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Flamsteed Astronomy Society

I wonder ...

...why the world’s Prime Meridian at Greenwich isn’t in the same place as the “GPS Meridian” used by today’s satellite navigation systems?

Mike Iveson writes — “Whilst doing the solar viewing on Saturday I was asked a question by a member of the public which interested me so much I told him I’d try and get an answer put on our web page.   He had one of those GPS gadgets and wanted to know why there is no mark for the “True” Meridian??   A bit confused I said it was just over there and pointed to the line on the courtyard floor.  Yes he said but that’s not a true position and proceeded to show me that the true meridian was about 100 metres east of the line……..


Do you know anything about this?  Do you know of a web site that we could post that gives information?”

Good question, Mike.    Here’s something of an answer.


It is true that a GPS receiver held over the Meridian Line at Greenwich will NOT read longitude 0˚ 00΄ 00˝   The WGS84 prime meridian is in fact 102.5 meters to the east of the 1884 Prime Meridian at Greenwich.  WGS84 stands for “World Geodetic System 1984” and is used by today’s GPS satellite Global Positioning System.


The World’s Prime Meridian marked at Greenwich is the “Airy Meridian” which is aligned to the eye-piece of George Airy’s Transit Circle that had been in use since 1851.  It was adopted by the international community in 1884 at a conference in  Washington DC.  It is no less than the fourth meridian marked at Greenwich.   The other meridians are all to the west, marching down the Meridian Building, and were established by Flamsteed, Halley, and Bradley.  It was therefore something of a tradition for Astronomers Royal to build new meridian walls (which they did to be able to continue observations while installing new instruments.  The geography of the Observatory hill and courtyard really forced them progressively eastward).


There was no “natural” or scientific reason for the Prime Meridian to be at Greenwich.  It could have been placed at any location where a world-class observatory could refine position measurements.  Agreement for Greenwich came from realisation that 70% of the world’s shipping was using Greenwich charts.  (Mainly thanks to Maskelyne and the Nautical Almanac.  After Harrison, chronometers had remained too expensive to become widespread very quickly).  It didn’t hurt that the US Railroads had effectively standardised on the Greenwich meridian in 1883 when they adopted Dowd’s time zone system for their standard timetables.   At the 1884 conference the delegate from France agreed not to oppose the adoption of Greenwich on the condition that Britain adopt the metric system.  This the Government of Britain promised to do in 1884, and firmly plans to keep that promise when the time is right.

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Martin Male writes -


The GPS question is one of Datum. Due to difficulties in obtaining accurate positional information before the GPS system was launched, folks around the world based their positions on the best info available, which then became adopted as "gospel" on locally produced maps and charts.


Not too much of a problem when you are navigating using a sextant as you'd be really lucky to fix your position within a mile or so at sea, but with GPS this give rise to the problem of hazards not being where they are charted on your particular datum set..... another problem is when you try to match different datums on very large scale maps and charts; they won't "overlay". The prudent navigator will take these innacuracies into account and use his echo sounder/ radar and Mk1 eyeball and brain when approaching a hazard!


The Ordnance Survey (OSGB 1936) datum will be some way off the the top of my head about 120 meters...There are a pletheora of different datum systems in use around the world, some as much as a mile out! The GPS set in question will (should/ must?) have a menu feature that will allow you to change the datum...try this and see what I mean!!


Most GPS sets have WGS84 as their "default" setting. The World Geodesic System 1984 was an attempt to standardize all this before the advent of GPS.


Jane and I are keen sailors, We have a small sailing cruiser on the Medway, so this sort of thing is more than academic for us!!! I'm also an Auxiliary Coastguard so have a "professional" interest in this too.