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Just an interesting footnote to military history:
from: Viking navigation: Sunstruck | The Economist
Viking navigation
Sunstruck
How Norsemen found their way round in cloudy weather
Nov 5th 2011 | from the print edition
Centuries before Columbus, Viking adventurers ruled the North Atlantic. They sailed as far as America without the aid of magnetic compasses, which was no mean feat. They were, however, assisted in their travels by another sort of magical device. According to the sagas they had stones which could point to the sun, even when the sky was cloudy.
No such sunstone has survived. But Guy Ropars of the University of Rennes, in France, thinks he knows what they were. He and his colleagues have been experimenting with a mineral called Iceland spar. Their results, just published in the Proceedings of the Royal Society, suggest they are on to something.
The passage of sunlight through the air polarises it. That means light from the sky itself points towards the sun, if you have the necessary equipment to detect the polarisation. Dr Ropars has shown that a piece of Iceland spar is sufficient.
Iceland spar is a form of calcite that splits light into two beams. If the light is polarised, there is only one way to orient the crystal to produce beams of equal intensity. Find this orientation by looking through the crystal at the sky at a time when you can see the sun, mark the suns direction on the crystal, and your mark will always point towards the sun when you match the beams from even a tiny patch of blue in an otherwise overcast sky. Dr Roparss experiments suggest the method is accurate to within 5°. That is good enough for navigation of the sort the Vikings managed.
Though no sunstones have survived from Viking days, despite the frequency of ship burials of Viking chiefs, there is one tantalising find from a more recent shipwreck. This is a large calcite crystal recovered from a vessel that went down off the coast of Alderney, in the Channel Islands, in Elizabethan times. Several centuries underwater have rendered the Alderney crystal opaque, but Dr Ropars and his team are now examining it, and believe it may be Iceland spar. Dr Ropars suspects it was being used as a sunstone because the magnetic compasses of the day were thrown out of kilter by iron cannon.
The true nature of the sunstone will probably not be settled until and unless one turns up in either a sunken Viking vessel or a ship burial. Perhaps, though, they not only permitted the Vikings to reach America, but also helped save England from the Spanish Armada.
from the print edition | Science and technology
from: Viking navigation: Sunstruck | The Economist
Viking navigation
Sunstruck
How Norsemen found their way round in cloudy weather
Nov 5th 2011 | from the print edition
Centuries before Columbus, Viking adventurers ruled the North Atlantic. They sailed as far as America without the aid of magnetic compasses, which was no mean feat. They were, however, assisted in their travels by another sort of magical device. According to the sagas they had stones which could point to the sun, even when the sky was cloudy.
No such sunstone has survived. But Guy Ropars of the University of Rennes, in France, thinks he knows what they were. He and his colleagues have been experimenting with a mineral called Iceland spar. Their results, just published in the Proceedings of the Royal Society, suggest they are on to something.
The passage of sunlight through the air polarises it. That means light from the sky itself points towards the sun, if you have the necessary equipment to detect the polarisation. Dr Ropars has shown that a piece of Iceland spar is sufficient.
Iceland spar is a form of calcite that splits light into two beams. If the light is polarised, there is only one way to orient the crystal to produce beams of equal intensity. Find this orientation by looking through the crystal at the sky at a time when you can see the sun, mark the suns direction on the crystal, and your mark will always point towards the sun when you match the beams from even a tiny patch of blue in an otherwise overcast sky. Dr Roparss experiments suggest the method is accurate to within 5°. That is good enough for navigation of the sort the Vikings managed.
Though no sunstones have survived from Viking days, despite the frequency of ship burials of Viking chiefs, there is one tantalising find from a more recent shipwreck. This is a large calcite crystal recovered from a vessel that went down off the coast of Alderney, in the Channel Islands, in Elizabethan times. Several centuries underwater have rendered the Alderney crystal opaque, but Dr Ropars and his team are now examining it, and believe it may be Iceland spar. Dr Ropars suspects it was being used as a sunstone because the magnetic compasses of the day were thrown out of kilter by iron cannon.
The true nature of the sunstone will probably not be settled until and unless one turns up in either a sunken Viking vessel or a ship burial. Perhaps, though, they not only permitted the Vikings to reach America, but also helped save England from the Spanish Armada.
from the print edition | Science and technology
