The Prime Meridian is the reference line at 0 degrees longitude from which longitude is measured eastward and westward. The internationally recognised modern Prime Meridian passes through the United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana before reaching Antarctica. The line crosses both land and water, and the list refers to sovereign countries whose land territory is intersected by the modern Greenwich Meridian.
The Prime Meridian is not an arbitrary line running through Greenwich simply because Greenwich was historically important; its modern position is based on the International Reference Meridian used by contemporary geodesy, which differs slightly from the historic line through the Greenwich Airy Transit Circle. The distinction is important because modern satellite based positioning systems such as WGS84 define longitude through a geocentric reference framework rather than the nineteenth century astronomical meridian at Greenwich.
The United Kingdom is the first country crossed by the conventional Prime Meridian when travelling south from the Arctic region. The line passes through Greenwich in London, where the historic Greenwich Observatory became internationally associated with 0 degrees longitude. The International Meridian Conference held in Washington in 1884 selected the Greenwich meridian as the international prime meridian for longitude and timekeeping purposes.
Greenwich Mean Time subsequently became an important global reference for civil time, although modern international timekeeping uses Coordinated Universal Time, based on atomic time and adjusted through leap seconds historically used to keep it close to Earth's rotation. The Prime Meridian and Greenwich are therefore closely connected historically, but modern geodesy distinguishes the conventional Greenwich astronomical meridian from the precise reference meridian used by satellite positioning systems.
After leaving the United Kingdom, the Prime Meridian crosses France. It passes through several regions of the country before continuing southward into Spain. The line then crosses the Mediterranean Sea and reaches North Africa, where it passes through Algeria. It continues south through Mali, Burkina Faso and Togo before reaching Ghana on the Gulf of Guinea.
The route illustrates that the Prime Meridian is a longitudinal reference extending from the North Pole to the South Pole rather than a line associated only with Europe. In Africa, the line crosses a broad range of geographical environments, including Mediterranean influenced northern regions, the Sahara, the Sahel and tropical West Africa. The climate and landscapes therefore change dramatically along the meridian despite longitude remaining fixed at 0 degrees.
The Prime Meridian should be distinguished from the Equator. The Equator is a latitude line at 0 degrees latitude and divides the Northern and Southern Hemispheres, while the Prime Meridian is a longitude line at 0 degrees longitude and is used to distinguish eastern and western longitude. Together they form the reference point at 0 degrees latitude and 0 degrees longitude, located in the Gulf of Guinea off the coast of West Africa.
The intersection is at sea rather than on land. Latitude is measured north and south from the Equator, while longitude is measured east and west from the Prime Meridian. These two reference systems create the global geographic coordinate grid used in mapping, navigation, surveying and geographic information systems.
Historically, the Greenwich meridian became dominant because Britain was a leading maritime and scientific power and because Greenwich had already become widely used by navigators. By the nineteenth century, a large proportion of nautical charts and navigational practice used Greenwich as the reference for longitude. The 1884 International Meridian Conference formalised the choice for international purposes.
Twenty five nations participated, and the conference adopted Greenwich as the prime meridian for longitude and a universal day beginning at midnight Greenwich mean time. The decision did not mean that every country immediately abandoned alternative meridians, but Greenwich subsequently became the standard international reference. Modern international timekeeping has evolved considerably since then, while the geographical convention of 0 degrees longitude remains.
A technically important distinction exists between the historical Greenwich meridian and the modern International Reference Meridian. When the Earth's shape and gravitational field were modelled using modern geodetic methods, the meridian defined by the World Geodetic System 1984 does not coincide exactly with the line marked at the historic Greenwich Observatory. The difference is roughly 100 metres on the ground in the Greenwich area.
This occurs because older astronomical reference systems were based on local observations and a particular vertical reference, whereas modern geodesy uses a geocentric Earth model derived from global measurements. For everyday maps and educational geography this difference is normally negligible, but it matters in high precision surveying, navigation and geodesy.
The Prime Meridian crosses countries with very different geographical and political environments. In Europe it passes through the United Kingdom, France and Spain. In Africa it passes through Algeria, Mali, Burkina Faso, Togo and Ghana. The meridian crosses the Sahara in Algeria and Mali and then enters the more tropical environments of West Africa. It also passes through the Mediterranean Sea, the Gulf of Guinea and other maritime areas along its route.
The countries crossed by a meridian are not necessarily connected politically or economically; the line is a geometric reference rather than a physical boundary. No country is inherently more associated with the Prime Meridian simply because the line passes through it, although Greenwich has a special historical association because of the development of international longitude standards.
The modern Prime Meridian is therefore commonly described as crossing the United Kingdom, France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana. The line continues north through the Arctic Ocean and south through the Atlantic and Southern Ocean to the South Pole. The exact physical location used by modern geographic systems can differ slightly from the historic Greenwich line because of the distinction between astronomical and geodetic reference frames. The Prime Meridian remains fundamental to longitude, global mapping, navigation and timekeeping, while Greenwich remains its best known historical symbol.
