10.22 考研英语源外刊模拟试题【难度6.0】

The leap second, an adjustment that many in the computing world regard as a nuisance, is to be replaced by a leap hour. Delegates from 69 countries approved the change at a meeting in France of the organisation that keeps the world’s standards of time, weight and distance, and it is due to start next year. The aim is to spare computers and other systems the risk of choking on minutes that run a second too long, or even a second too short.

The familiar leap year, devised more than 2,000 years ago, stops the calendar from drifting away from the Earth’s yearly orbit of the sun. The leap second is a far more modern invention. Since the arrival of atomic clocks, the second has been defined by the vibrations of a caesium atom rather than by the Earth’s rotation. Such clocks are remarkably steady, and modern computer networks, satellites, financial trading and power grids rely on their timekeeping. The Earth’s spin is much less dependable, since it is disturbed by the planet’s molten core, the movement of the oceans and the winds. Overall the planet is turning more slowly, and since 1972 officials have inserted 27 leap seconds to hold standard time close to astronomical time.

This worked for over half a century. Then scientists found fresh grounds for worry: during the last ten years the Earth has begun to spin faster, so the authorities could soon need their first negative leap second, which takes a second away from the clock. The estimated 30 per cent chance of that within a decade brought the vote forward.
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26. According to Paragraph 2, leap seconds have been added since 1972 because
A. atomic clocks gradually drift away from true time
B. the calendar year is not a whole number of days
C. the Earth’s rotation is irregular and generally slowing
D. computer systems need regular corrections to stay accurate

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