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    Re: At the centre of time
    From: Paul Hirose
    Date: 2009 Oct 21, 15:01 -0700

    Brad Morris wrote:
    > Aside from the political envy expressed by one reviewer as to why the prime 
    meridian shifted, what were the reasons for that shift?
    The National Maritime Museum, which runs the old Greenwich Observatory
    site, has this explanation:
    "The Airy Transit Circle at Greenwich was originally the zero of
    longitude for the World. It is not at zero longitude in the WGS84
    system, and the WGS84 system agrees with other modern systems (e.g. the
    International Terrestrial Reference Frame) to better than a metre. So at
    what stage was this origin lost from the International frames?
    "First it must be recognised that the only way that the position of any
    location can influence a terrestrial reference frame is if some
    positional measuring instrument is operated at the site, and the
    accuracy with which this position can be located in the frame is limited
    by the accuracy of the observations.
    "The latest astronomical observations made at Greenwich which
    contributed to the international terrestrial coordinate systems of that
    time were the observations made on the Small Transit instrument. These
    observations continued up to 1958. The observations made were the times
    and zenith angle of transit of stars, and were used to control Greenwich
    Time. Time signals from Greenwich were compared with time signals from
    other countries that were making similar observations, and from this a
    system of longitudes of the contributing observatories was obtained. The
    measured zenith angles give a system of latitudes. This coordinate
    system was compiled and maintained by the Bureau International de
    l'Heure (BIH). The zero point of this system was adjusted to give a
    constant value to the mean of all of them, rather than a precise value
    of zero for Greenwich, and so already at that time Greenwich had an
    offset of about 8 metres from the zero longitude of this system."
    So 50 years ago the prime meridian had already lost its physical
    connection to Greenwich and was starting to drift away.
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