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    Re: At the centre of time
    From: Douglas Denny
    Date: 2009 Oct 20, 16:59 -0700

    "....As part of this exercise,  a check was made of the link between the two 
    coordinate systems which depended on the survey made (I think in the late 
    nineteenth century {someone will be able to correct me on this no doubt} ..."
    I'll correct myself:  I meant of course late _eighteenth_ century as the Airey 
    instrument was established in the late nineteenth.
    The power of the internet is amazing,  here is the information about the 
    Greenwich meridian(s), in all it's glory at:-
    And a relevant quote from the above link :-
    "Even today, it can be confusing as there are four Meridians all passing 
    through the Old Royal Observatory.
    The earliest is Flamsteed's, named after the first Astronomer Royal, which was
    established in 1675. In 1725, Edmund Halley, the second Astronomer Royal
    established a second Meridian.
    The third was defined by another Astronomer Royal, James Bradley, in the mid
    18th century, and is still used as the basis for map-making in Britain today.
    The fourth Meridian was established in 1851 by yet another Astronomer Royal, 
    Sir  George Airy, who set up new measuring equipment in a room alongside 
    Bradley's  original equipment. It is the positioning of this neighbouring 
    equipment, just 5.79  metres (19ft) away, which eventually became the basis 
    for international time. Due to the convergence of meridians of longitude 
    towards the poles, Bradley's Meridian is  5.9m west of Airy's where they 
    cross the South Coast of England, and 5.5m west  where they cross the East 
    As the pace of development and travel accelerated in the 19th century, it became
    clear there would have to be a common, world-wide standard for timekeeping. In
    1884, 25 countries reached agreement at a conference in Washington, USA, that
    Airy's Greenwich Meridian would be adopted as the `Prime Meridian' - zero
    degrees - from which time could be set and from which other points of longitude
    could be calculated. Over a period of many years, countries which had not
    necessarily been party to this original agreement accepted and adopted the
    So since 1884, the Airy line has been The Greenwich Meridian, although for
    practical mapping purposes in Britain (excepting hydrographic charts) the 
    Bradley line continues in use as the zero meridian. The difference between 
    the two is known and well defined - and is important scientifically - but for 
    most day-to-day purposes has no real consequence."
    Douglas Denny.
    Chichester England. 
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