The prime meridian sets the zero point for measuring longitude.  What’s so special about it?

The Royal Observatory, Greenwich.

In the GIS courses we’ve recently been talking about coordinate systems and their importance in GIS.  They are absolutely fundamental as they allow us to do one of the key things that makes GIS useful: overlaying data.  As we’ve seen before, there are lots of different coordinate systems out there.  Here is the land of the long white cloud, things are pretty simple with just two primary ones to choose from: New Zealand Transverse Mercator and latitude/longitude.  We’ll focus on the latter one here.

Coordinate systems allow us to unambiguously say where something is on the surface of the earth.  Back in the day, when ships were the main mode of international transport, this was particularly important with hazards and reefs.  Well, it’s still important today for that matter:

Well before the days of GPS, and even before decent charts, mariners had to find their way around reefs and pesky land masses.  Early charts were made for these purposes and a coordinate system was necessary to get things in the right place.

We can go all the way back to the Greeks and the polymath Eratosthenes in particular to get to the start of these ideas.  He had a lot going on – first to measure the circumference of the earth, and its tilt, and first to put forth the idea of parallels of latitude and meridians of longitude.  So it’s an idea that’s got legs.

As most will know, latitude and longitude use angles to specify a location on earth relative to some datum (Ed. that’s a loaded word when it comes to geography!).  Latitude’s the slightly easier one to think about as its datum is the equator.  This imaginary plane slices the globe into northern and southern hemispheres and has a physical meaning – it’s halfway between the north and south poles and perpendicular to the earth’s axis of rotation, so there can only be one.  With this as a datum, we can measure the angle between the equator and a point (taking the centre of the earth into account).  Hopefully, the figure below helps illustrate this:

We can now see how latitude measurements can be north or south of the equator.  Longitude has a zero datum as well, but here it gets a bit more complicated.

The datum for longitude is the Prime Meridian, allowing us to locate things to the east or west of this north-south line.  Here’s the kicker: unlike the equator, the location of the prime meridian is completely arbitrary – there’s nothing physically special about its current location.  It could literally be anywhere, so long as it runs north-south through the poles!  So what’s so special it?

Since 1851, today’s Prime Meridian has been a north-south line passing through the Royal Observatory at Greenwich, west of London.  Geographers (and normal people, too) will often make the pilgrimage down the Thames to stand in two hemisphere’s at once:

(No, those aren’t my shoes.  I haven’t made it to Greenwich yet, but I have made the pilgrimage to Broadwick Street.  Next time…)

If we dig into the history a bit, we can see that there have been lots of different prime meridians over time.  An article on Wikipedia lists no less than 40; several around London, but also ones in Paris, Amsterdam, Washington D.C., Philadelphia, the pyramids of Giza, Jerusalem, Kyoto and the list goes on.  Here’s a very rough map of some of them – it was hard to get all of them on here!  Especially around Europe:

The current Prime Meridian passes through eight countries:

It’s a wonder that Michael Palin didn’t do a television series by travelling along the meridian (Ed. perhaps you could consider a new career?), but in Pole to Pole he did travel along the 30ºE meridian to cover as much land as possible.  One interesting prime meridian is the one looked upon as “neutral”, meaning it passed through as few countries as possible:

So how did all these prime meridians come to pass?

It’s notable most of the places listed above are national capitals – and this partially answers the question.  Individual countries often made their own nautical charts and so chose a country-centric prime meridian – there was no international agreement on where it should be.  By the 1850s many countries had adopted British admiralty charts based on the Greenwich meridian for navigation.  In 1884, an international conference was held to agree on a single Prime Meridian and, lo and behold, the British one won out.  It helped that the US had recently adopted Greenwich as the basis for their national timekeeping.  So not only did this make Greenwich the zero point of longitude, it also set the place up as the keeper of the global clock – enshrined in Greenwich Mean Time.  This was a good development as timekeeping wasn’t consistent either.  Travelling by train in, say, Europe, often meant changing the time on your watch at every international border crossing so this conference standardised the prime meridian and gave the world time zones.  Just as Einstein taught us, space and time are linked!  This decision is literally what puts us on the other side of the world (thank you very much!) by plonking the international date line 26 odd degrees to the east of us (Ed. and very odd degrees they are).

The current location of the Prime Meridian is a convention, something we’ve all agreed to (well, no one asked me, but I’m okay with it), just like most countries agree that a red traffic light means stop and green means go.  It doesn’t have any intrinsic physical meaning and having one is a vast improvement over having many.  So in that sense, it is pretty primo.

It doesn’t look like many of us will be making the trek to the Royal Observatory any time soon, sadly, but if you do, by all means go and straddle those two hemispheres.  While you’re there, do go see John Harrison’s marine chronometers, the instruments that made it possible for mariners to determine their longitude at sea – a fascinating story in itself that deserves a lot more attention (Ed. Oh no – that probably means another blog post…).  While straddling the hemisphere’s would be pretty exciting, the next level would be having a foot in all four hemispheres, though it might be a bit more challenging…


(In all good conscience, I can’t quite leave it here.  The Prime Meridian is, ummm, not quite in the right place…  Apparently, Sir George Airy wasn’t aware of WGS84 in 1851…what a bummer.  Can’t let that get in the way of a good story though.  As I said at the start, where it is is arbitrary so in the end, it’s all good.)