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    Re: Understanding a 1902 Sumner
    From: David C
    Date: 2017 Jan 20, 19:11 -0800

    I have made a first attempt at reducing Blackburne's sights. The latitude is within 0.4' of the GPS position but the longitude is over 2° in error.  I will append details of my working.

    • Is my procedure correct?
    • Have I made an arithmetical error?
    • Have I applied Eot with the correct sign?

    Sight 1

    Dec  2° 56'  54"  N    (2° 56.9')

    Ho  44° 14' 04"   (44° 14')

    MTG   13h  17m  31s

    EoT   5m 13.3s

    It is pre-1925 so Greenwhich time starts at noon. Therefore GHA = GAT. I tried adding and subtracting EoT - adding gave a more sensible answer i.e. GAT = MTG + EoT.

    GAT =  13h  22m  44.3s

    From Norie's  Time to Arc tables  I get

    GHA = 200° 41'

    Sight 2

    dec  2° 59'  24"   N    (2° 59.4')

    Ho   25°   45'  30"   (25°  45.5')

    MTG  15h   51m   18s

    EoT    5m   11.3s

    From which GAT = 15h  56m  29.3s

    From Norie  GHA = 239°  7.25'

    Solution

    When I insert these numbers into the Vector Solutions 2CoP app the position is

    41° 17.2' S

    172°  11.8' E

    The GPS position is

    41°  17.57'  S

    174°  47.36'

       
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