Sundials were the first “clocks”, The obelisk throws his work on the ground. The arc of a circle, which the top of the obelisk draws on the ground from sunrise to sunset, was divided. This division indicated the hours.
The shadow, that the sun throws during its daily routine, became the first sundial. From the time the sun rises until it sinks, the shadow of a tree continuously changes its length and direction. Then you could determine the times of day. The first sundials were still simple. They consisted of a stick, which one stuck vertically in the earth, or a tall stone or an obelisk. Later the sundial was improved considerably by Arab mathematicians.