The Shepherd Gate Clock is the clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

 mounted on the wall outside the gate of the Royal Greenwich Observatory building in Greenwich

Greenwich

Greenwich is a district of south London, England, located in the London Borough of Greenwich.Greenwich is best known for its maritime history and for giving its name to the Greenwich Meridian and Greenwich Mean Time...

, London

London

London is the capital city of :England and the :United Kingdom, the largest metropolitan area in the United Kingdom, and the largest urban zone in the European Union by most measures. Located on the River Thames, London has been a major settlement for two millennia, its history going back to its...

. The clock, an early example of an electric clock

Electric clock

An electric clock is a clock that is powered by electricity instead of powered manually or by other sources of energy, specifically in order to wind the mainspring or to drive the pendulum or oscillator.-Classification:...

, was a slave mechanism controlled by electric pulses transmitted by a master clock inside the main building. The 'network' of master and slave clock

Slave clock

In telecommunication, a slave clock is a clock that is coordinated with a master clock. Slave clock coordination is usually achieved by phase-locking the slave clock signal to a signal received from the master clock...

s was constructed and installed by Charles Shepherd in 1852. The clock by the gate was probably the first to display Greenwich Mean Time

Greenwich Mean Time

Greenwich Mean Time is a term originally referring to mean solar time at the Royal Observatory in Greenwich, London. It is arguably the same as Coordinated Universal Time and when this is viewed as a time zone the name Greenwich Mean Time is especially used by bodies connected with the United...

 to the public, and is unusual in using the 24-hour analogue dial.

Origins

The original idea for the clock network came from the Astronomer Royal, George Airy. With the arrival of the railway network, a single time standard was needed to replace the various incompatible local times then in use across the country. Airy proposed that this standard time would be provided by the Royal Observatory. His idea was to use what he called 'galvanism' or electric signalling to transmit time pulses from Greenwich to slave clocks throughout the country, and perhaps to Europe and the colonies too. The new undersea cable recently installed between Dover

Dover

Dover is a town and major ferry port in the home county of Kent, in South East England. It faces France across the narrowest part of the English Channel, and lies south-east of Canterbury; east of Kent's administrative capital Maidstone; and north-east along the coastline from Dungeness and Hastings...

 to Calais

Calais

Calais is a town in Northern France in the department of Pas-de-Calais, of which it is a sub-prefecture. Although Calais is by far the largest city in Pas-de-Calais, the department's capital is its third-largest city of Arras....

 in 1851 raised the possibility of sending time signals between England and France - this would allow longitude differences to be measured very accurately, for the first time.

Charles Shepherd

In 1849, Charles Shepherd Junior (1830–1905), an engineer and son of a clockmaker, patented a system for controlling a network of master and slave clocks using electricity (or galvanism, as it was called). Shepherd installed the public clocks for the Great Exhibition which opened in May 1851. In October 1851, Airy wrote to Charles Shepherd asking for proposals and estimates, including a request for the following clocks:

One automatic clock. One clock with large dial to be seen by the Public, near the Observatory entrance, and three smaller clocks, all to be moved sympathetically with the automatic clock.

Airy also wanted the existing Greenwich time ball

Time ball

A time ball is a large painted wooden or metal ball that drops at a predetermined time, principally to enable sailors to check their marine chronometers from their boats offshore...

 to be electrically operated, so that its descent at 13:00 was synchronized with the master clock inside the observatory.

By August 1852, Shepherd had built and installed the network of clocks and cables in the observatory. Costs were considerably higher than the original estimates. Shepherd had estimated £40 for the master clock and time ball apparatus, and £9 for each sympathetic clock. The total costs included £70 for the master clock, and £75 for the wall clock by the gate.

Transmitting Greenwich time

The master clock, at first called the Normal Clock or Master Clock, but later known as the Mean Solar Standard Clock, sent pulses every second to the sympathetic or slave clocks in the Chronometer Room and the Dwelling House (Flamsteed House) and to the Gate Clock. A pulse was also sent to the time ball at 13:00. The signals were also transmitted along cables from Greenwich to London Bridge. At London Bridge, a time signal was distributed at less frequent intervals to clocks and receivers throughout England.

Airy's report to the Observatory's Board of Visitors in 1853 explained the function of the Shepherd master clock:

This clock keeps in motion a sympathetic galvanic clock in the Chronometer room, which, therefore, is sensibly correct; and thus the chronometers are compared with a clock which requires no numerical correction.

The same Normal Clock maintains in sympathetic movement the large clock at the entrance-gate, two other clocks in the Observatory, and a clock at the London Bridge Terminus of the South-Eastern Railway.

It sends galvanic signals every day along all the principal railways diverging from London. It drops the Greenwich Ball and the Ball on the Offices of the Eastern Telegraph Company in the Strand.

All these various effects are produced without sensible error of time; and I cannot but feel a satisfaction in thinking that the Royal Observatory is thus quietly contributing to the punctuality of business through a large portion of this busy country.

By 1866, time signals were being sent to Harvard University

Harvard University

Harvard University is a private Ivy League university located in Cambridge, Massachusetts, United States, established in 1636 by the Massachusetts legislature. Harvard is the oldest institution of higher learning in the United States and the first corporation chartered in the country...

 via transatlantic cable.

The Gate Clock originally indicated astronomical time, in which the counting of the 24 hours of each day starts at noon. The clock was changed in the 20th century to indicate Greenwich Mean Time, in which the counting of the 24 hours of each day starts at midnight. Currently, the Gate Clock continues to show Greenwich Mean Time, and it does not show daylight saving time

Daylight saving time

Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

. The clock is now controlled by a quartz mechanism inside the main building. The master clocks are still on display, but are not functional.

The Timeball Museum in Deal contains another slave clock once connected to the Greenwich master clock.

In 1853, Shepherd was appointed '1st Class assistant to the Electric Telegraph Establishment' working for the East India Company and responsible for overseeing the establishment of a telegraph network for the Indian Government. In 1883, he installed a battery-powered clock in Hornblotton church, Somerset. The mechanism has now been moved to the London Science Museum.

During World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

, the dial was damaged beyond repair by a bomb. The current dial is a modern copy.

External links

• National Maritime Museum
• Shepherd screensaver a software emulation running as a Mac OS X screensaver