Universal Time (UT) is a time standard based on Earth's rotation. There are several versions of Universal Time, which differ by up to a few seconds. The most commonly used are Coordinated Universal Time (UTC) and UT1 (see § Versions). All of these versions of UT, except for UTC, are based on Earth's rotation relative to distant celestial objects (stars and quasars), but with a scaling factor and other adjustments to make them closer to solar time. UTC is based on International Atomic Time, with leap seconds added to keep it within 0.9 second of UT1.[a]
Prior to the introduction of standard time, each municipality throughout the clock-using world set its official clock, if it had one, according to the local position of the Sun (see solar time). This served adequately until the introduction of rail travel in Britain, which made it possible to travel fast enough over long distances to require continuous re-setting of timepieces as a train progressed in its daily run through several towns. Greenwich Mean Time, the mean solar time on the Prime Meridian at Greenwich, England, was established to solve this problem: all clocks in Britain were set to this time regardless of local solar noon.[b] Chronometers or telegraphy were used to synchronize these clocks.
Standard time was originally proposed by Scottish-Canadian Sir Sandford Fleming at a meeting of the Canadian Institute in Toronto on 8 February 1879. He suggested that standard time zones could be used locally, but they were subordinate to his single world time, which he called Cosmic Time. The proposal divided the world into twenty-four time zones, each one covering 15 degrees of longitude. All clocks within each zone would be set to the same time as the others, but differed by one hour from those in the neighboring zones. The local time at the Royal Observatory in Greenwich was announced as the recommended base reference for world time on 22 October 1884 at the end of the International Meridian Conference.[c] This location was chosen because by 1884 two-thirds of all nautical charts and maps already used it as their prime meridian. The conference did not adopt Fleming's time zones because they were outside the purpose for which it was called, which was to choose a basis for universal time (as well as a prime meridian).
In 1935, the term Universal Time was recommended by the International Astronomical Union as a more precise term than Greenwich Mean Time, because GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. In some countries, the term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.
Based on the rotation of the Earth, time can be measured by observing celestial bodies crossing the meridian every day. Astronomers found that it was more accurate to establish time by observing stars as they crossed a meridian rather than by observing the position of the Sun in the sky. Nowadays, UT in relation to International Atomic Time (TAI) is determined by Very Long Baseline Interferometry (VLBI) observations of distant quasars, a method which can determine UT1 to within 15 microseconds or better.
The rotation of the Earth and UT are monitored by the International Earth Rotation and Reference Systems Service (IERS). The International Astronomical Union also is involved in setting standards, but the final arbiter of broadcast standards is the International Telecommunication Union or ITU.
The rotation of the Earth is somewhat irregular and also is very gradually slowing due to tidal acceleration. Furthermore, the length of the second was determined from observations of the Moon between 1750 and 1890. All of these factors cause the modern mean solar day, on the average, to be slightly longer than the nominal 86,400 SI seconds, the traditional number of seconds per day.[d] As UT is thus slightly irregular in its rate, astronomers introduced Ephemeris Time, which has since been replaced by Terrestrial Time (TT). Because Universal Time is determined by the Earth's rotation, which drifts away from more precise atomic-frequency standards, an adjustment (called a leap second) to this atomic time is needed since (as of 2019[update]) 'broadcast time' remains broadly synchronised with solar time.[e] Thus, the civil broadcast standard for time and frequency usually follows International Atomic Time closely, but occasionally step (or "leap") in order to prevent them from drifting too far from mean solar time.
Barycentric Dynamical Time (TDB), a form of atomic time, is now used in the construction of the ephemerides of the planets and other solar system objects, for two main reasons. First, these ephemerides are tied to optical and radar observations of planetary motion, and the TDB time scale is fitted so that Newton's laws of motion, with corrections for general relativity, are followed. Next, the time scales based on Earth's rotation are not uniform and therefore, are not suitable for predicting the motion of bodies in our solar system.
There are several versions of Universal Time:
The table shows the dates of adoption of time zones based on the Greenwich meridian, including half-hour zones.
Apart from Nepal Standard Time (UTC+05:45), the Chatham Standard Time Zone (UTC+12:45) used in New Zealand's Chatham Islands and the officially unsanctioned Central Western Time Zone (UTC+8:45) used in Eucla, Western Australia and surrounding areas, all time zones in use are defined by an offset from UTC that is a multiple of half an hour, and in most cases a multiple of an hour.