A、It measures the length of a day and a night. B、it measures the movement of the sun each day. C、It measures the passing of hour

At present, human beings have used many devices to measure time. Perhaps, the sundial was, one of the earliest and simplest devices. A sundial can measure the movement of the sun across the sky each day.
    The sundial worked very well only when the sun was shining. So other ways of measuring the passing of time were invented at that time. One device was the hourglass. It used a thin stream of falling sand to measure time.
    By the 1700s, people had already developed mechanical clocks and watches. And today many of our clocks and watches are electronic.
    So we have devices to mark the passing of time. But what time is it now? Clocks in different parts of the world do not show the same time at the same time. As international communications and travel grew, it became clear we need a way to establish a common time for all parts of the world.
    In 1884, an international conference divided the world into 24 time areas or zones. Each zone represents one hour. The astronomical observation in Greenwich, England, was chosen as the starting point for the time zone.
    Most people have no trouble in agreeing that time moves forward. Some scientists believe there is one reason why time moves forward. It is clear we need a way to establish a common time for all parts of the world with scientific law: the second law of thermodynamics. The law says disorder increases with time.