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How do we know the time? A clock, a watch or a cell 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 wof telling the time.
About 5,500 years ago, the Egyptians ithe sun clock. This was a tall stone structure. Its shadow (影子)showed the movement of the sthey were able to determine（测定）midday.
About 3,500 years ago, the Egyptians made a sundial. It was smaller than the sun clock and could measure (测量)the time for half a day. On cdays or at night it was impossible to tell the time with a sun clock or a sundial.
Water clocks were the fclocks not to use the sun. The idea is simple. Water flows（流动） from one container（容器）to another. When the water reaches a certain l it moves a lever (控制杆)and this 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 fto keep accurate（精确的）time.
In the 13th century, the mechanical clock was invented. This was more accurate, bwas expensive to make. Over the next few centuries it was developed. For example, springs(发条)were added around 1500. This iaccuracy and allowed clocks to be smaller.
In 1927, the first quartz clock was developed. Clocks became cheaper to build and own. People began depending on them more and more to run businesses, markets and so on.
More recently, in1956, came the digital clock. And nowadays satellites（人造卫星）send our cell phones the time to the exact second. There has been a lot of progress in timekeeping but some things never c. Man of us still have trouble getting out of bed on time and not missing appointments.
