The Shepherd Clock, Royal Observatory, Greenwich, England, 1852. The first clock ever to show Greenwich Mean Time (GMT) directly to the public. It shows 24 hours on its face, so at 12 noon the hour hand points straight down. Public Standards of Length and OS Benchmark appear below… [1280×854] [OC]

…This type of clock is known as ‘sympathetic’ because it depends on another highly accurate one, known as the ‘motor’ clock, inside the main Observatory building. The time shown is accurate to half of a second. In summer, Britain converts to British Summer Time (BST), which is an hour ahead of GMT, and the clock then appears one hour ‘slow’.

While the Shepherd Gate Clock is the one most visitors see, the motor clock is the true technological breakthrough. Between 1852 and 1893, the time was sent via telegraph wires to London, Edinburgh, Glasgow, Dublin, Belfast and beyond, eventually supplying hundreds of local time signals.

Before this system’s introduction, each individual town or village kept local time using a sundial, which meant half an hour’s difference between the eastern and western sides of the country. Greenwich time meant the nation’s time could be synchronized.

By 1866, time signals were sent from Greenwich to Harvard University in Massachusetts via the new transatlantic submarine cable, making it the first worldwide time network.

In terms of the distribution of accurate time into everyday life, this is one of the most important clocks ever made.

The origin of the clock and its name take us onme year back, to 1851, and the Great Exhibition held in Hyde Park, London, where one of the world’s first ever electric clock systems was installed by Charles Shepherd of Leadenhall Street. It consisted of a central ‘motor clock’ sending regular electrical impulses to several ‘sympathetic’ dials. The Astronomer Royal saw the potential advantages of such a system and ordered one to be made for the Royal Observatory in Greenwich. The Shepherd system was supplied to Greenwich Observatory the following year and remained the basis of Britain’s time-distribution system for the next 70 years. The Shepherd Clock is named after its inventor.

 

**Public Standards of Length**

These Standards on a brass placard were first mounted outside the Observatory main gates in January 1859 to enable the public to check measures of length and officially verify measuring sticks. The stated length is the distance between the inner faces of the two D-shaped studs. 19th-century scientists would make the pilgrimage here to verify the precision of their measuring sticks.

The Public Standards measure the official lengths of the yard (one), foot (two and one), and inch (six and three). It was first placed in 1866, and the placard received a “recalibration” every 10 years by the Standard Department of the Board of Trade. The Standards were determined with an obsessive degree of craftsmanship and were designed to be used to their fullest accuracy at 15.5° Celsius. Any warmer or colder and the rulers’ thermal expansion and contraction gives off a misleading measurement.

In addition to their functional use as public backup rulers, the Standards of Length also had a very important public relations purpose. The work of the Royal Observatory was at times criticized as an expensive scientific boondoggle, lacking in military or economic purpose. The Observatory was eager to highlight the real world benefits of their work, and the plaque was one high visibility way to drive the point home.

The notion of a public Standard is very much a product of that time. As the Industrial Revolution spread across the globe, a fortuitous scientific-economy complex spurred the development of increasingly accurate commercial and navigational technology. The exact length of a yard gained increased importance in an era of interoperable parts and mass production.

 

**Ordnance Survey Bench Mark**

The small plate that appears incomplete (sorry) at the bottom right of the image (marked ‘G 1692’) is an Ordnance Survey benchmark (OSBM). It records the height of the mark above sea level. Dating from the 1940s, this particular mark is a replacement for an older one that once existed nearby. Ordnance Survey (OS) benchmarks and their heights haven’t been regularly maintained for over 40 years. The figures are a guide for the accuracy standards when benchmarks were being maintained.

The recorded height of G1692 is 154.70 feet above mean sea level, specifically referenced to the Ordnance Datum at Newlyn (Mean Sea Level at Newlyn). This value is confirmed by a plaque located right under the clock face.

As usual, my apologies for inaccuracies and mistakes.