Royal Observatory, Greenwich

Charles II commissioned the Royal Observatory in 1675 to help ships find their longitude. A century of star tables, a standard time signal and an 1884 conference later, its meridian became the zero line for the world.

A practical purpose

On 4 March 1675 the king appointed John Flamsteed his Astronomical Observator, with the task of finding the longitude at sea by rectifying the tables of the motions of the heavens and the places of the fixed stars. Christopher Wren designed the building on a hill in Greenwich Park; the foundation stone was laid on 10 August 1675 and Flamsteed moved in the next year. The Observatory was Britain's first purpose-built scientific institution.

The longitude problem is one of timing: a ship that knows the local time and the time at a reference place can compute its distance east or west. Astronomers proposed lunar distances; clockmakers, led by John Harrison, built marine chronometers; the Observatory tested both. Nevil Maskelyne, Astronomer Royal from 1765, published the Nautical Almanac to support the lunar method.

The prime meridian

Airy Transit Circle, first used in 1851, defined the meridian of Greenwich as the instrument's cross-wire; its position is marked by a brass strip in the courtyard. In 1833 a time ball was installed on the roof; it still drops at 13:00 so that ships on the Thames could set their chronometers. Railways brought standardised time from Greenwich across Britain in the 1840s.

The International Meridian Conference in Washington in October 1884 adopted the Greenwich meridian as the prime meridian of the world, with 22 votes in favour. The decision was a standard, not a discovery: it was chosen because most of the world's shipping already used Greenwich charts.

Away from the smoke

London's smoke and light degraded observation, and from 1948 the Observatory's departments moved to Herstmonceux Castle in Sussex. The Royal Greenwich Observatory, as an institution, closed in 1998. The site is now part of Royal Museums Greenwich, with the Meridian Line, Flamsteed House and the Great Equatorial Telescope. The area is part of the Maritime Greenwich World Heritage Site.

What 1884 decided and what it did not

The 1884 conference chose a meridian for the world's charts and for a world system of time zones. It did not force any country to change its own time at once, and France abstained and went on using Paris time for decades. Greenwich Mean Time, based on the sun's mean motion, was later replaced by Coordinated Universal Time, kept by atomic clocks, and the prime meridian itself is now defined by the International Reference Meridian, about a hundred metres from the brass line in the courtyard, because satellite geodesy placed the modern reference frame slightly differently. The courtyard line remains the symbol, but the reference has moved to the global network.

Instruments and technologies

  • Airy Transit Circle A telescope on a horizontal axis fixed in the plane of the meridian, with a graduated circle; the time of a star's transit and its altitude give position and set the clock.
  • Marine chronometer testing Chronometers were rated against the Observatory's clocks over months, in different temperatures, to find their daily error.
  • Time ball A ball on a mast dropped at a fixed time, visible from the river, to give a visual signal that did not depend on hearing.

The people

  • John Flamsteed First Astronomer Royal; produced a catalogue of about 3,000 stars.
  • Edmond Halley Second Astronomer Royal; predicted the return of the comet that bears his name.
  • Christopher Wren Architect of the observatory building.

Timeline

  • 1675 Foundation stone laid on 10 August.
  • 1676 Flamsteed moves in.
  • 1833 Greenwich time ball installed.
  • 1851 Airy Transit Circle first used.
  • 1884 International Meridian Conference adopts Greenwich as the Prime Meridian.
  • 1948 Departments begin moving to Herstmonceux, away from London smoke and light.
  • 1998 The Royal Greenwich Observatory institution closes.

Key lessons for observation today

  • A clear public mission can set a standard The brief was longitude and time. The product that outlasted the instruments was a reference everyone could use. Reference frames are infrastructure.
  • Standards are chosen as much as found The meridian was chosen because shipping already used it. Adoption, not elegance, decides which standard wins.
  • Test the instrument against a reference The Observatory's chronometer trials rated every candidate under the same conditions. A modern qualification programme does the same for every instrument.

Sources

  • Royal Museums Greenwich, history of the Royal Observatory
  • Wikipedia, Royal Observatory, Greenwich
  • Wikipedia, Edmond Halley
  • UNESCO World Heritage Centre, Maritime Greenwich