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    Re: Margett's Tables/graphical longitude.
    From: Francis Upchurch
    Date: 2016 Jan 15, 03:17 -0800

    Thanks Ken,

    I managed to find a copy of the Sheperd Tables  (well, you can copy individual pages, not the whole thing, but that is fine for what I need) .http://babel.hathitrust.org/cgi/imgsrv/download/pdf?id=mdp.39015080307997;orient=0;size=100;seq=18;attachment=0.

    Yes, I see now that the Margetts Curves come straight from these remarkable 1772 tables which were apparently paid for by the Longitude Board, but they never became widely accepted. I'm surprised Margetts is not more transparent about where he stole the data! Even 5 of the egs in the Margetts book are exact copies of the egs at the begining of the Sheperd tables from 1772!

    I've just compared the process using the 2 .The Shepherd Tables took me  about 20 minutes, with minimal maths (addition of funny things called parallactic logs, whatever they are?). Definitely viable for a non mathematician navigator circa 1772. The same eg using the Margetts only took about 5-7  minutes with the same result. That is almost as quick as the calculator check of about 3 minutes. ( I've here used the Letcher formula which only requires the same 4 inputs of LDsd, Hsd, hsd, Lunar HP from daily pages. I've preloaded this formula into a TI-36XPro calulator, as recommended by Greg. (Thanks again Greg). It gives immediate results of  P' Parallax correction with just the 4 inputs. You then just have to add the small refraction correction from Letcher's graphic or derived from the NA).

    So far, I'm getting roughly identical results from calculator and Margetts, so I assume the original Shepherd (actually Lyon?) Tables gave correct clearance procedure , even if the final reults may have been off due to early inaccuracies of the basic moon position predictions in the NA?).

    I'll next try with my own lunars and 2016 NA and see what we get. Should be interesting.

    Best wishes.

    Francis

       
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