NHK World Heritage 100 Series (en anglais)

Texte narratif (en anglais)

The Challenge to Find Longitude/Maritime Greenwich

Greenwich is located in east London on the River Thames. The Royal Observatory was established here in 1675. Astronomical observation was believed to be an essential key for the development of a navigation system that would help the British Empire gain control over the seas. Long-distance voyages were fraught with danger at the time. Ships often lost their exact location due to a lack of knowledge in plotting longitude. Both latitude and longitude have to be measured correctly to identify the exact location of a ship. Latitude could be calculated by the position of certain stars but correct longitude was difficult to figure out. To calculate longitude, people needed to know the local time both on board ship and at their home port.

The earth rotates 360 degrees in 24 hours. So one hour - a twenty-fourth of 360 degrees - is 15 degrees longitude. A clock indicating the correct time at the homeport would allow people on the ship to identify their longitude by the time difference between where they were and the port’s time. To establish local time at sea, a clock had to be adjusted to noon when the sun reaches its highest position. Then the time difference between the homeport and the ship can be replaced with longitude, at the ratio of 15 degrees per hour. As long as the correct time difference is calculated it is possible to locate oneself on a map regardless of how long or in which direction the ship travels. The problem was that all clocks were pendulum-operated and it was impossible to know the exact time at homeport on sea-going vessels. John Harrison, a Yorkshire carpenter, invented a clock which showed the correct time in Greenwich and was useable on any of the seven seas.

Greenwich was defined as zero degrees longitude at the International Meridian Conference held in 1884. The moment the sun crosses this meridian is considered noon. This is how Greenwich Mean Time was established. Greenwich became the centre of world time and made a significant contribution to the prosperity of the British Empire.