Reading Passage 1
You should spend about 20 minutes on Questions 1–13, which are based on the passage below.
Keeping time
A For most of human history, there was no need to know the time to the minute. People rose with the sun, worked while it was light and rested when it grew dark. The first ‘clock’ was the sky itself: the movement of the sun across the day, and of the moon and stars across the year, told farmers when to plant and harvest and priests when to hold festivals.
B The earliest instruments for measuring time made use of shadows. Ancient Egyptians placed tall stone pillars, called obelisks, where the length and direction of their shadows marked the passing of the day, and later developed small portable shadow clocks. Sundials of many designs spread across the ancient world. Their obvious weakness was that they were useless at night or on cloudy days, and that the length of an ‘hour’ changed with the seasons, since daylight was divided into twelve equal parts whatever its length.
C To measure time when the sun was not shining, people turned to water. A water clock, or clepsydra, was a container from which water dripped slowly through a small hole; marks on the inside showed how much time had passed as the level fell. In ancient Athens, water clocks were used in law courts to limit the length of speeches. In China, the engineer Su Song built a giant water-driven astronomical clock tower in 1088, more than ten metres high, which rang bells and struck gongs to announce the hours.
D The first mechanical clocks appeared in Europe in the late thirteenth century, mostly in monasteries and church towers, where monks needed to know the times for prayer. They were driven by a falling weight, and their speed was controlled by a device called an escapement, which allowed the gears to move forward in small, regular steps. These clocks were large, expensive and not very accurate, often gaining or losing a quarter of an hour a day, and many had no face at all, simply ringing a bell on the hour.
E A great leap in accuracy came in 1656, when the Dutch scientist Christiaan Huygens built the first pendulum clock. Galileo had earlier noticed that a pendulum of a given length swings in almost exactly the same time, however wide its swing. By using a pendulum to regulate the escapement, Huygens reduced the error of the best clocks from about fifteen minutes to less than fifteen seconds a day. Minute hands, previously unnecessary, now began to appear on clock faces.
F Accurate clocks became a matter of life and death at sea. To calculate their longitude — their position east or west — sailors needed to know the exact time at their home port and compare it with local time on board. Pendulum clocks did not work on a rolling ship, and in 1714 the British government offered a large prize for a solution. It was eventually won by a self-taught carpenter, John Harrison, whose marine chronometer, completed in the 1760s, lost only a few seconds during a voyage across the Atlantic.
G Today the most accurate clocks use the vibrations of atoms. Atomic clocks are so precise that they would lose less than a second in millions of years, and they are essential to modern life: satellite navigation systems, mobile phone networks and the internet all depend on them. Yet in a sense we have come full circle. The official length of a second is no longer based on the turning of the Earth, which is slightly irregular, but on the behaviour of atoms of the element caesium.