Step by step
The heart of a clock is the escapement, which releases energy from a weight or a spring in small, equal portions. Gears then move the hands.
Accuracy
A pendulum swings with an almost constant period, and that made clocks more accurate. For navigation at sea, John Harrison built precise marine chronometers in the 18th century. Today's atomic clocks drift by one second in millions of years, and without them GPS would not work.
The Prague astronomical clock
The Prague astronomical clock (the Orloj), dating from 1410, is one of the oldest working astronomical clocks in the world. It shows the time, the positions of the Sun and the Moon, and a calendar.