The first thing man measured was time. His first unit of time was the day. The regularly of sunrise and sunset made him count the days.
He probably kept a record of the passing days by making marks on a tree-trunk. He may also have noticed how the shape of the moon gradually changed from day to day. And cut a special mark on the tree for the full moon. Eventually he would have noticed that the full moon seemed to occur at regular intervals –always after he had cut about thirty day-marks.
How did man learn to divide the day into smaller units of time?
The Egyptians, and later the Greeks and the Romans, had a water clock. In its simplest form it consisted of a vessel like a modern bucket from which water slowly dripped through a hole. As the water level dropped, the time was measured by marks on the inside of the vessel.
Arab astronomers invented the sundial, on which they made hour markings. The changing direction of the shadow indicated the time. A sundial, however, could not be used when it was cloudy.
Another was the hourglass, or sand –glass, which was commonly used in churches.
Later on, mechanical clocks started being used. These clocks did not have hands or a face but rang an alarm every hour. They were put up in public places. Smaller ones were made for homes. Pendulum clocks and cuckoo clocks were also early kinds of mechanical clocks. But clocks and wristwatches were made much later.
Mummy and papa often set an alarm in the clock so that they can wake you up in time to get ready for school. The sound of the alarm ringing in the morning wakes people up. This clock is known as an alarm clock. The alarm sounds at whatever time you want it to. This is the inside of an alarm clock. Does not it look complicated?
Today, most clocks or watches run on batteries or electrical cells. They use the natural vibration in a quartz crystal to keep time. The modern atomic clock is the most accurate known to man.