# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Precomputing sextant observations at sea
From: Gary LaPook
Date: 2014 Jan 28, 16:32 -0800
Here is an excerpt from his book which shows him using both pecomputed altitudes and ones freshly worked on this Bygrave slide rule.

gl

From: Francis Upchurch <francisupchurch@gmail.com>
To: garylapook@pacbell.net
Sent: Monday, January 27, 2014 10:33 PM
Subject: [NavList] Re: Precomputing sextant observations at sea

Thanks to Douglas Dennny for sending the Chichester observer book extract. Looking at the original chart (from Hughes,also in Chichester' 2 books, Lonely Sea and Sky and Alone across Tasman Sea) with all the pencil markings, including wind drift etc, I still have 3 main questions.
1) how did he get the exact position of his "square box fixes" on some of the LOPs?
2) why did he need to use the Bygrave to calculate new LOPs mid flight if he already had them prepared? (I now think the answer is that he needed to do extra "pre-computed" LOPS at e.g. 0400GMT and did these before 0400, so needing new Bygrave calculations. What do you think Gary?
3)How did he work out the bearing from the turning point to Howe Is along the final LOP? His pencil markings (bottom left) give bearing 193.3degs resulting from calculation 180+24..3=204.3,-11(?variation 11E?)=193.3degs.I thought the true bearing would have been Az(68degs) +90deg=158deg true. Could anyone explain his calculation? I must be missing something basic! Good job he didn't or he would have missed and drowned!
Francis
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