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On tuesday the 19th, Vic wrote:

>I saw this posted on sci.geo.satellit-nav today. Any comments?
>
>>I was reading about the Prime Meridian in Greenwich, England. It is
>>marked by Airy's Transit Circle at the Royal Observatory. In this age of
>>  centimeter accuracy, does continental drift affect the position of the
>>prime meridian? It is moving at 2 to 5 cm / year, relative to the mantle.
>
>Vic

Indeed I found it an interesting question.
I had a look at the following websites:

http://maia.usno.navy.mil/conventions/ierscon.ps
[that is a large postscript file that you do not want to download over a
slow connection]
http://hpiers.obspm.fr/eop-pc/products/bulletins/explanatory.html
http://www.iers.org/iers/earth/itrs/itrs.html
http://www.ordnancesurvey.co.uk/literatu/info/cr107.html

From these I derived the following brief answer.

The position of the Prime Meridian is fixed to Airy's transit circle.
However, the Prime Meridian is no longer the basis of longitude measurement
on earth. Modern coordinate systems (including WGS84) keep the orientation
of the 'mean earth', to put it loosely. They make use of an International
Reference Meridian that has little (if anything) to do with the Prime
Meridian through Airy's Transit Circle. The International Earth Rotation
Service puts it this way:

   The time evolution of the orientation is ensured by
   using a no-net-rotation condition with regards to
   horizontal tectonic motions over the whole earth.

The longitude of the transit circle--meridian pair, measured from the
International Reference Meridian, changes slowly indeed, mainly due to
continental drift. There is also a displacement by tidal action but that is
a periodic one. Other displacements include e.g., ocean loading
(deformation of the earth due to mass displacements caused by oceanic
tides; note that this is not the same as tidal deformation of the earth
itself) and postglacial rebound.

Hope this helps,

Steven

-----------------------------------------------------------
Steven Wepster                          wepster@math.uu.nl
                                        tel +31 30 253 1531
Mathematisch Instituut                      +31 61 251 4380
Universiteit Utrecht
PO Box 80.010
3508 TA Utrecht
The Netherlands
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