Humanity has drawn a small number of lines on the planet and agreed to treat them as absolute. Zero degrees longitude. Zero degrees latitude. Sea level plus one mile.
Every one of them was measured by hand, with the best instruments available at the time. Every one was very nearly right. And every one, checked against satellites two centuries later, turns out to be slightly out.
The monuments are still there, because moving a line is easier than moving a monument.
LONDON — 0° longitude, off by about 102 metres.
In 1884, twenty-six countries agreed that longitude zero would run through the Airy Transit Circle at the Royal Observatory in Greenwich. The brass strip in the courtyard is one of the most photographed objects in Britain.
The modern reference meridian — the one satellites use — falls about 102 metres east of it. The reason is not sloppiness. Greenwich's line was defined astronomically, using the local vertical, and local gravity in that part of London pulls very slightly off true. When the reference shifted from stars to satellites, the line moved and the brass did not.
→ London
QUITO — 0° latitude, off by about 240 metres.
Mitad del Mundo, north of Quito, marks the equator as fixed by the French Geodesic Mission of 1736 under Charles-Marie de La Condamine. They had come to South America to prove the Earth bulges at the equator, and they proved it.
GPS puts the true line about 240 metres north of their marker. On a planet 40 million metres around, that is an error of roughly 0.0006%.
The nearby Intiñan Museum, established afterwards to mark the "real" line, may itself be out by around fifty metres.
→ Quito
DENVER — one mile above sea level, and the marker has moved three times.
The Colorado State Capitol steps carry the marker. In 1909 it was the 15th step, engraved One Mile Above Sea Level. In 1969, after students from Colorado State University remeasured, it moved to the 18th. In 2003 it moved again, to the 13th, where it is today.
Modern GPS suggests the Capitol area is closer to 5,277 feet — about three feet short of a mile. So none of the three may be right.
→ Denver
ACCRA — on the line, and the monument is gone.
The Prime Meridian passes directly through Tema, 25 kilometres east of Accra, at 05°40′N 00°00′W. Ghana is the closest country on Earth to 0°, 0° — the point where the meridian crosses the equator, in the Gulf of Guinea just off its coast.
The nearest land to that point is inside a container terminal at Tema, and a monument called The Meridian Rock was placed there to mark it.
It has since been lost.
→ Accra
And one that is right.
ST. JOHN'S sits at 52°42′ west. Newfoundland's time zone, UTC−3:30, is centred on 52°30′ west. Nobody built a monument to it and nobody remeasured it. The city's clock is within about one minute of its own solar noon, which makes it the most accurately timed place on this site.
The line that nobody marked is the one that turned out to be right.