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Royal Observatory, Greenwich

The Royal Observatory at Greenwich was founded in 1675 by Charles II, with John Flamsteed appointed as the first Astronomer Royal. The observatory's work was to track the moon and stars, and to eventually publish a record of where the moon would be in relation to the stars on any given day. This was not done out of scientific curiosity, but as a means of improving navigation at sea.

In the seventeenth century there was an urgent need to work out a way of calculating longitude, that is positions east or west of a fixed point, while at sea and out of sight of land. Many sailors had died because they could not accurately plot their position. The British government, knowing the importance of dependable sea navigation to a country which relied on seabourne trade, were desperate to find a solution. The nature of the problem was well understood. It was known that for every fifteen degrees travelled eastward, local time moves forward one hour. Similarly for every fifteen degrees travelled westwards, time moves backward one hour. Therefore, if a ship's navigator had local time for two points, he could use the time difference to calculate longitude.

The initial idea was to try and fix these two different times using moon and star positions. The moon's position in relation to set stars could - with a lot of complicated maths - be used to calculate local time back at a ship's home port. The ship's local time at sea could then be calculated by observing the sun. The difference between these two times would then allow longitude to be calculated.

With a huge amount of work, this method of working out longitude did eventually work, after a fashion. But it was not easy, and at times when the sky was obscured, impossible. Meanwhile a clockmaker named John Harrison was trying to solve the problem of longitude by creating a clock that would tell the time accurately at sea. One clock would be set to local time at a ship's home port, and one would be set to a ship's local time at sea, worked out by observing the sun. Longitude could then be easily and quickly calculated using the difference in time between the two clocks. But production of a clock able to tell time accurately at sea represented a huge technical challenge. Sea going clocks had to endure constant movement, wide temperature ranges and moisture laden air. In 1730 Harrison submitted plans for a sea clock to the Board of Longitude, which was offering a £60,000 prize for an idea that would allow accurate calculation of longitude. Work started on the first sea clock, Harrison Number 1, or H1, and continued for seven years. Harrison ran into opposition from astronomers working at the Royal Observatory, who looked down on his "mechanics" status. But with grants from the Board of Longitude, a sea clock design was slowly refined. Finally in 1760 Harrison produced his revolutionary H4, much smaller and neater than the three models which had preceded it. H4 was taken to sea and used to successfully calculate longitude.



The Royal Observatory from Greenwich Park

Eventually it was the successors of Harrison's clocks that became standard pieces of equipment at sea allowing calculation of longitude. The question then arose as to where longitude should be calculated from. Where was the imaginary "home port" for all the world's ships, from which longitude lines could be plotted on standardised charts? Nature sets the first meridian of latitude at the equator, where the sun travels through the sky directly over head. Points north and south can then be plotted by observing the position of the sun each side of this central line. But longitude is a man made construct. Anywhere in the world could be chosen as a Prime Meridian from which all other lines of longitude east or west are calculated. Since work on moon and star charts had been done at Greenwich, charts had already been produced plotting positions east and west of Greenwich. In this way Greenwich provided an unofficial prime meridian. In 1884 the International Meridian Conference held in Washington DC made this common practice official. The Greenwich meridian became the Prime Meridian of the world. Days begin at Greenwich. GMT even extends into outer space, where it is called Universal Time, and is used by astronomers to time their observations.

Accurate clocks weren't only relevant to navigation at sea. Precise time keeping was also an important feature of the Industrial Revolution. Mechanical processes required precise ordering of time. People no longer followed the sun and the seasons through an agricultural year. Days were now divided up into organised time to suit an industrial world. This dependence on accurate timekeeping has only increased in the computer age, since computers require precise time keeping to work effectively. A poignant comparison between modern time keeping precision and methods of the past is provided by the Observatory's "time ball". The time ball dropped down a short pole daily at 1pm on the Royal Observatory roof. Its original purpose was to provide a time signal to ships on the Thames. Today it serves more as a time signal to tourists, showing how times have changed.



"Time Ball" on the observatory roof

The Harrison clocks which had represented such a milestone suffered a sad fate. They were taken from their creator and stored at the Observatory for "testing" by Harrison's arch enemy, fifth astronomer royal Nevil Maskelyne. Testing actually consisted of hiding away the annoying contraptions which had undermined all that painstaking effort plotting of moon and star positions. These incredible clocks were rescued from a state of decay by Lieutenant Commander Robert T. Gould of the Royal Navy. Starting his unpaid work in 1920, and with no previous training, he worked quietly on the clocks for many years, restoring them, and in the process restoring himself from a breakdown suffered at the beginning of the First World War. H1, H2, H3, and H4 can now be seen on display at the Royal Observatory. H1, 2 and 3 are all working, their huge and intricate mechanisms open for visitors to view. H4, to avoid wear, is never wound up. Read more about the story of Harrison's clocks in Dava Sopel's book Longitude.


The Royal Observatory is a fascinating place to visit; there's Christopher Wren's observatory building, displays on the history of timekeeping, and a nearby Planetarium giving daily displays on the stars and planets. Greenwich Park in which the observatory stands is itself an historic environment. Also within walking distance is the National Maritime Museum, the Wren designed buildings of the Old Royal Naval College, and the old tea clipper Cutty Sark. But for me the greatest thrill of the observatory is the feeling that you are standing at some kind of rare crossing point. The Prime Meridian might be man made, but by some strange coincidence, the area of Greenwich has a very long history as a border between east and west. In the ninth century Britain was divided in two between a Saxon west and a Danish east. The border between these regions reached a crucial junction at the confluence of the rivers Thames and Lea. This point lies directly opposite the Observatory today on the Prime Meridian. It seems that an important meridian line has run through this part of the world for a long time. The coincidences seem intriguing. As well as being the place where east meets west, the Royal Observatory is also the place where the old world crossed into the modern world.


Opening Times: Please use contact details below.

The observatory tends to be busiest between 1pm and 4pm at weekends and quiet before 11.30am.


Address: The Royal Observatory, Blackheath Avenue, London SE10 8XJ

Directions: The observatory is in Greenwich Park. Car parking is difficult, so it is best to use public transport if possible. There is an Underground station at Greenwich. Alternatively there are regular river boat sailings from central London. This is a great way to get to Greenwich. Go to for details of sailings. Click on "River". Click here for an interactive map centred on the Royal Observatory.

Access: The Royal Observatory is at the top of a steep hill. Access into the observatory courtyard, the gift shop and the Meridian Line display is level, but the building itself is difficult for those with mobility problems. For those with sight problems there are Braille guides, large print guides and tactile plans.


telephone: 020 8858 4422

information line: 020 8312 6565


web site:





©2006 InfoBritain (updated 01/13)