The activities of early humankind did not require the measurement of time to be any more accurate than the knowledge of the length of a day and the forthcoming seasons. As human activity has both broadened and become more focused it has necessitated a more and more accurate interpretation of time. Today, the measurement of time has become quite specialised and takes in to account many factors, some of which are esoteric to the ordinary individual going about their daily business and, for example, waiting at a bus stop for the 3.30pm bus to town. To the astronomer trying to find the faintest smudge of light or radio waves from the far side of the universe, the accurate measurement of time is essential in order to point the equipment in precisely the right direction.
Currently the measurement of time falls into two broad categories: