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Stan # 7: Time and the Prime Meridian
13 January 1998
It's nice to know that there are a few things in life that you can rely on - like the Prime Meridian. Well, I'm afraid I'm going to upset you because sadly, this is not true. It keeps moving further to the east. Who moves it and how? Well it can't have been easy to move a very long stringy thing. Maybe it stretched when they moved it. And who got the contract? As it turns out, there are actually seven Prime Meridians. Now I'm all for choice and individual freedom, but I think this is going a bit too far. Does it matter? Well, the Millennium starts at the Prime Meridian, so if you're nimble you could celebrate it seven times, so yes it does matter to you party goers.
The first Prime Meridian was laid down by John Flamsteed, the first Astronomer Royal in 1685 when he marked the continuation of the cross hairs on his telescope on the ground. And as he worked at the Greenwich Observatory, that's where his telescope happened to be and therefore why we have the Prime Meridian there. However, Edmond Halley (of comet fame) found Flamsteed's telescope sinking into the ground and so established a new Prime Meridian 73 inches to the east. In 1750, James Bradley, another Astronomer Royal established a third meridian 436 inches further east, probably to impress his girlfriend. "What did you do today dear?" "Oh, I thought I would establish another Prime Meridian." "That's nice, now what would you like for tea?". This one was used for Ordnance Survey maps. Nice one Jimmy. Make a new Prime Meridian so all the maps need updating. Nice little earner.
George Airy (Astronomer Royal) had a bash in 1851 with his Prime Meridian a further 19 feet to the east and it was this line which was chosen at an international conference in Washington DC in 1881 as there were more Prime Meridians than you could shake a stick at world wide. Must have been interesting hearing the arguments for each. "My line's much nicer than yours!" "You show me your line and I'll show you mine." "Your line's too fat, mine's fashionably trim." How did they market their lines? Did they come in a range of colours? Could you buy replicas at your local Marks and Spencer?
The Ordnance Survey still uses Bradley's line, but the European oil industry uses a grid system based on German Army surveys carried out during World War Two and this Prime Meridian is a further 237 inches to the east. But this raises another question. How did the German Army manage to do this surveying during a war? Was there a temporary truce? Were the people of Britain threatened with theodolites? Was this the real reason why Hitler lost the War - too many army chaps surveying and not enough killing people?
Then there is WGS84 used by the GPS system which is a further 336 feet to the east and directly on top of this, ETRF89 which was defined in 1989 as being the same except that the two will slowly drift apart with the movement of the Earth's tectonic plates. In the real world, WGS84 will win simply because it is used more, especially by sailors. But Airy's line is the one which has a brass strip on the ground. WGS84 is marked by a tree and a rubbish bin. Tourists will
prefer the former as it makes for a better photo opportunity.
Time on the other hand, is controlled by the French (God help us) in the form of the International Earth Rotation Service (so they are the people responsible for spinning the earth!). The IERS compares the International Atomic Time produced by the precise ticking of atomic clocks with Universal Time based on observations of the Earth's rotation and produce a compromise time by adding the odd leap second here and there. This is then called Co-ordinated Universal Time which is shown by the clock at Greenwich. So, the Old Royal Observatory at
Greenwich is quite happy to take the satellite's word for it when it comes to measuring time, but not when measuring space. The proposed Millennium Dome to be built at Greenwich is going for the Airy line brass strip rather than the GPS line so it will be looking back rather than forward. All very curious.