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    Precomputing sextant observations at sea
    From: Gary LaPook
    Date: 2014 Jan 27, 02:04 -0800
    Posted with permission.

    ----- Forwarded Message -----
    From: Francis Upchurch
    Sent: Sunday, January 26, 2014 12:13 AM


    Hi Gary,
    Would you mind another question re Chichester's Tasman flight?
    I plan to sail the 35nm (6-7 hours) Penzance to Scilly in May, using his method of pre-computed hourly datum LOP lines as my "waypoints".I thought the method only required sun sights at close to the pre-computed times, then correct the Ho for change in alt v time diff. then draw an actual LOP parallel to the pre-computed but +or - xnm as per difference in Ho verses pre-computed Hc?
    I dont understand how Chichester gets his "square box fixes" on the chart in his Tasman book. I can see he is somewhere on the LOP but how does he know exactly where to put the square?(is it eye ball/drift/compass v and d?) Also he says he had to use the Bygrave to work out the new sun sights? Why if he only needs to draw a new LOP parallel to the pre-computed line corrected for change of alt v time difference? Is he using a new assumed position?
    I was also planning to make it even simpler/quicker by "reverse correcting" the pre-computed Hcs to essentially Hs so that I only have to compare raw Hs with the planned H, corrected only for time difference.
    Any thoughts appreciated and thanks for all your help.
    Francis
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