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

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Precomputing sextant observations at sea
From: Gary LaPook
Date: 2014 Jan 27, 02:07 -0800

----- Forwarded Message -----
From: Gary LaPook <
To: Francis Upchurch <f
Sent: Sunday, January 26, 2014 2:20 AM

Take a look at the more modern way of doing this.

Since you are not covering a lot of miles you can use just one AP. Pick a convenient one near the midpoint of your course and plot all the LOPs from there. Prepare a graph of the precomputed altitudes, twenty minute intervals are usually accurate enough. Since this is a multiple of four minutes the LHA of the sun will change by exactly 5 degrees so you can use the same AP for all the observations. BTW, you don't have to do the computations on exact whole minutes of time but can use any time you want to use to place your Assumed longitude where you want it. Then just space the other computations at exact 4 minutes spacing. To make this clear. If you calculated the GHA at 10:00:00 as having so many minutes that the assumed longitude would be 30 minutes too far to the east then just do the computation for 10:02:00 and then for 10:22:00, 10:42:00 etc. Before plotting the graph, apply the observation corrections with reversed signs so you actually plot Hp (precomputed) not Hc and then compare your Hs directly with the graph.

Chichester didn't make a graph, he made a table instead. He then made the adjustment for off time shots by multiplying the number of minutes from the tabulated times by the rate of change. You can get these correction factors from the MOB tables in H.O. 249 r just subtract the earlier from the next Hp and divide by the number of minutes between them. . So you might make a table like this:

10:00:00 Z   Hp  34° 15'   rate  +  3.0' per minute
10:20:00 Z   Hp  35° 16'  rate   + 3.2' per minute.

etc.

I will review Chichester's work again and get back to you.

gl

From: Francis Upchurch <francisupchurch@gmail.com>
To: garylapook@pacbell.net
Sent: Sunday, January 26, 2014 12:13 AM
Subject: Re: Poor Line of Position Computer

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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