Greenwich Mean Time (GMT) is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is regularly used to refer to Coordinated Universal Time (UTC) when this is viewed as a time zone. It is also used to refer to Universal Time (UT), which is a standard astronomical concept used in many fields and is sometimes called Zulu time.
Noon Greenwich Mean Time is not necessarily the moment when the noon sun crosses the Greenwich meridian and reaches its highest point in the sky in Greenwich because of Earth's uneven speed in its elliptic orbit and its axial tilt. This event can be up to 16 minutes away from noon GMT (this is called the equation of time). The fictitious mean sun is the annual average motion of the true Sun.
The term GMT has been used with two different conventions for numbering hours. The old astronomical convention before 1925 was to refer to noon as zero hours, whereas the civil convention during the same period was to refer to midnight as zero hours.
As the United Kingdom grew into an advanced maritime nation, British mariners kept at least one chronometer on GMT in order to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees (this convention was internationally adopted in the International Meridian Conference of 1884). The chronometer on GMT did not affect shipboard time itself, which was still solar time. But this practice, combined with mariners from other nations drawing from Nevil Maskelyne's method of lunar distances based on observations at Greenwich, eventually led to GMT being used worldwide as a reference time independent of location. Most time zones were based upon this reference as a number of hours and half-hours "ahead of GMT" or "behind GMT".
The daily rotation of the Earth is somewhat irregular and is slowing down slightly, atomic clocks have a more stable timebase. On 1 January 1972, GMT was replaced as the international time reference by Coordinated Universal Time, maintained by an ensemble of atomic clocks around the world.
Universal Time (UT) is a timescale based on the rotation of the Earth. It is a modern continuation of Greenwich Mean Time (GMT), the mean solar time on the meridian of Greenwich, and GMT is sometimes used loosely as a synonym for UTC. In fact the expression "Universal Time" is ambiguous, as there are several versions of it, the most commonly used being UTC and UT1. All of these versions of UT are based on sidereal time, but with a scaling factor and other adjustments to make them closer to solar time.
Standard time, divided the world into twenty-four time zones, each one covering exactly 15 degrees of longitude. All clocks within each of these zones is set to the same time as the others, but different by one hour from those in the next zone. The local time at the Royal Greenwich Observatory in Greenwich, England was chosen as standard at the 1884 International Meridian Conference, leading to the use of Greenwich Mean Time in order to set local clocks. This location was chosen because in 1884 two-thirds of all charts and maps already used it as their prime meridian.
In 1928, the term Universal Time was adopted internationally as a more precise term than Greenwich Mean Time, because the GMT could refer to either an astronomical day starting at noon or a civil day starting at midnight. The term Greenwich Mean Time persists in common usage to this day in reference to civil timekeeping.
UTC, Coordinated Universal Time) is an atomic timescale that approximates UT1. It is the international standard on which civil time is based.
UT1, is the principal form of Universal Time. UT1 is the same everywhere on Earth, and is proportional to the true rotation angle of the Earth with respect to a fixed frame of reference. Since the rotational speed of the earth is not uniform, UT1 has an uncertainty of plus or minus 3 milliseconds per day.
