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 and 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. This was a tall stone building. Its shadow (影子) showed the movement of the sun. So people were able to know midday. The Egyptians made a sundial about 3,500 years ago. It was smaller than the sun clock and could let people know the time for half a day. On cloudy days or at night it was impossible to tell the time with a sun clock or a sundial. Water clocks were the first clocks not to use the sun. The idea is simple. Water flows (流动) 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. These clocks were popular in the Middle East and China. But they failed to make people know the exact time.
In the 13th century, the mechanical clock was invented. This was more exact, but it was expensive to make one. Over the next few centuries it was developed. For example, springs (发条) were added around 1500. This enabled people to get the correct time and allowed clocks to be smaller. In 1927, the first quartz clock was developed. Clocks became cheaper to build. It's also cheaper for ordinary people to own a clock. People began depending on them more and more to run businesses, markets and so on. More recently, in 1956, came the digital clock. And nowadays satellites send our mobile phones the time to the exact second.
There has been a lot of progress in timekeeping. 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.
