birefringent crystal such as calcite usually two different strong beam of sunlight to be visible while looking through. By twisting and turning the stone can be oriented so that the two beams are equal in strength. The viewing direction is then exactly in the direction of the light source, ie the sun. To the effect also contributes to the polarization of the sunlight.
A possible mineral is calcite, a frequently occurring in Scandinavia stone. Such a stone was found in a salvaged before the Channel Island of Alderney wreck of an Elizabethan warship that was sunk in 1592. That the sun stone still in the 16th century - and centuries after the invention of the compass - was carried on board a ship, the scientists explained by the fact that little was known about the functioning of the compass at that time.
Assessment
Position determination of the sun at completely overcast skies and light snow flurries with the help of a natural sólsteinn as in the lore is not possible with the aid of a polarizing filter or a birefringent mineral, but probably a theatricality of the word. But both methods could be used in case of partial cloud cover and light, cloudy weather, the polarizing filters even when the sun stands just below the horizon.