Royal Greenwich Observatory
The Royal Greenwich Observatory, originally built as a workplace for the Astronomer Royal, is located on a hill in Greenwich Park in Greenwich, London, overlooking the River Thames. The Prime Meridian, to which longitude refers, goes 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.
Greenwich Mean Time was at one time based on the time observations made at the Royal Greenwich Observatory, before being superseded by Coordinated Universal Time. While the Royal Greenwich Observatory no longer hosts a working astronomical observatory, it remains 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.
The Royal Greenwich Observatory is now called the Royal Observatory Greenwich, and is part of the National Maritime Museum.
