Every year, thousands of visitors to the Royal Observatory in Greenwich stand astride a metal strip in the courtyard, one foot in the western hemisphere, one in the east, and have their photo taken at longitude zero.
The photos are almost correct. The strip is off by about 102 metres.
The line tourists stand on
The strip in the courtyard marks the Greenwich Meridian: the line that passed through the Airy Transit Circle, the telescope the seventh Astronomer Royal, George Biddell Airy, built in 1850. From 1851, the crosshairs of that instrument defined longitude zero. When 25 nations met in Washington in October 1884 and voted on a single meridian for the world, the line through Airy's telescope won, with France abstaining and keeping Paris on its maps for decades afterward.
For ninety years, every ship's chart and every railway timetable anchored itself to a telescope on a hill in south-east London. In 1984 the role passed to the IERS Reference Meridian, the zero of the satellite-based system the whole world now navigates by. And at the latitude of Greenwich, that new zero runs about 102 metres east of the old line. Past the courtyard wall. Into the park. Where there is no strip, and nobody queues.
So the classic visitor photo has a small error in it: the true longitude zero is over your left shoulder (if you face north), somewhere among the trees.
Why the line is "wrong"
The old explanation, repeated for years, was that the satellite systems simply placed their zero somewhere else, or that timekeeping errors crept in. Neither holds.
The real answer is in how Airy built his telescope. It was aligned using a basin of mercury, which is to say it was aligned to the local vertical: the direction gravity actually pulls at Greenwich. But gravity at Greenwich does not pull straight toward Earth's centre, because the planet is not a perfect sphere and its mass is not evenly spread. The local vertical is tilted, very slightly, from the ideal line a satellite-based coordinate system assumes.
A 2015 analysis by Stephen Malys and colleagues checked the offset against this "deflection of the vertical" and it matches, within measurement uncertainty. No global rotation of longitudes happened; nobody mislaid a hundred metres. The 1884 meridian and the modern one are both correct. They are correct in two different ways, and at Greenwich those two ways differ by 102 metres.
The other wrong thing people say about the line
Two smaller corrections, since they come up in the same conversations:
- The line is not brass anymore. The original brass strip has been replaced with stainless steel. (At night since December 1999 the meridian is also marked by a green laser shining north over London, which is the only meridian marker most Londoners will ever see.)
- It is not even the only historic meridian at the Observatory. Halley's and Bradley's transit rooms sit nearby, each with their own earlier zero. The famous strip just marks the one that won.
Why it matters, and why it does not
Nothing practical changed. Ships navigate by satellite; the offset of a courtyard strip affects nobody's arrival time. What the 102 metres measure is a boundary between two ways of pinning down the world: one anchored to a plumb line in a particular room in London, one anchored to Earth's rotation axis as satellites see it. The first is a place. The second is an abstraction. The strip stands on the place; the maps now agree on the abstraction; and the 102 metres between them is the honest record of the difference.
The queue for a photo on the line is still worth it. Just know that the actual zero, as your phone defines it, is a quiet stretch of parkland with no queue at all.
Sources: Royal Museums Greenwich, "What is the Prime Meridian - and why is it in Greenwich?"; S. Malys et al., "Why the Greenwich meridian moved", Journal of Geodesy (2015); Derek Howse, "Greenwich Time and the Longitude" (1997). Walked and checked on site during my Slow Study, September 2026.