The meridian line is an imaginary line which runs from the North Pole to the South Pole. By international convention it runs through the main telescope at the Royal Observatory in Greenwich, England. As the Earth's crust is moving slightly all the time, the exact position of the Meridian shifts
slightly too.

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, it became necessary to set an international time standard.

The Greenwich Meridian was chosen as the Prime Meridian of the World in 1884 at the International Meridian Conference, where forty-one delegates from 25 nations met in Washington DC. 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 two abstentions (France and Brazil). Algeria, a French dependent, objected to the phrase "Greenwich Mean Time" and proposed "Paris Mean Time diminished by 9 mins 21 secs" instead. 

Why Greenwich?

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 per cent 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 will be the official starting point for the new Millennium.