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航海

Crystal gazing
水晶球观海术

A mineral found in a shipwreck was an ancient form of compass
在沉船里发现的矿石是古代的一种指南针

THIS may look like a nondescript lump of rock, but it is, in fact, a sunstone.
很难形容这块石头的外观，其实它是一块日长石，至少法国法国雷恩大学的Guy Ropars和他的同事是这么看的。

That, at least, is the opinion of Guy Ropars of Rennes University, in France, and his colleagues.
磁罗盘出现之前，维京水手使用这种被视为神物的石头。

Sunstones are legendary items supposed to have been used by Viking sailors in the days before magnetic compasses.
据说即使阴天或者那个炽热的星球还没有高出地平线的时候，

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.
透过日长石观看天空，就能显示太阳的方向。

Dr Ropars thinks sunstones were real, and were actually crystals of Iceland spar, a form of calcite that polarises light.
博士Ropars认为日长石真的有这种作用，它实际上是冰洲石的晶体，是可以将光极化的一种方解石。

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.
来自天空的光线被极化，他在2011年发现，透过冰洲石能观察到偏振方向，太阳方向和这个方向的夹角小于5度。

Dr Ropars also believes the use of sunstones persisted until at least the 16th century.
Ropars博士还相信直到16世纪，人们还在使用日长石。

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.
教会档案中有使用日长石的记录，尽管磁罗盘已经出现，但有些原因当时的人们还不了解，比如接近船上铁铸加农炮，罗盘会失灵，所以大家认为它靠不住，就继续使用日长石。

He thinks this block of mineral is such a stone.
他认为这块矿石就有这些用途。

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.
1592年11月29日，一位海军上将报告说有一艘未命名的英国战舰在英吉利海峡里靠近奥尔德尼岛海岸的地方沉没，这块石头就是在失事船只里面发现的。

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.
四百年的水下岁月磨灭了它的透光性，但是它的形状和密度和冰洲石不差分毫。在冰洲石一米之内还发现了一副两脚规，用它可以在海图上测量距离。

Dr Ropars's latest research, just published in the Proceedings of the Royal Society, used spectroscopy to confirm the stone's composition as calcite.
Ropars博士的最新研究成果刚刚发表于《英国皇家学会学报》，他使用光谱学分析了这块石头的成分，确认这是一块方解石。

He also did further experiments with a recently mined Iceland-spar crystal of the same size.
他还用最近开采的冰洲石晶体做进一步的试验，试验用的石头和原来那块大小一样。

He and his colleagues found they could locate the direction of the sun even more accurately than before: to within 1°.
他和同事们发现用这块石头更能准确定位太阳的方向，偏差不到1°。

The Alderney crystal, as it is known, is thus almost certainly a sunstone.
几乎可以确定这块所谓的奥尔德尼水晶是日长石。

It didn't save the ship, though.
尽管它没能挽救这艘船的命运。