Excerpt:
Before we continue describing the evolution of ways to mark the passage of time, perhaps we should broadly define what constitutes a clock. All clocks must have two basic components:
a regular, constant or repetitive process or action to mark off equal increments of time. Early examples of such processes included the movement of the sun across the sky, candles marked in increments, oil lamps with marked reservoirs, sand glasses (hourglasses), and in the Orient, knotted cords and small stone or metal mazes filled with incense that would burn at a certain pace. Modern clocks use a balance wheel, pendulum, vibrating crystal, or electromagnetic waves associated with the internal workings of atoms as their regulators.
a means of keeping track of the increments of time and displaying the result. Our ways of keeping track of the passage of time include the position of clock hands and digital time displays.
The history of timekeeping is the story of the search for ever more consistent actions or processes to regulate the rate of a clock.