Local time is determined by the Sun’s place in relation to the local meridian, with noon being when it crosses that line. Since the Earth turns once in twenty-four hours, noon will be one hour later every fifteen degrees to the west.
This proved to be a problem with the emergence of railroads. In 1847, the British railways agreed to keep Greenwich time. In North America, time was standardized on the main cities along a line. In 1878, Sandford Fleming advocated the use of time zones. By 1884, many railways adopted the system, with Greenwich as the prime meridian.
In 1884, astronomers agreed at a conference in Washington to accept the Greenwich system. However, there were some who preferred that Paris or Berlin be the prime meridian. Most countries adopted the system very soon after. It took France another twenty years to accept it.
The new time became known as Greenwich Mean Time, with time anywhere in the world being measured from Greenwich. Today, the system is often referred to as Coordinated Universal Time. The time zones are not exactly 15° wide. They may be more or fewer because of political boundaries or areas of concentrated population. Approximately 180° east or west of the prime meridian (0°) lies the International Date Line. On crossing it, there is a day either gained or lost.