Stand in the courtyard of the Royal Observatory in Greenwich and you can put one foot in the eastern hemisphere and one in the western. A line runs across the flagstones — brass, then a strip of illuminated fibre by night — and beside it a plaque announces that this is longitude zero, the Prime Meridian of the World, the seam from which east and west are measured and against which the planet keeps its time. Tourists queue to be photographed straddling it. Almost none of them know that if they check the satnav on their phone, it will politely disagree, placing true zero about a hundred metres away, in the shrubbery.
Latitude has a natural starting line: the equator, fixed by the spin of the Earth, equidistant from both poles. Longitude has no such gift. There is no natural east–west zero; any meridian running pole to pole is as good as any other, so the choice of a starting line is a pure human convention. For centuries it was a mess. French charts measured longitude from Paris, others from Cadiz, from El Hierro in the Canary Islands, from Washington. A ship's stated longitude depended on whose maps its captain was using — an intolerable state of affairs for a world knitting itself together by rail, telegraph and steamship.
The vote of 1884
The fix was arranged at the International Meridian Conference, held in Washington in October 1884 at the invitation of the United States. Delegates from twenty-six nations gathered to choose a single prime meridian for the whole world. Greenwich had a powerful practical claim: by then the great majority of the world's shipping already used sea charts based on the Greenwich meridian, and the observatory's astronomical work was widely trusted. The conference resolved to adopt the meridian passing through the transit instrument at Greenwich as the world's zero of longitude, and to reckon longitude east and west from it up to 180 degrees.
The vote was lopsided but not unanimous. The resolution passed overwhelmingly; San Domingo, today the Dominican Republic, voted against, and — the detail every quiz reaches for — France and Brazil abstained. French pride in the Paris meridian ran deep, and France clung to its own reference for decades, defining its time by reference to Paris mean time rather than simply naming Greenwich. The conference also laid the groundwork for a universal day and for the system of time zones the world still uses; Greenwich Mean Time became the reference against which those zones were offset.
There was a quiet piece of horse-trading behind the choice. The Americans, whose railways were already coordinating their timetables around Greenwich-based reckoning, are often said to have supported Greenwich in return for tacit acceptance of the metric system's international standing — a nudge that softened the blow for France. Whatever the private bargains, the practical case was overwhelming: adopting the meridian that most mariners already trusted spared the world the labour of redrawing every chart afloat.
The line that moved
Which brings us back to the phone in the shrubbery. The meridian marked on the Greenwich flagstones is the historical astronomical line, defined by the Airy Transit Circle telescope installed in 1851. But modern positioning does not use a telescope pointed at the stars; it uses satellites and a mathematical model of the Earth's shape, the reference frame behind the Global Positioning System. When you ask a GPS receiver for longitude zero, it sends you to a line lying roughly 102 metres east of the brass strip.
The gap is not an error in either system. The old line was defined relative to the local direction of gravity — the vertical picked out by a plumb line and a spirit level at Greenwich — and the Earth's gravity field is lumpy, tugged slightly out of true by the uneven distribution of mass beneath the surface. The satellite reference frame ignores that local tug and works from the planet's overall shape. The small deflection between the two accounts almost exactly for the hundred-metre shift. Both meridians are correct; they simply answer slightly different questions about what straight down means.
So the tourist astride the famous line is standing on a nineteenth-century decision about telescopes and plumb lines, while the device in their pocket honours a twentieth-century decision about satellites and geometry. The zero of the world turns out to be not one line but a conversation between two — and, as with so much of geography, the line we drew says as much about us as about the Earth.
Quiz nuggets
- The Prime Meridian, longitude zero, was fixed at Greenwich by the International Meridian Conference in Washington in October 1884.
- Twenty-six nations attended; the vote adopted Greenwich, San Domingo voted against, and France and Brazil abstained.
- Longitude, unlike latitude, has no natural zero, so the starting line is a human convention — before 1884, Paris, Cadiz and other meridians competed.
- The historic Greenwich line was defined by the Airy Transit Circle telescope, installed in 1851.
- GPS places longitude zero about 102 metres east of the brass line, because satellites ignore the local gravity deflection the old line was based on.