Timekeeping

Mankind first developed a sense of time from observations of nature. For the short term, he observed the movement of heavenly bodies—the sun and moon held particular importance, heralding the seasons and the months. For the long term, birth and death events—of themselves and their livestock—marked the passage of time. One of the earliest inventions was the astrolabe, which astronomers used to track stars and planets. The first such instrument may have been made in the second century by the greatest astronomer of ancient times, the Greek Hipparchus, and was brought to perfection by the Arabs. Early artificial means of timekeeping to provide an estimate of the hour were all analogue in nature, whether passive, like the sundial, or dynamic, like the sandglass or water-clock, which measured time by rate of flow. The sundial probably began with a stick thrust into the ground; the position of its shadow corresponded to the hour of the day. Very elaborate sundials have been constructed which compensate for the sun’s relative position during the year, and ingenious pocket versions have been popular from time to time. However, all such instruments are worthless if the day is cloudy, and after the sun goes down. Therefore, particularly for stargazers, independent means of time estimation were important. The simple sandglass, in which sand is made to run through a small opening, was adequate only for short durations. However, running water can power a mechanism indefinitely. The greatest water-clock ever made was a building-sized astronomical device, constructed by Su Sung in China in 1094, to simulate the movements of sun, moon and the principal stars. Chinese philosophers thought that because water flows with perfect evenness, it is the best basis for timekeeping. However, in this belief they were wrong; the secret of accurate timekeeping is to generate and count regular beats, which is a digital rather than an analogue process. This may be surprising, and not just to the ancients in China, because except for intra-atomic events time can be regarded as a continuous phenomenon.