What does a sundial measure according to the passage?

Humans have used may devices to measure time. The sundial was one of the earliest and simplest. A sundial measures the movement of the sun across the sky each day.
    The sundial worked well only when the sun was shining. So other ways of measuring the passing of time were invented. One device was the hourglass. It used a thin stram of falling sand to measure time.
    By the 1700s, people had 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 needed 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 observatory in Greenwich, England, was chosen as the starting point for the time zone.
    Most people have no trouble agreeing that time moves forward. Some scientists believe there is one reason why time only moves forward. It is a will-known scientific law: the second law of thermodynamics. The law says disorder increases with time.