GMT is Greenwich Mean Time, the solar time at the Prime Meridian, which is the line of zero degrees longitude between the Eastern and Western hemispheres. Greenwich Mean Time was adopted as the official time by the Railway Clearing House in 1847, and most railway companies in Britain recognized it soon afterwards. It became the basis of an international system of time zones in 1884, following the International Meridian Conference in Washington, D.C., though the standardization of this system took decades and still contains anomalies.
No one person is responsible for GMT, but perhaps the man who did most to establish its use was Dr. Nevil Maskelyne, who held the post of Astronomer Royal at the Greenwhich Observatory from 1765 to his death in 1811. Maskelyne was extensively involved in the preparation of naval charts, in which he invariably used the Greenwich Prime Meridian. In eighteenth-century Europe, most countries had their own prime meridians from which they calculated time. It is due to British maritime supremacy in the eighteenth and nineteenth centuries that the Greenwich Prime Meridian and Greenwich Mean Time came to be adopted internationally. Greenwich is close to central London and is the site of the Royal Observatory, making it the obvious place within Britain for the location of the Prime Meridian and the base for GMT.