Greenwich Mean Time is the mean solar time at the Royal Observatory in Greenwich, south-east London, and it is the reference from which the world’s time zones are counted. Say it GREN-itch.
Why Greenwich
Not by decree, but by usage. Establishing longitude at sea requires a fixed reference meridian, and each nation initially used its own — Paris, Cadiz, Naples, Washington. By the late nineteenth century, Britain’s naval and merchant dominance meant that around two-thirds of the world’s sea charts already used Greenwich.
The International Meridian Conference in Washington in 1884 formalised it, adopting Greenwich as the prime meridian by 22 votes to 1, with France abstaining and continuing to use Paris time for decades afterwards out of what was widely understood to be pique.
Railway time
Before the railways, every British town kept its own local solar time. Bristol was about ten minutes behind London, Oxford about five. This was perfectly workable when travel took days.
Trains broke it immediately. A timetable is meaningless if every station keeps a different clock, and there were genuine collisions attributable to the confusion. The Great Western Railway adopted London time across its network in 1840, other companies followed, and by 1855 the great majority of British public clocks showed GMT.
Some clocks were fitted with two minute hands during the transition, one for local time and one for railway time. Legal recognition did not arrive until 1880.
British Summer Time
Britain moves its clocks forward an hour from the last Sunday in March to the last Sunday in October. During that period the country is on BST, which is GMT+1; the rest of the year it is on GMT itself.
The idea was campaigned for by William Willett, a builder who was irritated at how many people slept through summer mornings, and it was adopted in 1916 to save coal during the war. Willett died the year before it happened.
Britain has experimented with alternatives. From 1968 to 1971 it stayed on GMT+1 all year, which was popular in the south and deeply unpopular in Scotland, where winter sunrise moved past nine in the morning. It was abandoned, and proposals to try again resurface every few years and founder on the same objection.
UTC
Strictly, the modern global standard is Coordinated Universal Time, based on atomic clocks rather than the earth’s rotation, which is slightly irregular. UTC and GMT are within a second of each other and are used interchangeably in everyday speech, but timekeeping bodies use UTC.
Visiting
The Royal Observatory at Greenwich is open to the public, and the meridian line is marked in the courtyard so that visitors can stand with a foot in each hemisphere — which is precisely what everyone does.
The Shepherd Gate Clock on the observatory wall was the first to show GMT directly to the public and still does. Also there: John Harrison’s marine chronometers, the clocks that solved the longitude problem and are among the most important objects in the history of navigation.
A red time ball on the observatory roof has dropped at exactly one o’clock every day since 1833, so that ships on the Thames could set their chronometers by it.