The Royal Observatory in Greenwich is the home of Greenwich Mean Time (GMT) and the Prime Meridian of the world.
What is a meridian?
A meridian is a north-south line selected as the zero reference line for astronomical observations. By comparing thousands of observations taken from the same meridian it is possible to build up an accurate map of the sky.
The line in Greenwich represents the Prime Meridian of the World - Longitude 0º. Every place on Earth is measured in terms of its distance east or west from this line. The line itself divides the eastern and western hemispheres of the Earth - just as the Equator divides the northern and southern hemispheres.
Where is the meridian?
In 1884 the Prime Meridian was defined by the position of the large 'Transit Circle' telescope in the Observatory’s Meridian Observatory. The transit circle was built by Sir George Biddell Airy, the 7th Astronomer Royal, in 1850. The cross-hairs in the eyepiece of the Transit Circle precisely defined Longitude 0° for the world. As the earth’s crust is moving very slightly all the time the exact position of the Prime Meridian is now moving very slightly too, but the original reference for the prime meridian of the world remains the Airy Transit Circle in the Royal Observatory, even if the exact location of the line may move to either side of Airy’s meridian.
What is the latitude of the Observatory?
Airy's Transit Circle lies at Longitude 0°, by definition, and Latitude 51° 28' 38'' N.
The creation of Standard Time
Since the late 19th century, the Prime Meridian at Greenwich has served as the reference line for Greenwich Mean Time. Before this, almost every town in the world kept its own local time. There were no national or international conventions which set how time should be measured, or when the day would begin and end, or what length an hour might be. However, with the vast expansion of the railway and communications networks during the 1850s and 1860s, the worldwide need for an international time standard became imperative.
The deciding conference
The Greenwich Meridian was chosen as the Prime Meridian of the World in 1884. Forty-one delegates from 25 nations met in Washington DC for the International Meridian Conference. By the end of the conference, Greenwich had won the prize of Longitude 0º by a vote of 22 to 1 against (San Domingo), with 2 abstentions (France and Brazil).
There were two main reasons for the choice. The first was the fact that the USA had already chosen Greenwich as the basis for its own national time zone system. The second was that in the late 19th century, 72% of the world's commerce depended on sea-charts which used Greenwich as the Prime Meridian.
The decision, essentially, was based on the argument that by naming Greenwich as Longitude 0º, it would be advantageous to the largest number of people. Therefore the Prime Meridian at Greenwich became the centre of world time, and was the official starting point for the new Millennium.