Prime meridian

What it is and why it runs through Greenwich

Latitude has a natural zero, the equator, but longitude doesn’t: any meridian could be 0°. For centuries each country used its own (Paris, Cádiz, Washington…), until in October 1884 the International Meridian Conference in Washington, D.C., picked the one through the Royal Observatory in Greenwich, London.

The vote was 22 in favor, one against (San Domingo, today’s Dominican Republic) and two abstentions: France and Brazil. France kept using the Paris meridian and didn’t adopt Greenwich for timekeeping until 1911, and even then without naming it: its legal time was “Paris mean time, retarded by 9 minutes and 21 seconds.”

It runs from pole to pole and is 12,430 mi (20,004 km) long. Together with the 180° meridian on the far side of the world, it splits the Earth into the Eastern and Western Hemispheres.

Today’s line runs 102 meters from the observatory’s

In the observatory courtyard, a brass strip marks the meridian of Airy’s Transit Circle, the telescope that fixed it in 1851. But stand on it with a GPS and you won’t see longitude 0°: today’s zero line, the IERS Reference Meridian used by GPS and every modern map (WGS84), runs about 102 m (335 ft) to the east.

The ground didn’t move. The telescope was aligned with the local vertical, the direction of a plumb line, and a plumb line doesn’t point exactly at Earth’s center: the surrounding rock pulls it slightly aside. At Greenwich that deflection is worth 5.3 seconds of longitude, and at that latitude 5.3″ is about 102 m. Malys and colleagues explained it in 2015 in the Journal of Geodesy.

Where it runs: the countries, north to south

From the North Pole it crosses the Arctic Ocean and the North Sea, enters England, cuts through East London and heads out into the English Channel. Then it runs through France, Spain, Algeria, Mali, Burkina Faso, Togo and Ghana, crosses the Gulf of Guinea and the South Atlantic, and ends in Antarctica, in Queen Maud Land, which Norway claims.

All told, 6,720 km runs over land (34%), 82 km over lakes and 13,202 km over the sea. The longest stretch over land is in Antarctica, 1,284 mi (2,066 km) across the ice; outside it, the longest is in Algeria, 963 mi (1,550 km), almost all of it across the Sahara.

Where it meets the border between Togo and Ghana, the line zigzags from one to the other: Togo, Ghana, Togo again and finally Ghana. In Ghana it crosses Lake Volta, one of the largest reservoirs in the world, for 51 mi (83 km). Niger falls 10 mi (17 km) short: its border doesn’t reach the meridian.

Towns on the line, country by country

These are the towns and cities whose center lies within 3 km (1.9 mi) of the prime meridian, according to GeoNames: up to the ten most populous per country, listed north to south. In Algeria and Mali the line crosses mostly empty desert, so the closest town is listed instead. For Spain, the list shows the municipalities with more than 10,000 people that the line actually crosses, from official boundaries.

In England the line comes ashore in Holderness, near Withernsea, passes Cleethorpes, Louth and Boston, crosses East London (Chingford, Walthamstow, Stratford and, of course, Greenwich) and leaves at Peacehaven, on the south coast. In Spain it crosses 57 municipalities, from Fanlo in the Pyrenees to Altea on the Mediterranean.

Cities near the prime meridian

The closest large city is Tema (Ghana), the country’s main port, 0.7 mi (1.1 km) from its center. Central London lies 5.1 mi (8.2 km) to the west, and Accra, Ghana’s capital, 15 mi (24 km) away.

Greenwich Mean Time and solar noon

The sun crosses each meridian 4 minutes later for every degree you go west. That’s why solar noon in a city is earlier than in Greenwich if it lies to the east, and later if it lies to the west, whatever its clocks say.

Greenwich Mean Time (GMT) was the world’s time reference for a century. Today official time is based on UTC, kept by atomic clocks; in winter, UK time matches it. Spain, almost entirely west of Greenwich, has used Central European Time, an hour ahead, since March 1940.

Prime meridian facts

Where the prime meridian meets the equator, at coordinates 0, 0, there’s only sea: the spot lies in the Gulf of Guinea, 386 mi (621 km) from the coast of Ghana. Mapmakers jokingly call it “Null Island,” because that’s where data with blank coordinates ends up.

The meridian crosses London’s eastern boroughs rather than its center, which lies 5.1 mi (8.2 km) to the west of the line.