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Greenwich Village has now been absorbed in the general London sprawl, but still retains a character of its own. From the river the restored three-masted tea clipper Cutty Sark is a prominent landmark, and further east the extensive buildings of the National Maritime Museum and Royal Naval College catch the eye. Behind the museum rise the slopes of Greenwich Park, and on the top of the rise stand the famous Royal Greenwich Observatory. 
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The Royal Greenwich Observatory is over 300 years old, the building being founded in 1675. It has a long and distinguished history, and although the astronomers have now moved to the clearer air of Herstmonceux in Sussex, much remains to interest the visitor. The old building is called Flamsteed House, and now contains a display of old astronomical apparatus. On the turret on the roof is a time-ball which drops at 1 o’clock every day. It was originally installed to allow ships to set their chronometers. The National Maritime Museum gives a comprehensive survey of British naval history, and contains displays of chronometers and various old navigational instruments, together with documents, photographs and other material of great historical interest. At the Observatory there is also a brass strip set into the ground marking ‘the prime meridian’ – longitude 0°. Most visitors like to be photographed standing with a foot on either side!
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The observatory
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Latitude and longitude
The choice of Greenwich as the place through which the prime meridian (0°) passes was made in 1884. Events leading up to this decision extended back over many years, and make interesting reading. The publication of the British Nautical Almanac in 1767 meant that Greenwich began to be used as the prime meridian on the maps and charts of many nations from the late 1700s.
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Latitude and longitude
Investigate Use an atlas to discover the latitude and longitude coordinates of your own town or city.
Finding longitudes is a much more difficult problem. As earth rotates on its axis once every 24 hours, a difference in longitude corresponds to a difference in measured time. The relationship between longitude and time is illustrated in the accompanying diagram. So 24 hours corresponds to 360°, four minutes to 1°. Today, with accurate time-keeping and radio communication, there is little problem in determining longitudes. But without instant communication, how would one determine the time at some place on the other side of the world? One method would be to record the time of an eclipse seen simultaneously at both places. Later, the times recorded at the two places could be compared and the difference in the longitudes found. This method would of course be no help to the captain of a ship lost at sea! The method also requires instruments that keep accurate time.
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Pendulum clocks A swinging pendulum possesses a natural period of oscillation, because its motion is controlled by gravity which is virtually constant. It is known that the time T of a complete double swing of a pendulum of length l is given by T = 2 A clock provided for the Royal Observatory in 1750 was fitted with a ‘grid-iron’ pendulum devised by John Harrison. Harrison had found that for a given temperature change, the ratio of expansion of brass to steel is approximately 3:2. He then devised the system illustrated, where expansion by the steel rods is compensated by expansion of the brass rods. A different idea was to use a ‘bottle’ of mercury for the bob of the pendulum; properly designed, any expansion in the shaft of the pendulum is compensated by a rise in the level of the mercury.
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(l/g), where g stands for the acceleration caused by the force of gravity: 980 cm/sec2. The accuracy of the pendulum clock then depends directly on ensuring that the length of the pendulum remains constant. In particular, the period of the pendulum should not be affected by changes of temperature.|
Around 1700 there were a number of tragic shipping disasters, a cause of which was the low standard of accuracy of navigational practice at the time. Up until then, the longitudinal position of a ship was generally determined by the lunar distance method, in which the position of the moon was determined using a predecessor of the sextant. But such methods were often inaccurate. So in 1714 the British House of Commons proclaimed a reward of £20 000 to anyone who could provide a means of accurately determining longitude at sea.
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Chronometers
Since the longitude corresponds to the overhead position of the sun, an accurate chronometer allows us to find longitude at sea. John Harrison (1693–1776) eventually received the whole of the 1714 Longitude award, but not without difficulty. His first marine time-keeper (above) kept good time, but Harrison constructed four more. The last two were large watches which performed very well. London watchmaker, Larcum Kendall, made an authorized copy of Harrison’s fourth time piece (left) which was successfully tested during Captain James Cook’s second voyage, never being more than 8 seconds out in its daily reckoning.
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