National Physical Laboratory

The World Time System

Co-ordinated Universal Time, or UTC, is the time scale used all over the world. Based on atomic clocks, with regular corrections to keep it close to the traditional solar time, this stable and precise standard is used worldwide in technologies ranging from global telecommunications to satellite navigation.

Defining the solar day

The rotation of the Earth on its axis, and its orbit around the Sun, has served as the basis for timekeeping since the dawn of history. The day was divided into 24 hours, each of which contained 60 minutes, which in turn were broken down into 60 seconds.

Because the length of the apparent solar day (as shown, for example, by a sundial) varies in a regular way during the year it became necessary to average out this effect and define a mean, or average, solar day. This explains the name Greenwich Mean Time (GMT), a time scale in which the length of the day is calculated so that the mean position of the sun at noon for the whole year is directly above the Greenwich Meridian (longitude zero).

Over the centuries the accuracy of time measurement has steadily improved and it has become clear that there are irregularities in the Earth's rate of rotation. In effect, the seconds of Universal Time (UT1, as GMT is now officially known) vary a little in length in order to keep in step with the changes in the Earth's rotation. This makes UT1 an imprecise method of keeping time.

Moving from solar time (UT1) to atomic time (TAI)

In 1955 the first atomic clock, which was much more regular than the Earth itself, or any other type of clock then in existence, was brought into operation at the National Physical Laboratory. Constructed by Louis Essen and Jack Parry, it was based on measurements of a particular vibration of the caesium-133 atom.

Over the next few years the frequency (or rate) of the NPL caesium clock was compared with the astronomical second calculated by the United States Naval Observatory (USNO), and as a result of this work in 1967, by international agreement, the second was defined in the International System of units of measurement (SI) as the duration of 9 192 631 770 periods of that particular vibration of the caesium-133 atom.

This SI second, is believed to always be the same amount of time when measured under certain defined conditions. It is used to make a time scale called International Atomic Time (TAI), which is a simple count of SI seconds labelled conventionally using minutes, hours and days.

However, as TAI is not linked to the Earth's rotation, a clock and calendar based on TAI will gradually become more and more out of step with UT1. Traditional navigation, using observation of the sun, moon and stars, is one application that requires knowledge of UT1 to within a few seconds. Another is the use of UT1 by astronomers as a measure of the Earth's rotation angle, allowing them to move their telescopes to aim accurately at any object. For all other uses of timekeeping, TAI is a much more stable and reliable time source.

Leap seconds

Although TAI is a stable and accurate source of time, the lack of a link between it and the solar day causes difficulties for those who want or need to keep accurate time linked to the Earth's rotation, particularly astronomers. The solution adopted for this problem was to construct a second atomic time scale called 'Coordinated Universal Time' (abbreviated in all languages as UTC) as the basis of international timekeeping.

From 1 January 1972, the seconds of UTC have been exactly the same length as those of TAI, and they occur at the same instants. UTC is also kept within 0.9 seconds of UT1 by the insertion of extra seconds as necessary (known as positive leap seconds). It could happen that seconds would need to be removed (negative leap seconds), but so far all leap seconds have been positive. This compromise time scale combines all the regularity of atomic time with most of the convenience of UT1 (or GMT), and most countries have adopted it as the legal basis for time.

A leap second occurs at the same instant throughout the world, when the familiar 'six pips' radio time signal gains an extra pip before the long pip marking the hour, to become a 'seven pip' signal. So, the minute containing the leap second becomes a 61-second minute.

Leap seconds are generally applied to UTC at the end of December or June, or in theory in March or September if needed. The International Earth Rotation and Reference Systems Service (IERS) take the decision based on analysis of results from several different methods for monitoring changes in the Earth's rotation, and the result is announced around 6 months in advance.

A worldwide time scale

The outcome of this whole process is the international time scale, UTC, which the Bureau International des Poids et Mesures (BIPM) publishes in its Circular T each month. It is an atomic clock-based time scale, based on the outputs from a large number of caesium clocks in institutions around the world, and an integrated time scale, meaning that it is a simple count of the basic unit of time duration, the second. UTC time is described in the universally understood units of minutes, hours and days.

Who sets the time?

Since 1988, the generation of UTC has been the responsibility of the BIPM, located near Paris. The BIPM collects clock comparison and time transfer results from around 55 National Measurement Institutes (NMIs) around the world, including NPL, allowing around 260 atomic clocks to contribute to UTC.

Last Updated: 22 Dec 2016
Created: 26 Oct 2011

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