Many places once used their own local time based on the Sun's position. As railways connected distant cities, timetables and safe dispatching created pressure for common time standards.
Time zones organize differences in longitude into shared time regions. Modern standard time also depends on precise clocks, astronomical measurement, and international coordination.
History
Until the latter part of the 18th century, time was normally determined in each town by a local sundial. Solar time is calculated with reference to the relative position of the sun. This provided only an approximation as to time due to variations in orbits and had become unsuitable for day-to-day purposes. It was replaced by local mean time, which eliminated the variation due to seasonal differences and anomalies. It also took account of the longitude of a location and enabled a precise time to be applied.
Such new-found precision did not overcome a different problem: the differences between the local times of neighbouring towns. In Britain, local time differed by up to twenty minutes from that of London. For example, Oxford Time was five minutes behind Greenwich Time, Leeds Time six minutes behind, Carnforth eleven minutes behind, and Barrow almost thirteen minutes behind. In India and North America, these differences could be sixty minutes or more. Almanacs containing tables were published and instructions attached to sundials to enable the differences between local times to be computed.
Before the arrival of the railways, journeys between the larger cities and towns could take many hours or days, and these differences could be dealt with by adjusting the hands of a watch periodically en route. In Britain, the coaching companies published schedules providing details of the corrections required. However, this variation in local times was large enough to present problems for the railway schedules. For instance, Leeds time was six minutes behind London, while Bristol was ten minutes behind; sunrise for towns to the east, such as Norwich, occurred several minutes ahead of London. It soon became apparent that even such small discrepancies in times caused confusion, disruption, or even accidents.
Influence of the electric telegraph
The electric telegraph, which had been developed in the early part of the 19th century, was refined by William Fothergill Cooke and Charles Wheatstone and was installed on a short section of the Great Western Railway in 1839. By 1852 a telegraph link had been constructed between a new electro-magnetic clock at Greenwich and initially Lewisham, and shortly after this London Bridge stations. It also connected via the Central Telegraph Station of the Electric Time Company in the City of London, which enabled the transmission of a time signal along the railway telegraphic network to other stations. By 1855 time signals from Greenwich could be sent through wires alongside the railway lines across the length and breadth of Britain. This technology was also used in India to synchronise railway time.
Great Britain
Before the advent of the telegraph, stationmasters adjusted their clocks using tables supplied by the railway company to convert local time to London Time. In turn, train guards set their chronometers against those clocks.
The introduction of railway time was in the end swift despite not being straightforward. The Great Western Railway was the first to standardise its timetable on Greenwich Mean Time, in November 1840. One of the most vociferous proponents of standardising time on the railways was Henry Booth, Secretary of the Liverpool and Manchester Railway, who by January 1846 had ordered the adjustment of clocks to Greenwich Mean Time at both Liverpool and Manchester stations.
The Midland Railway adopted London Time at all of its stations on 1 January 1846. As a consequence, in February 1846 the town council of Nottingham ordered that the town clocks be furnished with three hands, two indicating local time and the additional one the railway and post-office London time.
On 22 September 1847, the Railway Clearing House, set up five years earlier to coordinate the distribution of revenue between railway companies, decreed that "GMT be adopted at all stations as soon as the General Post Office permitted it". From 1 December 1847, the London and North Western and the Caledonian Railways switched over.