- 2017年12月15日13:23 来源：互联网
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A mineral found in a shipwreck was an ancient form of compass
THIS may look like a nondescript lump of rock, but it is, in fact, a sunstone.
That, at least, is the opinion of Guy Ropars of Rennes University, in France, and his colleagues.
Sunstones are legendary items supposed to have been used by Viking sailors in the days before magnetic compasses.
Looking at the sky through one, it is said, would reveal the sun's direction even on a cloudy day or when that fiery orb was below the horizon.
Dr Ropars thinks sunstones were real, and were actually crystals of Iceland spar, a form of calcite that polarises light.
Light from the sky is polarised and, as he discovered in 2011, looking through a piece of Iceland spar reveals the direction of polarisation, and thus the direction of the sun, to within 5.
Dr Ropars also believes the use of sunstones persisted until at least the 16th century.
Their existence is mentioned in church records, and they would have been useful because although magnetic compasses were known by then, they were unreliable for reasons not then understood, such as proximity to the large amounts of iron in ships' cannons.
He thinks this block of mineral is such a stone.
It was found in a wreck believed to be that of an unnamed English warship which an admiral's report from November 29th 1592 says was lost off the coast of Alderney, in the English Channel.
Four centuries underwater have rendered it opaque, but it is the right shape and density to be Iceland spar, and it was discovered within a metre of a pair of navigational dividers of the sort used to measure distances on charts.
Dr Ropars's latest research, just published in the Proceedings of the Royal Society, used spectroscopy to confirm the stone's composition as calcite.
He also did further experiments with a recently mined Iceland-spar crystal of the same size.
He and his colleagues found they could locate the direction of the sun even more accurately than before: to within 1°.
The Alderney crystal, as it is known, is thus almost certainly a sunstone.
It didn't save the ship, though.