The Greenwich Time Service

The measurement of accurate star positions and the determination of accurate time are inextricably linked together. As a result, accurate time was available at the Observatory from the time of its inception. Although Greenwich Time had been used at sea since the introduction of the Nautical Almanac in 1767, it was not widely disseminated from Greenwich until the 1830s. Listed below are the main methods that have been used since that time.


The Greenwich Time Ball

The use of Time Balls to disseminate Time began in Portsmouth in 1829. A few years later, in 1833, the Astronomer Royal, John Pond, oversaw the setting up of a Time Ball at the Royal Observatory in Greenwich. It consisted of a large ball which rose to the top of a mast on the roof of Flamsteed House and dropped as a visual time signal at one o’clock precisely. Its main purpose was to enable navigators aboard ships moored on the River Thames and in the London Docks to set their chronometers before setting sail. The Greenwich Time Ball is still operational today.

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Manual distribution of time by the Belvilles

In 1836 the Observatory’s Second Assistant, John Henry Belville began a private weekly subscription service for London chronometer makers and others who needed accurate time but were unable to make use of the signal provided by the Time Ball. This involved the transportation of a pocket chronometer from the Observatory to the subscribers’ premises where it would be used to check and adjust their timekeepers. Following the death of Belville in 1856, the service was continued by his third wife Maria and then his daughter Ruth. It came to an end during the Second World War, probably in the autumn of 1940.

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Dissemination of time via the railway telegraph network

In 1849, Charles Shepherd patented a new form of electric clock whose potential prompted George Airy, (Pond’s successor as Astronomer Royal), to enter discussions with Charles Walker of the South Eastern Railway Company with a view to distributing Greenwich Time more widely via the railway telegraph network. In 1851 the Great Exhibition was held in Hyde Park in London. It was there, on 23 May and again on 24 June that Airy was able to see for himself, the electric ‘master-and-slave’ clock system installed by Shepherd for disseminating time to the exhibition’s numerous visitors. It consisted of a ‘master clock’ sending regular electrical (galvanic) impulses to a number of ancillary ‘slave’ dials. Airy, placed an order in December 1851 and by the following summer had a working system in place at Greenwich. It was to be at the heart of Britain’s time distribution system for the next 70 years. By 1853, the Observatory’s master clock was controlling a public clock at its gates, three other clocks within the Observatory (including one in the chronometer room), and a clock at the London-bridge Terminus of the South-Eastern Railway. It also sent galvanic signals every day along all the principal railways diverging from London as well as dropping the Time Ball at Greenwich and another on the Offices of the Eastern Telegraph Company in the Strand. The Gate Clock is still operational today. 

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Radio time signals

In 1924, the BBC began the broadcasting of a ‘six pip’ radio time signal, which until 1990, was generated at the Observatory. Anyone with a radio had access to accurate time directly from their own home. In 1927 the domestic time signals were supplemented by a more complex international service broadcast from Rugby.

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The Speaking Clock

The speaking clock gives access to accurate time via the phone system. A service was first introduced in Britain in 1936. It was (and still is) a commercial enterprise and unlike the radio time signals was never under the direct control of the Observatory.

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