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    Sumner lines
    From: Nigel Gardner
    Date: 2001 Feb 18, 3:41 AM

    The 'Sumner Line' was described by Captain Sumner in a book he published in
    1843 in Boston. Bound from Charleston to Greenock in the South-west
    approaches (South of Ireland) on 17 Dec 1837, not having had a sight for
    some time and thus unsure of his latitude, he got a sight about 10am,
    worked out the longitude from the sight for three different latitudes. He
    noticed that the three different points lay in a straight line which ran
    through the Smalls Light. He concluded "that the observed altitude must
    have happened at all three points, at the Smalls light and at the ship at
    the same instant of time". His method (calculating the longitude for two
    different latitudes and running aposition line through them) was in use
    certainly into the beginning of the 20thC although by that time Lecky and
    others were advocating calculating one longitude from a DR latitude and
    drawing  line at right angles to the sun's calculated azimuth.
    
    It might be well to remember the ethos of the age, as a previous
    contributor has commented, one has to get into the mind of the 18/19C
    mariner, "getting a fix" was not the be-all and end-all of things.  A
    longitude by the Prime Vertical at breakfast and latitude by mer-pass at
    lunchtime served most of their needs.
    
    NG
    

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