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    Re: NOW what?
    From: Don Seltzer
    Date: 2015 Aug 18, 18:36 -0400


    On Aug 18, 2015, at 4:32 PM, Frank Reed <NoReply_FrankReed@fer3.com> wrote:

    ...If we could determine the location of the prime meridian by astronomical observations at various points along its length, through England, France, Spain, and Africa, as well as points at sea, we would find that it snakes back and forth by up to a mile around the "true" prime meridian. And note that the astronomically determined prime meridian would have points both west and east of the true prime meridian. Greenwich's local circumstances place it 100 meters west of the true prime meridian. If the observatory had been built somewhere else along the prime meridian, then the shift could just as easily have been to the east..


    I am confused by 'true prime meridian'.  The PM is completely arbitrary, chosen originally for political reasons and convenience.  The new PM 100 meters east of the 19th century line is a mathematical convenience determined in the 1980's to minimize the errors as observed at different locations around the world.

    Don Seltzer
       
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