spaceSpace and Physics

Why Is The Greenwich Meridian Not Where It Should Be?

Gravity, the shape of the Earth, and satellites are to blame for the necessary correction.


Dr. Alfredo Carpineti


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

Alfredo (he/him) has a PhD in Astrophysics on galaxy evolution and a Master's in Quantum Fields and Fundamental Forces.

Senior Staff Writer & Space Correspondent

Photo of a pair of Feet stand on opposite sides of the Prime Meridian. Greenwich, London, UK.

When you are standing on the Prime Meridian at Greenwich, you are not standing on the zero meridian.

Image credit: GMaple Design/

Greenwich in London is home to the Prime Meridian, but just visiting the world-famous Royal Observatory won’t actually make you cross from one hemisphere to the other. That’s because the actual zero meridian is now off from where it was first established in 1884 by 102.5 meters (334 feet). You can blame improvements in technology and science for that. And, well, international cooperation.

A meridian is a line connecting one pole to another on a celestial body, maintaining the same longitude. Unlike an equator that can be physically defined, meridians in bodies that rotate are completely arbitrary. You can set one wherever you want, which is what people have been doing for thousands of years. Longitude gives you east-west coordinates, but it only works if you have a zero point. And that point is the Prime Meridian.

A satellite photo of the obervatory showing the difference in the two meridians.
The traditional Greenwich Meridian (dashed line) goes through the Royal Observatory of Greenwich and the actual zero meridian is a bit off (solid line).
Image credit: Malys et al., Journal of Geodesy 2015 (CC BY 4.0); Google Maps/Infoterra Ltd/Bluesky

Why is the Prime Meridian in Greenwich?

Countries established their prime meridians as they wished, but the game-changer moments took place in England. There, for centuries, its prime meridian passed through St. Paul's Cathedral in the center of London, until 1721 when it was moved to Greenwich.

Around the same time, the ways to measure longitude at sea were developed and the first Nautical Almanac was published by the Royal Observatory in 1767. All of that put Greenwich on the map as the place of reference. That’s where the measurements were taken from, and that is how ships would find their position in the world – an approach that was copied far and wide.

“By the time you get to the mid to late 19th century, it starts to get a bit confusing because you've now got other national observatories. You've got ones in Berlin, in Pulkovo [Russia], in Rio de Janeiro, in Oslo. Their observatory is zero degrees longitude, all their maps and charts are based on that reference,” Louise Devoy, Curator of the Royal Observatory Greenwich, told IFLScience.

“If you're an Englishman trying to do business with a Russian, but your maps have zero in a different place, it just becomes impossible. So that's why there was this big conference in 1884 to try and decide on one single prime origin that everyone could use as zero degrees longitude. By that stage, around 70 percent of shipping by tonnage was already using British maps and charts that were based on Greenwich. So it was a very pragmatic decision.”  

Why is the Prime Meridian in the wrong place?

The meridian is established by precisely timing the passage of certain stars overhead, and it was all well and good until a few decades ago. The employment of the Global Navigation Satellite System, or GPS, revealed a discrepancy between what has been established as the prime meridian and where the zero one actually should be.

At first, it was thought to be an issue caused by the internal protocols of the satellites or small but gradual timekeeping issues. However, that was not the case. A 2015 study showed that the problem had to do with gravity. As mentioned, the astronomical method requires stars to pass precisely overhead. But how confident can you be of where is "up"?

The usual approach is to find where is "down". That was done using gravity. After all, gravity pulls everything down towards the center of the planet. Well, sort of. The gravity on Earth is not exactly the same everywhere because the interior of the planet and its surface have variations. So, a line pointing down in New York might not be parallel to a line pointing down in Los Angeles.

“So the measurements that were made at Greenwich would have been affected by local variations in gravity because the gravitational pull is not constant around the whole Earth,” Devoy explained.


Navigation satellites move instead around the very center of the Earth, unaffected by the variation because there are so many of them taking many measurements at a distance that they just smooth over.

“And so that's why the whole system then starts to shift slightly. And that's why the satellite meridian is about 102 and a half meters east of the Greenwich meridian, just because it's accounted for those variations and centered them on the earth,” Devoy concluded.

The astronomical approach and the satellite approach were squared in the International Reference Meridian, which is now the zero meridian. Close, but definitely off from the Greenwich Meridian.


spaceSpace and Physics
  • tag
  • gravity,

  • navigation,

  • GPS,

  • prime meridian,

  • greenwich,

  • longitude