Archive for category: IELTS Reading Academic

B. It was the Ingest·distance repair job in history, and a triumph for the NASA engineers. But it also highlighted the astonishing power of the techniques developed by American communications engineer Claude Shannon, who had died just a year earlier. Born in 1916 in Petoskey, Michigan. Shannon showed an early talent for maths and for building gadgets, and made breakthroughs in the foundations of computer technology when still a student. While at Bell laboratories, Shannon developed information theory, but _Q29 shunned the resulting acclaim. In the 1940s. he single-handedly created an entire science of communication which has since inveigled its way into a host of applications, from DVDs to satellite communication to bar codes – any area, in short, where data has to be conveyed rapidly yet accurately.

C. This all seems light years away from the down-to-earth uses Shannon originally had for his work, which began when he was a 22-year—old graduate engineering student at the prestigious Massachusetts Institute of Technology in 1939. _Q32 He set out with an apparently simple aim: to pin down the precise meaning of the concept of ‘information’. The most basic form of information, Shannon argued, is whether something is true or false – which can be captured in the binary unit, or ‘bit’, of the form 1 or 0. _Q38 Having identified this fundamental unit, Shannon set about defining otherwise vague ideas about information and how to transmit it from place to place. ln the process he discovered something surprising: it is always possible to guarantee information will gel through random interference – ‘noise’ — intact.

D. Noise usually means unwanted sounds which interfere with genuine information. information theory generalizes this idea via theorems that capture the effects of noise with mathematical precision. In particular, Shannon showed that _Q27 noise sets a limit on the rate at which information can pass along communication channels while remaining error-free. _Q39 This rate depends on the relative strengths of the signal and noise travelling down the communication channel, and on its capacity (its’ bandwidth’). The resulting limit, given in units of bits per second, is the absolute maximum rate of error-free communication given signal strength and noise level. The trick, Shannon showed, is to find ways of packaging up – ‘coding’ – information to cope with the ravages of noise, while staying within the information carrying capacity ‘bandwidth’ – of the communication system being used.

E. Over the years scientists have devised many such coding methods, and they have proved crucial in many technological feats. The Voyager spacecraft transmitted data using codes which added one extra bit for every single bit of information; the result was an error rate of just one bit in 10,000 — and stunningly clear pictures of the planets. Other codes have become parts of everyday life – such as the Universal Product Code, or bar code, which _Q30 uses a simple error-detecting system that ensures supermarket check-out lasers can read the price even on. say, a crumpled bag of crisps. _Q40 As recently as 1993, engineers made a major breakthrough by discovering so-called turbo codes – which come very close to Shannon’s ultimate limit for the maximum rate that data can be transmitted reliably, and now play a key role in the mobile videophone revolution.

F. Shannon also laid the foundations of more efficient ways of storing information, _Q28 by stripping out superfluous (‘redundant’) bits from data which contributed little real information. As mobile phone text messages like ‘l CN C U’ show, it is often possible to leave out a lot of data without losing much meaning, As with error correction, however, there’s a limit beyond which messages become too ambiguous. Shannon showed how to calculate this limit, opening the way to the design of compression methods that cram maximum information into the minimum space.

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B. For centuries, transits of Venus have drawn explorers and astronomers alike to the four corners of the globe. And you can put it all down to the extraordinary polymath Edmond Halley. In November 1677, Halley observed a transit of the innermost planet Mercury, from the desolate island of St Helena in the South Pacific. He realized that from different latitudes, the passage of the planet across the Sun’s disc would appear to differ. By timing the transit from two widely-separated locations, teams of astronomers could calculate the parallax angle – _Q19 the apparent difference in position of an astronomical body due to a difference in the observer’s position. Calculating this angle would allow astronomers to measure what was then the ultimate goal; the distance of the Earth from the Sun. This distance is known as the ‘astronomical unit` or AU.

C. Halley was aware that the AU was one of the most fundamental of all astronomical measurements. Johannes Kepler, in the early 17th century, had shown that _Q20 the distances of the planets from the Sun governed their orbital speeds, which were easily measurable. But no-one had found a way to calculate accurate distances to the planets from the Earth. The goal was to measure the AU; then, knowing the orbital speeds of all the other planets round the Sun, the scale of the Solar System would fall into place. However, Halley realized that Mercury was so far away that its parallax angle would be very difficult to determine. As Venus was closer to the Earth, its parallax angle would be larger and Halley worked out that by using Venus it would be possible to measure the Sun`s distance to 1 part in 500. But there was as problem: transits of Venus, unlike those of Mercury; are rare. occurring in pairs roughly eight years apart every hundred or so years. Nevertheless, _Q22 he accurately predicted that Venus would cross the face of the Sun in both 1761 and 1769 – though he didn’t survive to see either.

D. Inspired by Halley’s suggestion of a way to pin down the scale of the Solar System, teams of British and French astronomers set out on expeditions to places as diverse as India and Siberia. But things weren’t helped by Britain and France being at war. The person who deserves most sympathy is the French astronomer Guillaume Le Gentil.

He was thwarted by the fact that the British were besieging his observation site at Pondicherry in India. _Q15 Fleeing on a French warship crossing the Indian Ocean, Le Gentil saw a wonderful transit – but the ship`s pitching and _Q21 rolling ruled out any attempt at making accurate observations. Undaunted, he remained south of the equator, keeping himself busy by studying the islands of Mauritius and Madagascar before setting off to observe the next transit in the Philippines. Ironically after travelling nearly 50,000 kilometres, _Q23 his view was clouded out at the last moment, a very dispiriting experience.

E. While the early transit timings were as precise as instruments would allow the measurements were dogged by the ‘black drop’ effect. _Q24 When Venus begins to cross the Sun’s disc, it looks smeared not circular – which makes it difficult to establish timings. This is due to diffraction of light. The second problem is that Venus exhibits a halo of light when it is seen just outside the Sun’s disc. _Q17 While this showed astronomers that Venus was surrounded by a thick layer of gases refracting sunlight around it, both effects made it impossible to obtain accurate timings.

F. But astronomers labored hard to analyze the results of these expeditions to observe _Q18 Venus transits. Johann Franz Encke, Director of the Berlin Observatory, finally determined a value for the AU based on all these parallax measurements: 153340,000 km. Reasonably accurate for the time, that is quite close to today’s value of 149,597,870 km, determined by radar, which has now superseded transits and all other methods in accuracy. The AU is a cosmic measuring rod, and the basis of how we scale the Universe today _Q14 The parallax principle can be extended to measure the _Q26 distances to the stars. If we look at a star in January – when Earth is at one point in its orbit – it will seem to be in a different position from where it appears six months later. Knowing the width of Earth`s orbit, the parallax shift lets astronomers calculate the distance.

G. June 2004’s transit of Venus was thus more of an astronomical spectacle than a scientifically important event. _Q16 But such transits have paved the way for what might prove to be one of the most vital breakthroughs in the cosmos – detecting Earth-sized planets orbiting other stars.

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