The Prime Meridian, the line between the Eastern and Western hemispheres at a longitude of zero degrees, is situated in Greenwich primarily because of the maritime power of the British empire in the nineteenth century. Over the course of the nineteenth century, long-distance travel by land and sea increased sharply. Great Britain, an island country with a worldwide empire, was the greatest naval power, with a large merchant navy, in addition to the Royal Navy's warships. It was John Harrison, an Englishman who, in the eighteenth century, had solved the problem of calculating longitude at sea, and, by the middle of the nineteenth century, most naval charts were drawn up with Greenwich at the center. Within Britain, Greenwich was the obvious choice, being fairly close to the center of London and the location of the Royal Greenwich Observatory, the scientific center of the Royal Navy.

The Prime Meridian is important for travel and timekeeping because it provides an international central point for cartography, navigation, and chronology. Before time zones were established at the end of the nineteenth and the beginning of the twentieth century (crucially by agreement at the International Meridian Conference in Washington DC in 1884), most towns and cities in the world decided for themselves on timekeeping standards. This lack of organization which would cause chaos in the era of international mass transit means that it is now more crucial than ever that there is a single international basis for navigation. The Prime Meridian is so called because it is the first meridian in both directions, as the International Date Line is the last.