Mankind was always conscious of the fact that time moves on, but for centuries it was difficult to measure the rate at which it did so.
At first the day was divided into three parts - morning, afternoon and night - each bounded by two of the three phenomena, sunset, dawn and noon. Dawn and sunset were impossible to miss, but noon was more difficult; it was the moment of the shortest shadows of the day, or for the ancient Romans, the arrival time of the sun between the Rostra and a spot called Graeco-sta- sis, the place where the Greek ambassador used to stand.
The origins of the 24-hour day, and the reasons for the choice of this particular number, are lost in the mists of time. We know, however, that after the number 24 became established, all the hours were not of equal length.
The two daily periods of light and darkness were each divided into 12 equal parts called temporal hours, with the inconvenient consequence that the length of an hour varied from day to night, and also with the changing seasons. This awkward arrangement is believed to have been revised at the suggestion of one Abul Hassan, a 13th century Arabian mathematician, who defined the hour as a fixed period based on "a 12th part of the daylight at the equinoxes".
The hours themselves were first measured by a sundial, when our ancestors learned:
To carve out dials, quaintly, point by point, Thereby to see the minutes how they run.
The first mechanical clocks are believed to have made their appearance in the 14th century, but they were clumsy affairs, often made by the village blacksmith; they were doing well if they kept time to the nearest hour, but the pace of life in those days was such that minutes, as units of time, did not bother people very much.
The first major innovation was the foliot balance, introduced about 1450. It was, in essence, a weighted horizontal arm swinging erratically to and fro, which slowed down the rate at which the mechanism unwound, but its effectiveness as a control device left much to be desired, and even the best of clocks would gain or lose 15 minutes per day.
In 1583, Galileo Galilei discovered the properties of the simple pendulum, and in 1656 Christiaan Huygens adapted the device to control the movement of a clock. The effect was dramatic; by harnessing the regular swing of the pendulum, clocks could now be constructed which kept time to an accuracy of 10 seconds per day.