In October 1884 and following fierce discussions, particularly between the French and British delegations, the International Meridian Conference adopted in Washington the Greenwich meridian as the prime meridian for the measure of Longitude.
Greenwich is situated in the southeast suburbs of London where the Royal Observatory bearing the same name was located until 1946.
The Greenwich meridian marks also the universal time or GMT (Greenwich Mean Time) and the starting point of every time zone in the world. As days last 24 hours, the globe is divided up into 24 time zones which longitudes differ of 15° (one hour). The centre of each time zone governs the time of each of them. GMT is mean solar time calculated at midday. The GMT system was adopted at an international level on January 1st, 1885.
However, time zones follow the borders of nation states, and some countries are not in step with universal time. For example, China has adopted a single time for its entire territory, while in India there is a time difference of five and a half hours.
On January 1st, 1982 and following a decision made by the International Telecommunication Union (ITU), universal time became UTC (Universal Time Coordinated), to correct universal time in which the duration of the day is not constant throughout the year due to the earth’s rotation on its axis. By international agreement, UTC time is Greenwich Mean Time, although their measurement differs :
GMT is measured from noon whereas UTC is measured from midnight. UTC is the legal basis of time in the world.
UTC itself is derived from International Atomic Time (IAT) from which it differs only by a whole number of seconds, currently 32. These intercalary seconds are added at the initiative of the International Earth Rotation Service to ensure that, on average as the years go by, the sun is at the Greenwich meridian at 12:00:00 UTC to the nearest 0.9 second.
Legal time is that in force in a country. Around 70 countries currently use summer time (more precisely Daylight Saving Time). During certain periods of the year, generally from April to September in the northern hemisphere and from October to March in the southern hemisphere, they add one hour. Japan is the only industrialized country not to use it.
The International Date Line is marked near the Greenwich's opposite meridian. Greenwich was preferred to Paris as prime meridian because of the fact that, among others, its opposite meridian does not cross any land.