# 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 29, 03:54 -0800
Something I found very interesting about Chichester's methods was the way he plotted his measured wind drifts on three heading which then allowed him to determine the actual wind that the plane was encountering. More modern navigators plot a vector diagram on a piece of paper or on the wind side of their E-6B computer to do this but this is the only time I have seen the vector diagram drawn directly on the chart. People have commented how difficult it would have been for him to manipulate the Bygrave while flying the plane at the same time. I have flown the exact same kind of aircraft and it is twitchy, you have to control it at all times, most airplanes will fly hands-off for a while but not the Gypsy Moth biplane. As difficult as it might be to use the Bygrave I am amazed that he managed to do the chart work in the open cockpit of the twitchy plane.

gl

From: Francis Upchurch <francisupchurch@gmail.com>
To: garylapook@pacbell.net
Sent: Tuesday, January 28, 2014 11:18 PM
Subject: [NavList] Re: Precomputing sextant observations at sea

Thanks Gary, yes I think I now understand how and why he did extra LOPs mid flight in addition to pre-flight pre-computed.. He needed more pre-computed LOPs for intermediate times like 0400GMT ?, some with new APs and therefore new Hcs via Bygrave?
The 2 remaining questions from the original chart are therefore:
1) how did he place his "square box fixes" along the new sun sight LOPs?
2) how did he calculate that final compass bearing at the turn off point to Howe Is? His pencil markings (bottom left) give bearing 193.3degs resulting from calculation 180+24.3=204.3,-11(?variation 11E?)=193.3degs, presumably magnetic? Where does the 24.3 degs come from?(combination of deviation,drift etc?) I think the -11degs is 11E variation,.I thought the true bearing would have been Az(68degs) +90deg=158deg true.
Anyways,fascinating stuff.
For my planned trip, the easiest approach looks to be simple graphs Hp & Az v time as you outlined.Although a short trip, I need 3 rock hopping waypoints where I need to miss the rocks by a mile and turn course, so I recon at least 3 LOPS..I'll show you the plan nearer the time.thanks again.I'll do the odd running fix as well ,just for fun.
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