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» Online Classroom   »   » Public Discussion of Cel Nav   » NAO/Long Term Almanac vs N(x) Sight Reduction Discrepancies

   
Author Topic: NAO/Long Term Almanac vs N(x) Sight Reduction Discrepancies
bterhart


 - posted March 12, 2018 12:38 PM      Profile for bterhart           Edit/Delete Post 
Hi,

In working through some practice problems, I've come across an example sight reduction in Kolbe's Long Term Almanac that I can't reconcile with the N(x) tables as published in Celestial Navigation: A Complete Home Study Course 2nd Edition

On Page 72 of Kolbe, observational data for Dubhe yields an Hc and Zn of 36 54 and 327.2 respectively. I can reproduce these results via calculator.

Using the N(x) tables with this observational data, I get wildly different result even with very careful interpolation. In reducing other practice sights, the N(x) tables have always produced accurate results.

I must be doing something wrong, but for the life of me I can't figure it out.

Here's my N(x) reduction:
GHA Dubhe (given): 118 47
Dec Dubhe (given): N61 33
DR Lat (given): 55 15N
DR Long (given): 2 43 W

LHA=GHA-WestLong=118 47 - 2 43 = 116 04
Assumed Long= 2 47, LHA therefore 116
Assumed Lat = 55N

N(v) = N(90-Dec) + N(t)
where t = 116 as LHA < 180
as t > 90, t=180-LHA=180-116=64
N(v) = N(90 - 61.55) + N(64)
N(v) = N(28.45) + N(64)
N(v) = 741.5 + 107
N(v) = 848.5
v = 25.34

N(w) = N(Dec) - N(90-v)
N(w) = N(61.55) - N(90-25.34)
N(w) = N(61.55)- N(64.66)
N(w) = 128.5 - 101.5
N(w) = 27
w = 76.7

u = 90 - w + Lat (Dec N and DR N, therefore same name, therefore + Lat)
u = 90 - 76.7 + 55
u = 68.3

N(Hc) = N(90 - v) + N(u)
N(Hc) = N(90 - 25.34) + N(68.3)
N(Hc) = N(64.66) + N(68.3)
N(Hc) = 101.5 + 74
N(Hc) = 175.5
Hc = 57.05
Hc = 57 03

The N(x) derived Hc of 57 03 is wildly different than the 36 54 given in Kolbe (and calculator sight reduction). What am I missing? Where's the error in my N(x) calculation or assumptions?

TIA
Bert

From: Nanaimo
David Burch


 - posted March 12, 2018 03:51 PM      Profile for David Burch           Edit/Delete Post 
Please look over these answers to see if that helps.


From: Starpath, Seattle, WA
bterhart


 - posted March 12, 2018 04:31 PM      Profile for bterhart           Edit/Delete Post 
Hi David,

Got it! Much appreciated David! I didn't make the translation when original t > 90 such that w = 180 -w. Looking through the explanations in the CelNav book, I don't find that note. Is it missing? Did I miss it?

Lastly, is there a link for the N(x) table checker that you're using?

Thanks again David : )

Bert

From: Nanaimo
David Burch


 - posted March 12, 2018 04:52 PM      Profile for David Burch           Edit/Delete Post 
This should be on table T-27, page 270 in the latest printing of that book. It is on the Emergency Card.

No that is not public that i know of, but I have on the list to do a short article on the N(x) solution and will double check it and make a link to it at that time. Also need to check with the author of the that utility.

Should be in about two weeks we can get to that. If that does not appear in our blog by then, please post a reminder

www.starpath.com/articles

From: Starpath, Seattle, WA
bterhart


 - posted March 12, 2018 11:14 PM      Profile for bterhart           Edit/Delete Post 
Hi Dave,

I've got the eBook version and there's nothing in the T-27 that references the modification to w if original t > 90.

I'll look forward to the upcoming article...

Thanks
Bert

From: Nanaimo
David Burch


 - posted March 13, 2018 10:26 AM      Profile for David Burch           Edit/Delete Post 
May I ask what format of ebook do you have and when did you purchase it?

This was updated at some point, but i do not recall when.

From: Starpath, Seattle, WA
bterhart


 - posted March 14, 2018 10:24 AM      Profile for bterhart           Edit/Delete Post 
Hi Dave,

I purchased a Kindle edition of the book May 31, 2016 from Amazon (along with the Radar, Inland and Coastal Nav, Starfinder, Modern Weather, and Emergency Nav).

Thanks,
Bert

From: Nanaimo
bterhart


 - posted April 05, 2018 05:37 PM      Profile for bterhart           Edit/Delete Post 
Hi Dave,

Here's that two week reminder you asked for...

Thanks,
Bert

From: Nanaimo
David Burch


 - posted April 05, 2018 06:24 PM      Profile for David Burch           Edit/Delete Post 
We are just at the last stage of finishing a new book so i cannot get to this for a bit longer, but i will write to the author of that app to see if it can be distributed. I think it can, maybe already has, but we have to check.
From: Starpath, Seattle, WA
bterhart


 - posted April 05, 2018 06:41 PM      Profile for bterhart           Edit/Delete Post 
Thanks for the update Dave : )
From: Nanaimo
David Burch


 - posted April 06, 2018 11:32 AM      Profile for David Burch           Edit/Delete Post 
I have contacted the author of that app, Stan Klein, and he is making a version we can distribute.

Thanks for motivating this work. I had almost forgotten that we had this valuable tool for learning the N(x) table.

From: Starpath, Seattle, WA


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