How do we know the time? A clock, a watch or a mobile phone can help us. However, many years ago there were no clocks, so knowing the time was not so easy. Over the centuries, people have developed different ways of telling the time.
About 5,500 years ago, the Egyptians invented the sun clock. That was a tall stone building. Its shadow showed the movement(运动) of the sun. So people were able to know midday(正午). The Egyptians made a sundial (日晷) about 3,500 years ago. It was smaller than the sun clock and could let people know the time for half a day. But on cloudy days or at night, it was impossible to tell the time. Water clocks were the first clocks not to use the sun. The idea is simple. Water flows(流) from one bottle to another. When the water reaches a certain level, it shows the hours. The Egyptians used water clocks about 3,400 years ago. But these clocks failed to make people know the exact time.
In the 13th century, the mechanical(机械的) clock was invented in Europe. This was more exact, but it was expensive to make one. Over the next few centuries it was developed. For example, springs were added around 1500, this made people get the correct time and allowed clocks to be smaller. In 1927, the first quartz(石英) watch was developed. Clocks became cheaper to make. More recently, in 1956, came the digital(数字的) clock. And nowadays satellites(卫星) send our mobile phones the time to the exact second. Many people use mobile phones to know about the time instead of wearing a watch.
Clocks are always changing but some things never change. Many of us still have trouble getting out of bed on time and not being late for school or work.
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