The earliest time measuring devices used either the shadow cast by the Sun or the rate which water runs out of a vessel. Both methods were in use before the earliest historical records. Early systems for dividing the day and night into 'hours' used a simple division into twelve parts regardless of season. The familiar use of hours, each of identical length, only came into use in the 15th and 16th centuries with the spread of mechanical clocks.
In the 17th century the pendulum clock was developed and by the 18th century clocks were sufficiently accurate for them to be employed at sea for determining longitude and for scientific time measurement. The fundamental accurate timekeeper, however, was the rotation rate of the Earth and clocks were kept on time by means of astronomical observations.
The development of the pendulum clock reached its climax early this century with the Free Pendulum clocks which had an accuracy of about a second a year. Their use indicated the non-uniformity of the Earth's rotation. They were succeeded by quartz crystal controlled clocks and subsequently by clocks using an atomic transition of very accurately known frequency. The second was redefined using this frequency and clocks can now be made which are accurate to one second in thousands of years.
The most accurate clocks available use an atomic transition in a caesium vapour, which defines a very accurately known frequency. This frequency is then divided down to give seconds, minutes etc. Several atomic clocks are used to define a local time standard time service. There are many separate time services throughout the world and a combined mean version of their time measurement is used as International Atomic Time (TAI).
Our natural concept of time is linked to the rotation of the Earth and we define the length of the day as the time for the Earth to rotate once relative to the Sun. Unfortunately the Earth does not rotate at exactly a constant rate. Due to physical processes, such as mass motions in the atmosphere, the rate varies and so a timescale, called Coordinated Universal Time (UTC), has been adopted which differs from TAI by an integer number of seconds. This difference is made in order to keep UTC within 0.9 seconds of the time determined by the mean position of the Sun (UT1). As the Earth's rotation rate varies the difference between UTC and UT1 changes and, in order to keep the difference small, leap-seconds are inserted at the end of the year, or on June 30. The current difference between UTC and TAI, following the leap second on 1999 Jan 1, is 32 seconds.
The rotation of the Earth is measured using pulsed laser beams which are reflected by artificial satellites in Earth orbit. The location of the satellite laser telescope can be determined relative to the satellite with an accuracy of a few centimetres from one pass. As the satellite's orbit in space is known this allows the precise determination of the location of the telescope and hence the rotation of the Earth.
In everyday life we use the time displayed on our clocks and watches. In the UK we use Coordinated Universal Time (UTC), the modern equivalent of Greenwich Mean Time (GMT), which is never more than half a second different from it. In the summer we adjust our clocks to British Summer Time, BST, which is one hour in advance of GMT.
Universal time relates to the time on the Greenwich meridian. For any other place the difference in longitude measured in hours, etc., gives the difference between noon (UTC) and local noon. When setting up a sundial this difference must be applied in order to get the sundial to register noon when the Sun is on the meridian at Greenwich rather than on the local meridian.
The world is divided into time zones which are usually an integral number of hours different from UTC and which correspond to local time somewhere in the countries within that zone.
In the summer many countries including the UK and the US adopt a daylight saving time which is in advance of their local time.
Just as the Sun rises in the east, passes across the sky through the meridian (the line from the zenith to the south point) and sets in the west, so do all astronomical objects. This apparent motion is, of course, due to the Earth's rotation. It is thus apparent that the position of any object in the sky is time dependent.
The Earth rotates relative to the Sun once a day but also in the course of a year, due to its orbit, makes one additional rotation around the Sun. Thus, relative to the stars, there is an extra rotation once a year. This amounts to a difference in the position of the stars in the sky by about one degree, or four minutes of time, when viewed at the same time on two successive days.
Astronomers keep clocks running at this slightly different rate, called sidereal time, which indicates the time at which any particular star will cross the meridian.
Because the Earth moves around the Sun in an elliptical path and because the Earth's equator is inclined to this path the Sun does not cross the meridian (the line from the zenith to the south point) at exactly noon each day. This is the difference often noted between clock time and that shown by a sundial.
The reasons for this and the value of the difference between clock time and that shown by a sundial are given in the equation of time fact file.
Accurate atomic clocks are maintained on the web by: