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    Re: Fw: Letcher page 103
    From: Frank Reed
    Date: 2010 Feb 18, 19:07 -0800

    Henry,

    Great review of Letcher's "Self-Contained..." It's one of my favorites.

    Amazingly I first browsed through a copy of this book way back in 1978 about three months after I first learned basic celestial navigation. The Seaport Planetarium at Mystic Seaport back then had two categories of volunteers: teenagers like me (I was 15 at that time) and comfortably retired older men. One of the latter was working on a review of Letcher's book, and he had carefully marked up a copy with notes on numerous errors which he had found in it. My recollection is that these were errors in detail --occasional mis-placed decimal points or missing signs. I believe I can locate that copy and at some point I will transcribe his list of errors for other owners of the book.

    You're right that Letcher's particular method of clearing lunars is fairly conventional, but it's concise and he has a few neat tricks. The formula he gives is just a variant of the standard series expansion. At that time, in modern books, there was almost nothing else available so it was a nice contribution to the subject. The small inaccuracies of his formula can be overcome by extending it in relatively simple ways. In historical terms, his formula would fall about halfway between Turner's Tables and Thomson's Tables (so in timeline terms, around 1825), which are both based on the same equations with the latter including a few more of those extra details that make the solution more accurate.

    I, too, recommend picking up a copy. There are many used copies available on abebooks.com, amazon.com, and I've seen it on ebay, too. It's a nice way to get the H.O. 208 tables, and Letcher has lots of great advice on many topics of celestial navigation from basic sextant use through such esoteric topics as lunars. He writes great prose. Also the book has many good technical illustrations. And finally, for the recent thread, if I remember correctly, Letcher also had a physics background. We're a pox on navigation. :-)

    -FER


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