When people began to measure time for themselves they turned to other natural processes already familiar to them from their daily lives. These were processes which took time to happen and therefore could provide them with a measure of time. Simple things like the flow of water, the trickle of sand, the gradual lengthening of a shadow all suggested ways of measuring time. 

Ultimately the motive power which lay behind all these ways of measuring time was gravity. The gravitational constant which governed the movement of the planets on a cosmic scale also powered the water clock, the sand glass and, later, mechanical weight driven pendulum clocks.  It also of course guaranteed the regularity of the relative motions of the Sun, the Moon and the Earth, and this enabled the development of a wide range of instruments that exploited those motions by measuring the varying length and position of the shadows which they cast. 

Unlike the 'natural' cycles of day and night, or the seasons, these measuring devices could be calibrated to provide an abstract measure of time such as the hour. This development marked a crucial stage in the ever increasing ability of human beings to exercise control over the world around them rather than simply adapting and responding to it.