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Royal observatory, Greenwich

Flamsteed House, the original part of the Observatory, was designed by Sir Christopher Wren probably with the assistance of Robert Hooke and was the first purpose-built scientific research facility in Britain. It was built for a cost of £520 (£20 over budget) out of largely recycled materials on the foundations of Duke Humphrey's Tower, which resulted in the alignment being 13 degrees away from true North, somewhat to Flamsteed's chagrin.

It housed not only the scientific instruments to be used by Flamsteed in his work on stellar tables, but over time also incorporated a number of additional responsibilities such as the keeping of time and later Her Majesty's Nautical Almanac Office.

Two clocks, built by Thomas Tompion, were installed in the 20 foot high Octagon Room, the principal room of the building. They were of unusual design, each with a pendulum 13 feet (3.96 metres) in length mounted above the clock face, giving a period of four seconds and an accuracy, then unparalleled, of seven seconds per day.

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This old astronomical prime meridian has been replaced by a more modern prime meridian.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. Thus on current definitions the brass strip now marks about 5.4 arcseconds West.

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).

All info on internet.

Detail of the iron spiral staircase that leads up to the Upper Computing Room.