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    Re: Interesting question
    From: Steven Wepster
    Date: 2003 Aug 20, 14:25 +0200

    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.
    Indeed I found it an interesting question.
    I had a look at the following websites:
    [that is a large postscript file that you do not want to download over a
    slow connection]
    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 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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