Measuring Time
When it first became important for people to measure time they didn't have fancy wrist watches made in Switzerland. They needed to know when the winter was coming or when they could harvest fruit. For this they used the regular cycles of their environment. It was known to them that the Sun rises and sets in a regular and predictable manner. It was also known that the Moon, Stars, and Planets have cyclic motion through the heavens. They kept track of the passing of a day with sundials and the like. With this knowledge, the ancient peoples of Earth could predict the coming of the different seasons. There were many ancient peoples that devised very accurate calendars to keep track of time. The megalithic stones of Stonehenge in Great Britain are an example of such a calendar. The Mayans of Central America had a calendar based on the sidereal year that was at least as accurate as the calendars of today. Of course, these early measurements of time were based on the periodic motion of the Earth about its rotation axis and around the Sun. It is now known that these motions are not constant but vary in time. As history progressed methods of measuring time's passage became more focused on very regular oscillations of such things as springs and pendulums. Today official time is measured by atomic oscillations of cesium to an accuracy of parts per billion.