雅思阅读 89: The Viking Who Navigated by Hidden Light（靠隐形光导航的维京人）

改编自 Royal Society Open Science / arXiv（2011-2018）。雅思阅读 Section 3 难度，约 1050 词。 素材来源：https://arago.elte.hu/sites/default/files/VikingPol-review_PTRSB.pdf

Reading Passage

A. The Viking longships crossed the North Atlantic in the ninth and tenth centuries, reaching Iceland, Greenland and, centuries before Columbus, the coast of Newfoundland. How they found their way across open water, with no compass, in latitudes where the sun could be hidden for days by fog and storm cloud, has long been a puzzle. Magnetic compasses did not reach Europe until centuries later. Dead reckoning from the sun and stars works in clear weather but fails completely when the sky is a uniform grey. Yet the sagas themselves drop a hint. An Icelandic saga describes a king, Olaf, testing a man named Sigurd by asking him to locate the sun from a clouded sky. Sigurd looked at the sky, then was handed a crystal, which he held up and "saw where the beam of light fell" — and named the sun's position correctly. The object is called, in the Old Norse, a sólarsteinn, a "sun stone". For almost a thousand years that passage was read as legend. In the late 1960s, a Danish archaeologist named Thorkild Ramskou proposed that it might instead be a description of a real navigational instrument.

B. Ramskou's idea rested on a subtlety of light that most people never notice. When sunlight scatters off molecules in the upper atmosphere, it becomes partially polarised — the waves of light oscillate in preferred directions rather than randomly. The pattern of polarisation forms a vast, invisible bow around the sun: directly toward or away from it, light is unpolarised; at ninety degrees from it, polarisation is strongest. Crucially, this polarisation pattern survives clouds. Cloud droplets reduce the degree of polarisation but do not erase it entirely, especially when the cloud layer is thin. A navigator who could sense the direction of polarised skylight at two points on the sky, and who knew the simple geometry connecting those points to the sun, could triangulate the sun's position even when it was below the horizon or hidden by fog. The human eye, unfortunately, cannot see polarisation directly. An instrument was needed — and Ramskou guessed that a piece of Iceland spar, a transparent calcite crystal, might serve.

C. Iceland spar is remarkable. It splits a ray of light into two, each ray polarised in a perpendicular direction, so that a line seen through the crystal appears double. When the crystal is rotated in front of a patch of sky, the brightness of the two images changes as the crystal's internal axes align with the polarisation of the skylight. At one specific orientation, the two images have equal brightness — a point that a trained navigator could find by eye within a few seconds. From that orientation, using a simple table of angles, the direction of the hidden sun could be read. Later refinements, proposed by French physicists in 2011, showed that the same crystal can act as a "depolariser": rotated until the two images match, it reveals the direction of polarisation with surprising precision even under cloudy skies. Experiments with three candidate crystals — calcite, cordierite and tourmaline — showed that all three worked in clear skies, that cordierite and tourmaline were more robust under thin cloud, but that pure calcite recovered best when polarisation was weak. The Vikings, who traded across the North Sea, had easy access to Iceland spar eroded out of Icelandic cliffs.

D. The archaeological evidence is suggestive rather than conclusive. In 2018, a block of transparent calcite was recovered from the wreck of a sixteenth-century English warship, the Alderney, which sank in the Channel in 1592 — long after the Viking age but long before the compass was in common use for small sailing ships. The crystal was found alongside navigational dividers, and its presence is consistent with the sailors on board using it for "sky compass" navigation when the magnetic needle became unreliable in high latitudes. Viking-age graves and settlements have, however, produced no confirmed sunstone, and the sagas mention the object only in passing. Critics of the theory argue that the technique, while physically possible, is too demanding — requiring hundreds of careful measurements per voyage — for it to have been used routinely. Even if a single skilled navigator could do it, they ask, how often would a ship's crew actually have done so in a storm? Supporters reply that the sagas describe exactly such a specialist navigator, and that the technique need only have worked often enough to keep a ship roughly on course.

E. Recent optical modelling has sharpened the question. A 2017 study in Royal Society Open Science calculated what fraction of the sky dome would, under various cloud conditions, show enough polarisation for a navigator to read. On a clear day, almost three quarters of the sky is usable. Under heavy overcast, the usable fraction falls toward zero. There is also a faint but real visual effect, called Haidinger's brushes, that the human eye can perceive under favourable conditions: a tiny yellow-blue bow at the point where the eye looks ninety degrees from the sun. The most sensitive observers, the study found, could in principle navigate by Haidinger's brushes alone on clear days, without any crystal at all. Whether the Vikings noticed that faint bow, or held up a block of calcite, or both, cannot be decided from existing evidence. Archaeologists continue to scan Viking shipwrecks and settlement layers for any fragment of Iceland spar that can be dated securely. What can be said is that the technique is physically real, the materials were available, and the sagas describe it in detail. A thousand years later, a grey sky over the North Atlantic still hides its secret in the same faint polarised pattern — waiting, if anyone still knows how to look, for a pair of trained eyes.

Questions 1-4

Choose the correct heading for paragraphs B, C, D and E from the list of headings below.

List of Headings i. The hidden pattern of polarised skylight ii. How the crystal actually works as a navigational tool iii. Archaeological evidence and its limits iv. What modern optics can and cannot decide v. The Viking colonisation of Newfoundland vi. The discovery of the magnetic compass in Europe vii. Why calcite is used in modern electronics

1. Paragraph B: ____
2. Paragraph C: ____
3. Paragraph D: ____
4. Paragraph E: ____

Questions 5-8

Choose the correct letter, A, B, C or D.

5. Why can the sunstone still work under cloudy skies? A. Clouds emit their own polarised light from below. B. The polarisation pattern of skylight survives clouds, though weakened. C. The crystal sees through clouds using infrared light. D. Vikings carried compasses hidden inside the crystal.

6. How does a navigator use Iceland spar to find the sun? A. By melting the crystal to reveal a hidden inscription. B. By rotating the crystal until the two split images are equally bright. C. By comparing the crystal's weight with a known reference. D. By listening to a faint hum from the crystal.

7. What does the Alderney shipwreck find suggest? A. Vikings sailed the Channel in the sixteenth century. B. Calcite was used for sky-compass navigation long after the Viking age. C. Calcite was mainly used as a building material. D. The wreck contained a magnetic compass dated to 900 AD.

8. What did the 2017 Royal Society Open Science study calculate? A. The exact number of sunstones used by Viking ships. B. The fraction of sky usable for polarisation navigation under various skies. C. The speed of longships under different wind conditions. D. The age of the Alderney wreck.

Questions 9-13

Do the following statements agree with the claims of the writer?

Write:

• TRUE if the statement agrees with the information
• FALSE if the statement contradicts the information
• NOT GIVEN if there is no information on this

9. The saga describes King Olaf testing a man named Sigurd about the sun's position.
10. Polarisation of skylight is strongest when looking directly toward the sun.
11. Cordierite and tourmaline performed better than calcite under thin cloud.
12. A confirmed sunstone has been recovered from a Viking-age grave.
13. The Vikings reached Newfoundland before Columbus.

Questions 14-15

Complete the summary below using NO MORE THAN TWO WORDS from the passage.

Iceland spar, a transparent form of (14) __________, splits light into two rays; when rotated so that the two images match in brightness, it reveals the direction of (15) __________ skylight.

答案与解析

← 上一篇 | 返回雅思焦点 | 下一篇 →