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Precomputing sextant observations at sea
From: Gary LaPook
Date: 2014 Jan 27, 02:12 -0800

----- Forwarded Message -----
From: Gary LaPook
To: "francis@pharmdoc.co.uk"
Sent: Sunday, January 26, 2014 12:09 PM

If you are going to try to do running fixes then try using the MOO table (Motion Of the Observer) from HO 249 to adjust the observations. I posted a table for doing this on Navlist for boat speeds a while ago. I am attaching it now. I am also attaching the MOO tables and  the Motion Of the Body (MOB) tables from HO 249 in case you don't have it. The MOB table is similar to the correction table for "off-schedule" observations that Chichester used. He only had to compute it for a limited range of latitudes.

See:

I'm curious now about the Chichester plotting that you mentioned, it's been a long time since I looked at it.

Is it OK with you if I post our converstaion over on Navlist for the benefit of the other kids?

From: PharmDoc <francis@pharmdoc.co.uk>
To: 'Gary LaPook' <garylapook@pacbell.net>
Sent: Sunday, January 26, 2014 7:26 AM
Subject: RE: Poor Line of Position Computer

Oh, thank you so much Gary. That is perfect! I did not know about the graph method which looks much better. I realize it is aviation rather than nautical, but still rather magical.
When you say “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.

Why didn’t Chichester do this? (apart from dip which I guess was different for each obs due to altitude above sea level changes?)
Yes, I chose the 6-7 hour trip to mimic  the duration of the Chichester Norfolk- Howe trip. If it works, I’ll try it to cross Irish Sea to Cork 140nm! Last time I cheated with sun/moon fix which is easy! (why didn’t Chichester try to plan his very dangerous flight for a time/date when he could have got a couple of moon /sun fixes?).He would still need to run down the LOP at the end, but a good fix or 2 would have reduced the stress!
If you find time to figure out the question on Chichester’s “square fixes”, that would be great.
Keep up the good work on all this.
Francis

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