Latitude is given to you. The equator is where the Earth’s rotation puts it and the poles are where the axis comes out, so nobody has ever had to negotiate about latitude.
Longitude has no such anchor. Every line from pole to pole is exactly like every other one, so the question of which one counts as zero has no answer in nature. It is a decision, and for most of history each country made it for itself.
In October 1884, in Washington, twenty five nations sat down to make it once.
The Mess Before
Before 1884 there were dozens of prime meridians in active use. The British used Greenwich, the French used Paris, the Spanish used Cádiz, the Russians used Pulkovo, the Portuguese used Lisbon. Cartographers used whichever suited the map. Some charts used Ferro in the Canaries, a convention inherited from Ptolemy’s edge of the known world.
For a navigator this was a practical hazard. Your chart’s zero and your almanac’s zero and your chronometer’s reference had to agree or your position was wrong by the difference, and a mistake of a couple of degrees of longitude puts you on rocks. The whole apparatus of solving longitude by carrying accurate time to sea only works if everybody agrees which time you are carrying.
The pressure that finally forced the issue came from railways rather than ships. A railway timetable across a continent is impossible when every town keeps its own local noon. On 18 November 1883 the North American railroads simply imposed four standard time zones by agreement among themselves, and the day was known as the Day of Two Noons because clocks were reset at midday. Those zones were defined by offsets from Greenwich, which meant the United States had, in effect, already voted with its clocks before any conference was held.
Three Weeks in Washington
President Chester A. Arthur convened the International Meridian Conference at the invitation of the United States. It ran from 1 to 22 October 1884, with forty one delegates representing twenty five countries.
The prime mover behind the idea was Sandford Fleming, a Scottish born Canadian railway engineer, who had missed a train in Ireland in 1876 because of an ambiguous timetable and had been campaigning for a universal time ever since. He proposed standard zones and a single global day, and he lobbied for years to get governments into a room.
The conference passed seven resolutions. The important ones were the first three.
- A single prime meridian for all nations, replacing the national ones. Passed unanimously
- That meridian to be the one through the transit instrument at Greenwich Observatory. Passed 22 votes to 1, with two abstentions. San Domingo voted against. France and Brazil abstained
- A universal day beginning at mean midnight at Greenwich and counted through 24 hours, to be used for purposes where it is convenient, without displacing local time for local affairs
Note what is not in there. The conference did not create time zones. It defined a universal day and a common zero, and left each country to organise its own clocks, which they then did over the following fifty years at their own pace.
Why Greenwich Won
Not because of any astronomical merit. The argument was traffic.
By the 1880s something like seventy per cent of the world’s shipping tonnage was already navigating on charts referred to Greenwich, and the British Nautical Almanac was the most widely used set of astronomical tables at sea. The American delegation supported Greenwich for the same reason: their railways and their own coast survey already depended on it.
So the choice was a ratification of existing practice, and the practical case was overwhelming. Choosing any other line would have meant reprinting every chart and every almanac in use, at enormous cost, to gain nothing.
The Greenwich line itself was not an abstraction. It was defined by a physical object: the Airy Transit Circle, installed at the Royal Observatory in 1851, whose telescope swung in a fixed north south plane. Zero longitude was wherever that instrument pointed. A brass strip was later set into the courtyard along it, and people have been photographing themselves straddling it ever since.
France, and the Twenty Seven Year Sulk
The French position deserves more respect than it usually gets, because their objection was about principle rather than pride.
France argued for a neutral meridian, one running through no great power, and various candidates had been floated over the years including the Azores, the Bering Strait and the Great Pyramid. The reasoning was the same reasoning that had produced the metric system a century earlier: a universal standard should derive from nature or from neutrality, not from the commercial dominance of one empire. The metre had been defined as a fraction of the Earth’s own quadrant precisely so that it would belong to nobody.
Having lost the vote, France declined to adopt Greenwich. French legal time remained Paris Mean Time.
The resolution, when it finally came in 1911, is one of the great pieces of diplomatic face saving. France defined its legal time as Paris Mean Time retarded by nine minutes and twenty one seconds. That quantity is exactly the longitude difference between Paris and Greenwich expressed in time. French legal time became Greenwich Mean Time, calculated to the second, without the word Greenwich appearing anywhere in the statute. It stayed written that way until 1978.
The Line You Stand On Is the Wrong One
Here is the detail that makes the whole thing quietly funny.
Stand on the brass strip at Greenwich with a phone and the satellite navigation will report that you are about 102 metres west of zero longitude. The tourists queue for a photograph on a line that modern instruments say is in the wrong place.
Neither is a mistake. They are two different definitions of vertical.
The Airy Transit Circle was levelled with respect to local gravity, using a plumb line and a mercury basin, so its meridian plane follows the direction gravity actually points at Greenwich. Satellite systems use a mathematically smooth reference ellipsoid, and its normal at that point is not quite parallel to the local plumb line, because the Earth’s mass is not distributed evenly. The angle between the two, called the deflection of the vertical, is a few arcseconds. Projected onto the ground, a few arcseconds is about a hundred metres.
So the meridian has not moved. The definition of down has been refined. The same system that reveals the discrepancy also has to correct for the rate at which its own clocks run in orbit, which would have been an entirely unimaginable objection to raise in 1884.
What the Conference Was Really About
The nineteenth century kept running into the same problem in different forms: the world was becoming connected faster than its conventions could keep up. Britain had already deleted eleven days from its calendar in 1752 to stop being out of step with its neighbours. The metric system was an attempt to make measurement portable. Standard time was an attempt to make timetables possible.
The prime meridian is the purest case, because there is no fact of the matter at all. There is no true zero longitude anywhere in the universe, and there never will be. What the delegates achieved in three weeks in Washington was to agree on an arbitrary thing, which is much harder than agreeing on a true one, and it has held for a hundred and forty years.
Every satellite fix, flight plan and timestamp on Earth is still measured from a telescope mounting in south east London that was aligned with a bucket of mercury.
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