Home > Maritime Greenwich > Outstanding Universal Value >

Aerial view of the Royal Observatory building complex

The Royal Observatory

Attribute 4

The work of John Flamsteed and others permitted accurate measurement of the Earth’s movement and contributed to the development of global navigation. The choice of Greenwich for the Prime Meridian in 1884 means that the Royal Observatory sits at the meeting point of the eastern and western hemispheres on a map of the world. It is a popular photographing opportunity for visitors to straddle the line. After dark a green laser beams out northwards along the Meridian line.

The red time-ball

'Greenwich Mean Time’ has become a world standard since 1833, 1 o’clock in the afternoon (1300 hours) has been signalled by the raising and lowering of the red ‘time-ball’ at the Observatory. This was intended to be visible to ships in the Thames and the former London docks as a check for chronometers and remains a symbol of the Royal Observatory’s importance to navigation and seaborne trade.

John Harrison instrument

The Royal Observatory contains a number of instruments connected with its role as the centre of astronomical measurement, for ‘finding out longitude’ and other navigational improvements: many are those with which it was originally equipped. The timepieces constructed by John Harrison for establishing longitude are also housed there, as are many others reflecting the Observatory’s largely 19th century history as Britain’s national facility for testing the efficiency of new developments in the field of timekeeping.


  • Greenwich Hospital
  • Old Royal Naval College
  • Royal Borough of Greenwich
  • Royal Museums Greenwich
  • The Royal Parks
  • St Alfege
  • Trinity Laban
  • University of Greenwich
  • Visit Greenwich