The prime meridian is the only one of these seven lines that's a decision rather than a fact. The equator and the tropics are set by how the Earth spins and leans. Zero longitude was voted on, in Washington, in October 1884, and Greenwich won because 72% of the world's shipping tonnage already used charts based on it.
Eight countries sit on it, and the shape of the list surprises people. Everyone expects the United Kingdom and France. Fewer expect Algeria to take the longest stretch, or that the line crosses Ghana twice with 31 km of Togo wedged in between.
South of Ghana it runs 6,000 km down the Atlantic without touching anything, meets the Antarctic coast at about 71°S, and carries on to the pole.
Worked out by intersecting the line with national borders, so the order is the order you’d meet them tracing the line north to south. Tap any row to fly the map to that crossing.
| # | Country | Enters and leaves | Distance |
|---|---|---|---|
| 1 | 320 km | ||
| 2 | 780 km | ||
| 3 | 340 km | ||
| 4 | 1,570 km | ||
| 5 | 760 km | ||
| 6 | 430 km | ||
| 7 | 580 km | ||
| 8 | 30 km | ||
| — | not a country | 2,060 km |
Antarctica is included in the table because the meridian genuinely runs from its coast to the South Pole. It isn't a country, and lists that say "eight countries" are right to leave it out.
Stand on the brass strip in the courtyard of the Royal Observatory, open the GPS on your phone, and it'll tell you that you're at about 0.0015° west. Roughly 102 metres off. This bothers a surprising number of visitors, and the explanation is good.
The brass line marks the Airy Transit Circle, the telescope George Airy installed in 1850 and the instrument the 1884 conference adopted. Airy's meridian was defined by plumb line, levelled against local gravity. Gravity doesn't point at the centre of the Earth, because the planet's mass is lumpy, and at Greenwich the deflection runs to about five arcseconds.
Modern longitude comes from satellites, which don't care about local gravity. They reference the Earth's centre of mass through the IERS Reference Meridian, and the two definitions differ by exactly that gravitational deflection: 102.5 m on the ground. The real zero runs through the park east of the observatory, with no monument on it at all.
The International Meridian Conference met in Washington in October 1884 with delegates from 25 countries. The case for Greenwich was practical: the Royal Navy and most merchant fleets already used it, and the United States had adopted it for railway time zones the year before.
The vote was 22 in favour, one against, two abstaining. San Domingo voted no. France and Brazil abstained, and France kept using the Paris meridian until 1911, when it adopted Greenwich by defining French time as Paris Mean Time minus 9 minutes 21 seconds, which is Greenwich without saying so.
The rival candidates were serious: Paris, Cádiz, Washington, and a proposal for the meridian through the Bering Strait, which had the attraction of putting the date line in the Atlantic where almost nobody lives. It lost on the grounds Greenwich won. Nobody's charts used it.
The African stretch has the oddest crossing on any of these seven lines. Coming south from Burkina Faso the meridian enters Ghana, runs about 10 km, crosses into Togo for 31 km, then crosses back into Ghana and stays there for 565 km to the Gulf of Guinea.
That happens because the Ghana-Togo border in the Oti region wanders either side of 0° rather than following it. Most published lists give Ghana and Togo as single entries, which hides the double crossing.
In Britain the same meridian runs from the East Sussex coast near Peacehaven, where there's a monument on the cliff, up through Greenwich and out to the Yorkshire coast at Tunstall. About 320 km of it, and the only stretch of the line in the northern hemisphere that most people have ever stood on.
The meridian leaves Africa just east of Tema, Ghana's port city, where there's a monument. The intersection of the equator and the prime meridian in the Gulf of Guinea is sometimes called Null Island, and a weather buoy sits there, the only physical thing marking the most frequently mistyped coordinate pair in the world. Faulty geocoding dumps records at 0,0 constantly.
The catch is resolution. Ten thousand points for the whole world means smoothed coastlines, islands that vanish entirely, and entry points that can sit 20 or 30 km from where a surveyor would put them, which is why distances are rounded to the nearest 10 km. The order of the countries is solid. The last digit of any single figure isn't. Crossings the data is too coarse to hold get named under the table rather than quietly folded in.