Greenwich Mean Time

Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, London, calculated from the average time of the Sun's transit over the Prime Meridian. It was originally established for astronomical and navigational purposes, with the observatory's time signals first transmitted by telegraph in 1852. In 1847, British railways adopted GMT as their standard, and by 1880 it became the legal time for the whole of Great Britain. The 1884 International Meridian Conference in Washington, D.C., selected Greenwich as the prime meridian, making GMT the world's time reference.

GMT was essential for maritime navigation, as sailors used it to calculate longitude by comparing local time with Greenwich time. The Royal Observatory's time ball, installed in 1833, dropped daily at 1 p.m. to allow ships on the Thames to set their chronometers. GMT also became the basis for the worldwide system of time zones, with the Earth divided into 24 zones centered on the Greenwich meridian. Although UTC replaced GMT as the scientific standard in 1972, GMT remains in common use in the United Kingdom and many Commonwealth countries as civil time during winter.

Today, GMT is often used interchangeably with Coordinated Universal Time (UTC), but they differ subtly: UTC is an atomic time scale that does not observe daylight saving time, while GMT is a solar time that can be adjusted. In the UK, GMT is the standard time from late October to late March, with British Summer Time (BST) used in between. Many international organizations, including the BBC and the United Nations, still refer to GMT for scheduling, though UTC is the official standard. The term 'GMT' is also used in computing and broadcasting to denote time without a time zone offset.

One lesser-known fact is that the original Prime Meridian at Greenwich was defined by the Airy Transit Circle, built in 1850, and its position has been used for over a century. However, with the advent of satellite navigation, the International Reference Meridian (IRM) was established in 1984, which lies about 102 meters east of the Airy Transit Circle. Another obscure detail is that GMT was once measured from noon, not midnight, until 1925, when astronomers switched to midnight to align with civil time. Additionally, the Greenwich time signal was broadcast by radio from 1924, and the 'pips' heard on BBC radio are derived from this tradition.

Mean solar time

Time based on the average length of a solar day, as opposed to apparent solar time.

Prime Meridian

The line of 0° longitude, from which all other longitudes are measured.

UTC

Coordinated Universal Time, the primary time standard by which the world regulates clocks and time.