The original site of the Royal Greenwich Observatory (RGO), which was built as a workplace for the Astronomer Royal, was on a hill in Greenwich Park in Greenwich, London, overlooking the River Thames. The Prime Meridian, to which longitude refers, went through the observatory. It is marked by a brass strip in the courtyard and, since 2000, a powerful green laser shining north across London and Essex.
The observatory was commissioned in 1675 by King Charles II. Flamsteed House (1675-76), the original part of the Observatory, was designed by Sir Christopher Wren and was the first purpose-built scientific research facility in Britain.
In 1948 RGO moved to Herstmonceux Castle, near Hailsham in East Sussex in search of clearer skies. The Isaac Newton Telescope was built there in 1967, but was moved to La Palma in Spain in 1979. In 1990 the RGO moved again, to Cambridge. Following a decision of the Particle Physics and Astronomy Research Council it closed in 1998. The HM Nautical Almanac Office was transferred to the Rutherford Appleton Laboratory after the closure. Other work went to the UK Astronomy Technology Centre in Edinburgh. The castle now houses the International Study Centre of Queen's University, Kingston, Canada and the Observatory Science Centre.
Greenwich Mean Time was at one time based on the time observations made at Greenwich, before being superseded by Coordinated Universal Time. While Greenwich no longer hosts a working astronomical observatory, it remains a centre of excellence for modern astronomy.
A time ball still drops daily to mark the exact moment of 1pm (13:00) - this was installed by Astronomer Royal John Pond in 1833. There is a good museum of astronomical and navigational tools, notably including John Harrison's prize-winning longitude chronometer, H4.