“I am in Greenwich, and I am standing on the Prime Meridian. Zero degrees of longitude. This foot is in the eastern hemisphere, that foot is in the western. Except. My GPS tells me that I'm actually about 100 metres west of the meridian. It's actually a little way over there. And that's not an accuracy problem: the reading is correct. So what's going on? Well, first of all, the earth. It is shaped like a slightly squashed sphere. But that's an approximation: in reality, it is not a perfect spheroid -- and I'm not just talking mountains and valleys here, the entire thing is a little bit... lumpy. So if you're designing a latitude and longitude system, you cannot take into account all those lumps. Instead, you pick a mathematical, perfect spheroid, one that is as close as you can reasonably get. Now the Ordnance Survey, the wonderfully-named British government department that deals with maps, chose their spheroid in 1936, based on an earlier survey by Victorian Astronomer Royal, George Airy. All official British government maps, to this day, use that spheroid as their base. And it works brilliantly, incredibly well -- for the UK, where the measurements were taken.

Unfortunately, the world is lumpy, and Airy's measurements weren't quite as good for the rest of the globe. Over the years, dozens of other spheroids have been chosen around the world, each one fitting best to a particular country. But no-one minded that for a very long time, because the resulting maps were never used together. They were on paper, so any map that was accurate enough that the spheroid could make a real difference would only cover a tiny area of the world. And besides, you navigated using local landmarks, you never had a magic device that said here are some numbers, you are here. But then: the space race came along, and suddenly, we had satellites, and we had the need for one, standardised latitude and longitude for the world so you knew that your missiles were going to hit the right target. So in the fifties, over at Johns Hopkins University, in the US, the Applied Physics Laboratory and the US Navy set up the first satellite navigation system. And as the base point for their calculations, they used their latitude and longitude that they knew, based on their North American spheroid. Now, that spheroid then changed many times over the years as the scientific world refined its calculations, but that baseline stuck for American systems. And you can probably see where this is going. The GPS system, an American military invention that's now a global standard, uses a descendent of the American spheroid and baseline. The Royal Observatory, up on that hill, continues to use the British spheroid, because, well, our government uses that spheroid, because it's more accurate for geographic calculations within Britain, but also because it would be incredibly inconvenient to move that hill a hundred metres that way.

Who's right? Both of them. Neither of them. The truth is there can be no actual, zero meridian, first of all because the earth is lumpy, it's always going to be a little bit wrong, but mainly because the earth's tectonic plates move. According to GPS, Greenwich is moving, and so is pretty much every other point on land. Modern latitude and longitude are a mathematical abstraction, nothing more. So I really am in both hemispheres at once. It just depends how you measure it.”