The perception of time most likely developed through the observation of natural phenomena. Early humans survived without quantifying time. As behavior became more complex, increasingly advanced societies required timekeeping devices. The history of clocks may be thought of as a chronicle of human progress. Horologists have responded to specific technical and social problems by creating new methods of measuring time. Even in the modern era, technological needs determine clock accuracy.
For the earliest humans, time may have been related to the movement of the sun. Repetitive phenomena such as the sunrise and sunset could have led to a conception of time span. Eventually, primitive humans began to estimate the day's length. This period of light and darkness is known as the "natural day," or solar day. Obviously, the length of a solar day varies depending on the season. Early humans may have gained an appreciation for longer time intervals by watching the moon. A person keeping track of the waxing and waning moon would note that the seasons repeated themselves approximately every twelfth full moon. Such observations may eventually have led to delineations of the following time intervals: e.g., the solar day, the lunar month, and either the solar or astronomical year.
Two of the earliest civilizations to emerge included the Babylonians and the Egyptians. These cultures' religion was oriented towards sun worship. The Babylonians adopted the solar day and, like the year, divided it into twelve parts. According to Herodotus, these hours were passed on to Greece. In one of the earliest texts on the history of science, Herodotus discusses the origins of sun-dials, the 12-hour day, and land surveying. His passage proceeds as follows: "From this [the need to re-measure land in Egypt each year because of the Nile inundation] to my thinking, the Greeks learnt the art of measuring land; the polos and the gnomon, and th...