The matters of every-day life, much less the affairs of a complex civilization, could scarcely be carried on without some accurate and uniform system of measuring time. Nature herself furnishes measurements for certain divisions of time. The "two great lights" that God made, as the Bible tells us, were designed "for signs, and for seasons, and for days and for years." The revolution of the earth around the sun marks the year; the revolution of the moon around the earth determines the month; the rotation of the earth on its axis causes and measures day and night. But no object of nature distinguishes the hours of the day or the divisions of the hour.
Man requires a smaller unit of time than the day. He must divide the day into hours; the hours into minutes; the minutes into seconds. The division of the day into twenty-four hours is as old as authentic history. But the means for determining the hours and their subdivisions were at first quite crude and inefficient.
Perhaps the most primitive of all time-measuring devices was a stick or pole planted upright in a sunny place. The position of the shadow which it cast marked time. The sun-dial was a development of this simple device. It consisted essentially of two parts: a flat plate of metal marked off much like the dial of a modern clock or watch, and an upright piece, usually also of metal, fastened to the center of the dial. To make the direction of the shadow uniform for any given hour throughout the year, the upright piece was made parallel to the axis of the earth. As the earth rotated on its axis the shadow cast by the upright piece moved from point to point on the dial, measuring the flight of time. The sun-dial was in use among the earliest nations. Herodotus is authority for the statement that the Greeks borrowed it from the Babylonians. The sun-dial was obviously of no use on cloudy days or dark nights, and even in sunny weather it could not accurately or delicately indicate the passage of time. However, it continued in use so long that to the end of the seventeenth century the art of dialling was considered a necessary element of every course in mathematics.
Another ancient invention for measuring time was the water-clock. Water was permitted to drop from a small orifice in a containing vessel. The period required for emptying the vessel marked a unit of time. Its principle was the same as the common hour-glass, according to which time is measured by the slow dropping of sand from one receptacle into another. The water-clock was used by the ancient Chaldeans and the Hindoos, and also by the Greeks and Romans. Demosthenes mentions its use in the courts of justice at Athens.