Why the Greenwich meridian is the reference meridian ?

The term GMT, Greenwich Mean Time, dates back to 1675. It was only in 1880 that Greenwich was adopted as the world's legal time. Greenwich is now the global reference time, but where does this standard come from? We'll explain everything in this article.

GMT, “Greenwich Mean Time”: meaning 

The term GMT, short for "Greenwich Mean Time," refers to the mean time at Greenwich and dates back to 1675. At that time, establishing a mean time in a given village was intended to help sailors determine their longitude at sea. The Royal Observatory Greenwich, located in this London district, helped British sailors calculate their longitude relative to the Greenwich meridian. The acronym GMT designates Greenwich Mean Time, the meridian passing through the Royal Observatory Greenwich in England. While many railway companies gradually adopted Greenwich Mean Time, such as the Railway Clearing House in 1847, it was not until 1880 that it was officially accepted as the legal time in the United Kingdom.

In everyday language, the acronym GMT stands for mean solar time at the Greenwich meridian. This mean solar time served as the world's time reference for a long time before being gradually replaced by UTC, Coordinated Universal Time. The acronym GMT is still commonly used today as a synonym for time zone. In watchmaking, the acronym GMT is used to designate... watches which indicate on the dial, thanks to an additional hour hand, a second time zone, that is to say the exact time of a city located in another time zone, without it necessarily being that of Greenwich.

The birth of Greenwich Mean Time

Average time From a fishing village with a reference time zone, Greenwich Mean Time was born. However, at that time, no real division of the world into time zones had been proposed. Each nation defined its time in relation to the sun's path across the sky: when it reached its highest point, it was noon. Many cities then put themselves forward to become the world's prime meridian, notably London with the Greenwich district and Paris with the Saint-Sulpice district.

Finallyent, August 2, 1880, Following negotiations, Greenwich Mean Time was adopted as the world's legal time. Since then, Greenwich Mean Time has served as the world's time reference for most of the 20th century.^th century. If London won the battle of the timetable, it seems that the City of Lights has nevertheless won the war of watchmaking, as its walls contain many renowned master watchmakers, among them Charlie Paris!

The Greenwich Meridian put to the test of science

But think again! Recent studies have made some surprising revelations: The prime meridian (conventionally considered to mark zero longitude) is shifted 102.5 meters westward from where it should be. Therefore, Greenwich is not located at 0° longitude but at 00° 00′ 05.3″. Today, the correct term for GMT is UTC. UTC is even more precise than GMT! Discover more by visiting our watch blog, Where does UTC time come from and how is it calculated?