The Royal Observatory Greenwich

Address Greenwich Royal Park, London SE10. Official Site
Contact 0181-858 4422.
Hours Mon-Sun
Last admission
Charge includes admission to The National Maritime
, The Queen's House.
Greenwich Card
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The home of Greenwich Mean Time, where you can stand with a foot in both Western and Eastern hemispheres.

Since 1833 its red timeball has fallen daily at exactly 1300 hours to enable ships to set their clocks accurately. Britain’s first telegraph cable linked it to a similar timeball in Walmer on the south-east coast for the benefit of shipping in the English Channel. In keeping with this naval tradition, a cannon is sounded daily on the deck of the Cutty Sark at 1300 hours.

Since 1884, the world has set its clocks according to the time of day on the Meridian of Greenwich, longitude 0°--an imaginary line joining the North and South Poles through the dead centre of a specialised telescope installed at the Observatory in 1851. Today, the Observatory houses Britain’s largest refracting telescope.

Following its complete renovation in 1993, you can see a unique collection of historic timepieces and navigational instruments which tell the story of time and astronomy and the origins of the Observatory itself; you can walk around Sir Christopher Wren’s Octagon Room and the apartments of the Astronomer Royal; and you can enjoy regular shows in the intimacy of the Observatory’s tiny Planetarium and visit one of the country’s few camera obscuras in the courtyard.

History of the Royal Observatory

Charles II appointed John Flamsteed as his first Astonomer Royal in 1675 to devise ways of calculating time at sea--essential for the exploration and mapping of the globe. The following year saw the completion of the Observatory--one of the oldest extant observatories--which was later extended with additional buildings.

Edmond Halley of comet fame succeeded Flamsteed and in 1784 the third director, James Bradley, discovered the nutation of the earth (the slight oscillation of its axis). In 1884 the meridian (0° longitude) at Greenwich was chosen the world's Prime Meridian, from which East-West longitude and time zones are calculated.

Greenwich Mean Time

Greenwich Mean Time (GMT) is the time on the Greenwich meridian, used as the zero for longitudinal measurement, according to the Mean Sun. The Mean Sun is an imaginary body that moves around the celestial equator with constant angular speed, making a complete circuit with respect to the vernal equinox in one tropical year. GMT was established as the world standard in 1884. In 1928 it was also given the name Universal Time. The International Time Bureau in Paris now coordinates astronomical measurements and atomic clock readings from around the world to arrive at Coordinated Universal Time.

In 1919 the Observatory organized expeditions which confirmed predictions made by Einstein's theory of relativity. After World War II, the day-to-day working of the Observatory was transferred to Herstmonceux Castle, near Hailsham in Sussex (UK) from which reports of observations continue to be published.

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