WEBVTT
Kind: captions
Language: en-GB

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I am in Greenwich, and I am standing on the
Prime Meridian. Zero degrees of longitude.

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This foot is in the eastern hemisphere, that
foot is in the western.

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Except. My GPS tells me that I'm actually
about 100 metres west of the meridian. It's

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actually a little way over there. And that's
not an accuracy problem: the reading is correct.

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So what's going on?

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Well, first of all, the earth. It is shaped
like a slightly squashed sphere. But that's

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an approximation: in reality, it is not a
perfect spheroid -- and I'm not just talking

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mountains and valleys here, the entire thing
is a little bit... lumpy.

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So if you're designing a latitude and longitude
system, you cannot take into account all those

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lumps. Instead, you pick a mathematical, perfect
spheroid, one that is as close as you can

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reasonably get.

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Now the Ordnance Survey, the wonderfully-named
British government department that deals with

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maps, chose their spheroid in 1936, based
on an earlier survey by Victorian Astronomer Royal,

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George Airy. All official British government
maps, to this day, use that spheroid as their

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base. And it works brilliantly, incredibly
well -- for the UK, where the measurements

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were taken.

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Unfortunately, the world is lumpy, and Airy's
measurements weren't quite as good for the

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rest of the globe. Over the years, dozens
of other spheroids have been chosen around

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the world, each one fitting best to a particular
country.

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But no-one minded that for a very long time,
because the resulting maps were never used

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together. They were on paper, so any map that
was accurate enough that the spheroid could

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make a real difference would only cover a
tiny area of the world. And besides, you navigated

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using local landmarks, you never had a magic
device that said here are some numbers, you

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are here. But then: the space race came along,
and suddenly, we had satellites, and we had

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the need for one, standardised latitude and
longitude for the world so you knew that your

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missiles were going to hit the right target.

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So in the fifties, over at Johns Hopkins University,
in the US, the Applied Physics Laboratory

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and the US Navy set up the first satellite
navigation system. And as the base point for

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their calculations, they used their latitude
and longitude that they knew, based on their

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North American spheroid. Now, that spheroid
then changed many times over the years as

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the scientific world refined its calculations,
but that baseline stuck for American systems.

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And you can probably see where this is going.
The GPS system, an American military invention

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that's now a global standard, uses a descendent
of the American spheroid and baseline. The

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Royal Observatory, up on that hill, continues
to use the British spheroid, because, well,

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our government uses that spheroid, because
it's more accurate for geographic calculations

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within Britain, but also because it would
be incredibly inconvenient to move that hill

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a hundred metres that way.

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Who's right? Both of them. Neither of them.
The truth is there can be no actual, zero

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meridian, first of all because the earth is
lumpy, it's always going to be a little bit

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wrong, but mainly because the earth's tectonic
plates move. According to GPS, Greenwich is

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moving, and so is pretty much every other point
on land. Modern latitude and longitude are

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a mathematical abstraction, nothing more.
So I really am in both hemispheres at once.

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It just depends how you measure it.

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