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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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Re: Interesting Bowditch CelNav results was : Celestial Coordinator
From: Stan K
Date: 2011 Dec 10, 11:03 -0500
Kermit,

I worked through the numbers, and the difference in methods (Pub. 229 plotting from an "assumed position", and Law of Cosines plotting from the DR position) is definitely the reason for the differences in intercept values.  FWIW, when I used the Law of Cosines, I got intercepts of 11.5 nm for the Kochab sight, as you did, but 22.2 nm for the Spica sight, compared to your 22.1.  Maybe something to dow with how things are rounded?

Methods that use an assumed position (assumed latitude to the nearest whole degree, etc.) such as Pub. 229 or NASR often come up with large intercepts, sometimes as much as 30 nm, but the LOPs should come out in pretty much the same place.  Pub. 229 works to 0.1' resolution, so it should be a very good match, but NASR works to 1' resolution, so it could be a bit off.

Stan

-----Original Message-----
From: Antoine Couette <antoine.m.couette@club-internet.fr>
To: NavList <NavList@fer3.com>
Sent: Fri, Dec 9, 2011 9:57 pm
Subject: [NavList] Re: Interesting Bowditch CelNav results was : Celestial Coordinator

RE :
[17580] Re: Interesting Bowditch CelNav results was ...
Kermit,
The Bowditch reductions were done with Pub. 229. Without running the numbers, I suspect that you did yours with the Law of Cosines, which could explain the difference.
Stan
================================================================================
Dear Stan,

Yes, I did use direct trig computation from each individual DR position.
Difference between computation methods here will certainly yield significantly different end results, as I have quite often observed in the past. This is one reason why - once having number crunching power at my finger tips (Thanks again to HP !!) - I totally discarted such "approximate" computations methods some 35 years ago.
I had also thought about your suggestion as being a possible explanation for the differences found between Bowditch and own results.
Can this difference between methods explain just by itself a 0.9 NM difference in the intercept of Spica (20.2 NM vs 22.1 NM)? Well, maybe, after all, but ...
... could it also "reasonably" explain a ... 6.3 NM in the intercept of Kochab (5.2 NM vs 11.5 NM) ? Would this assumption "reasonably" make sense ???
So, it looks like I am definitely missing something here.
Any help from any body, simply through running by himself the same computations ? (hint : the GMT date and time are in the morning of May 17, 1995).
Thanks in advance to who-ever will bring some light here.
Best Regards to all

Kermit
Antoine M. Couëtte
PS : For any interested "Good Samaritan", I could publish here "rearranged" computational starting data as well as end results. Although (a bit) helpful - one never knows, because this might probably be where I mistakenly understood (one of) them - the Bowditch starting data are quite self explanatory.

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