GMT

Today I went to the Docklands and from there by DLR to Greenwich to see the Observatory.

From Wikipedia (pictures are mine):

The Royal Observatory, Greenwich (formerly the Royal Greenwich Observatory or RGO) was commissioned in 1675 by King Charles II. At this time the king also created the position of Astronomer Royal, to serve as the director of the observatory and to "apply himself with the most exact care and diligence to the rectifying of the tables of the motions of the heavens, and the places of the fixed stars, so as to find out the so much desired longitude of places for the perfecting of the art of navigation." It is situated on a hill in Greenwich Park in Greenwich, London, overlooking the River Thames.

Flamsteed House, the original part of the Observatory, was designed by Sir Christopher Wren and was the first purpose-built scientific research facility in Britain. It housed not only the scientific instruments to be used by John Flamsteed in his work on stellar tables, but over time also added a number of additional duties such as the keeping of time and later Her Majesty's Nautical Almanac Office.

British astronomers have long used the Royal Observatory as a basis for measurement: four separate meridians have been drawn through the building. The basis of longitude, the Prime Meridian, established in 1851 and adopted at an international conference in 1884, passes through the Airy transit circle of the observatory. It was long marked by a brass strip in the courtyard, now upgraded to stainless steel, and, since December 16, 1999, has been marked by a powerful green laser shining north across the London night sky.

This old astronomical prime meridian has been replaced by a more modern prime meridian. When Greenwich was an active observatory, geographical coordinates were referred to a local oblate spheroid called a datum, whose surface closely matched local mean sea level, called the geoid. Several datums were in use around the world, all using different spheroids, because mean sea level undulates by as much as 100 metres world-wide. Modern geodetic reference systems, such as the World Geodetic System and the International Terrestrial Reference Frame, use a single Earth-centered oblate spheroid. The shift from several spheroids to one world-wide spheroid caused all geographical coordinates to shift by many metres, sometimes as much as several hundred metres. The Prime Meridian of these modern reference systems is about 100 metres east of the Greenwich astronomical meridian represented by the brass strip.

Greenwich Mean Time (GMT) was at one time based on the time observations made at Greenwich (until 1954). Thereafter, GMT was calculated from observations made at other observatories which were still active. GMT is now often called Universal Time, which is now calculated from observations of extra-galactic radio sources, and then converted into several forms, including UT0 (UT at the remote observatory), UT1 (UT corrected for polar motion), and UTC (UT in discrete SI seconds within 0.9 s of UT1). To help others synchronize their clocks to GMT, a time ball was installed by Astronomer Royal John Pond in 1833. It still drops daily to mark the exact moment of 1 p.m. (13:00) year round (GMT during winter and BST during summer) [1].

Today the buildings include a museum of astronomical and navigational tools, notably including John Harrison's prize-winning longitude chronometer, H4 and its three predecessors. It is also home to the 28-inch Grubb refracting telescope the largest of its kind in the UK. The Shepherd Clock outside the observatory gate is an early example of an electric slave clock. In February 2005 construction work began on a £15 million redevelopment project which will provide a new planetarium and additional display galleries and educational facilities.