From Wikipedia, the free encyclopedia

The Shepherd Gate Clock is the clock mounted on the wall outside the gate of the Royal Greenwich Observatory building in Greenwich, London. The clock, an early example of an electric clock, was a slave mechanism controlled by electric pulses transmitted by a master clock inside the main building. The 'network' of master and slave clocks was constructed and installed by Charles Shepherd in 1852. The clock by the gate was probably the first to display Greenwich Mean Time to the public, and is unusual in using the 24 hour analog dial.

Contents 1 Origins 2 Charles Shepherd 3 Transmitting Greenwich time 4 See also 5 References 6 External links

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 to Calais 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.

In 1849, Charles Shepherd, of 53 Leadenhall Street, London, had patented a system for controlling a network of master and slave clocks using electricity (or galvanism, as it was called). Shepherd, an engineer and son of a clockmaker, had installed the public clocks for the Great Exhibition which opened in May 1851. In October, 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 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.

By August 1852, Shepherd's clock system was 'up and running'. 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, 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 via transatlantic cable.

The Gate Clock originally indicated astronomical time, in which the counting of the 24 hours of a day starts at noon every day rather than midnight. In the 20th century, the clock was changed to show Greenwich Mean Time, which it still does - it doesn't show daylight savings time. The clock is now controlled by a quartz mechanism inside the main building, and the master clocks are still on display, but not working.

Royal Greenwich Observatory

• Howse, Derek Greenwich Time and the Longitude 1997, Oxford University Press

• National Maritime Museum
• Shepherd clock screensaver a software emulation running as a MacOS X screensaver