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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » NAO sight vs 249 reduction

   
Author Topic: NAO sight vs 249 reduction
navi


 - posted May 30, 2017 10:15 AM      Profile for navi           Edit/Delete Post 
After having met and talk w David Burch (thanks David for agreeing to meet in Seattle!) I tried the NAO sigh reduction available in the Nautical Almanac using David's forms.

+ NAO is available in the Almanac
+ NAO can take all declinations (also over 29 deg)
- You have to look in the table 4 times instead of 2 times (249), twice for reduction + twice for corrections.
-Signs are very important and shifts depending on different factors

I got values very close to 249 when I got it all right. To use NAO using David's form makes it much easier.

To hone the skills I think one has to do 10 reductions with the NAO.

Just seen 229 once I think that one is somewhere between NAO and 249 in difficulty to use. (But 229 is more precise then the others.)

If one would like to increase safety it would be possible to use David's double form 104-106 on page 281 in his course book and reduce with BOTH 249 and NAO (dec until 29) and check that the results are close! Similarly for dec over 29 one could reduce with both 229 and NAO using the same form from page 181.

From: Chi
David Burch


 - posted May 30, 2017 11:02 AM      Profile for David Burch           Edit/Delete Post 
I agree with your general assessment.

I think with practice you will find the 229 closer to 249 than to NAO, but the key issue is the one you make: namely NAO is a viable alternative to the 249 tables, with several attractions. 1) it comes with every almanac, so no extra books are needed, and 2) it will do any sight. it will also do great circle sailings, and we have notes on that somewhere. I think they are included in the Kolbe Long Term Almanac, which also includes a set of NAO tables.

for those doing computer or calculator sight reduction, the NAO is the obvious choice for printed back up. (I know you are not using calculations.)

Do you have an estimate of the time required for sight reduction of the same sight by 249 and by NAO? It would be interesting to see that comparison for a few sights.

thanks

From: Starpath, Seattle, WA
Lou


 - posted May 11, 2018 10:50 AM      Profile for Lou           Edit/Delete Post 
I'm finding that many stars that are convenient for sights lie outside the 29º dec' limit of Vol.2/3.
I would like to use stars with declinations exceeding 29º and also, those not employed in 249 Vol 1, at the given LHAs.
Does this require using HO 229?
And if using NAO tables, is a celestial calculator always required?

From: San Diego
David Burch


 - posted May 11, 2018 11:49 AM      Profile for David Burch           Edit/Delete Post 
Pub 249 is for dec <29º. For higher dec you can use Pub 229 or NAO tables. NAO tables require just adding and subtracting numbers; no special calculator. We recommend using our NAO form which makes them very easy to use.
From: Starpath, Seattle, WA
Lou


 - posted May 13, 2018 08:06 AM      Profile for Lou           Edit/Delete Post 
Thanks.
I'll try it!

From: San Diego
Soulstice


 - posted November 07, 2020 01:03 PM      Profile for Soulstice           Edit/Delete Post 
Hello All, I am curious to know what the margin of error is between the 249 SR method and the NAO SR method. I have done several problems paying close mind to the signs numerical entry and have found differences of up to 8' when compared to the 249 SR method. Does anyone know what the average amount of error is or is there a way to determine the approximate amount of error incurred using the NAO SR method? In advance, thank you for your time and input.
From: Earth
David Burch


 - posted November 07, 2020 01:22 PM      Profile for David Burch           Edit/Delete Post 
These should agree most of the time within 1', and indeed the NAO tables Zn values will likely be more accurate than the 249.

We will need to track down the two sight reductions you are comparing and the very best way to do that is to use the SR comparison function in CelestialTools.exe we have a copy of this in our downloads section. It is PC only. That will tell you what all the individual intermediate values should be using both tables.

The function within CelestialTools you want is called "SR Methods & Fix" Open the program and then click the icon or press Alt+letter O.

He calls the NAO tables "NASR" but these are the same. This program was new at about the time they came out and the name had not been settled... might still not be amongst some!

From: Starpath, Seattle, WA
Soulstice


 - posted November 22, 2020 11:56 AM      Profile for Soulstice           Edit/Delete Post 
Greetings,
I am running into an issue with rather large differences in HC when obtained from 229/249 vs. the NAO tables. I am using the NAO reduction method for all sights in the "HI by Sextant" book. Please refer to problem #7 July 9th in the HI by Sextant book PP43 Jupiter sight WT 21h44m32s. The HC obtained in the book was 39 deg 48'.I obtained all of the same values as the book for LHA 17, Dec S10, and a-Lat 37 deg. When using the NAO table I worked the problem two different ways. If you look in the NAO table, "A" is 13 deg 30'. I worked the remainder of the reduction rounding "A" up and down since the minutes part is 30'. Rounding A bar up to 14 deg yields an Hc of 39 deg 28' a difference of 20' when compared to the books answer on PP43. Rounding "A" down to 13 deg yields an Hc of 39 deg 39' a difference of 11'. This again brings up my question of what is the expected margin of error using the NAO reduction method vs. 229/249 methods? I have seen this on numerous occasions and just wanting to see if an error is being made or if I should lower my confidence when using the NAO tables. In advance, thank you for any feedback.

From: Earth
David Burch


 - posted November 22, 2020 08:02 PM      Profile for David Burch           Edit/Delete Post 
It seems this question was answered in the above reply. That is, they should agree to roughly ± 1'. As for use of the NAO tables, did you run the NAO solution through the Celestial tools app? That should show the right values. In other words, you can run that for Pub 249, then Pub 229, then for the NAO tables and you will see what each provides.

If you are having trouble finding that free app or running it, let us know. It is a powerful tool to have, so we encourage its use. See above reply.

From: Starpath, Seattle, WA
Soulstice


 - posted November 25, 2020 07:03 PM      Profile for Soulstice           Edit/Delete Post 
Thank you for the swift reply. I am unable to open the CelestialTools.exe as I currently only run IOS. I do have the StarPilot app installed on my Ipad. There is a definite learning curve to this great tool. I am working on how to solve the averaging of sights. I also seem to be having some difficulty clearing data from the settings page in efforts to enter new sights. Any tips are always appreciated. Ill continue to work through the StarPilot with hopes of becoming more fluid with its use. Ultimately though my intent is to rely on as little electronic assistance as I can. Kind Regards....
From: Earth
Capt Steve Miller


 - posted November 26, 2020 06:15 AM      Profile for Capt Steve Miller           Edit/Delete Post 
In the Starpilot go to Celestial Fix and there you will find New Seq (Del ALL).
From: Starpath
Soulstice


 - posted December 02, 2020 01:12 PM      Profile for Soulstice           Edit/Delete Post 
Would anyone have any input on NAO discrepancy when compared to Pub 249 sight reduction methods using the data in this work form? Note: the corrections for Dip, IE, and Ha were omitted as the Hs here was obtained from the Stellarium App. None the less, the reductions should workout similar with the given data. Racking my brain trying to find out why. Any help is much appreciated.


From: Earth
Soulstice


 - posted December 02, 2020 01:30 PM      Profile for Soulstice           Edit/Delete Post 
I apologize, compressed photo in above post did not show. So we are looking at a DR 39 21.8’ N, -119 45.7 W. GMT 19h36m02s on December 2nd 2020. Sight Antares Hs 24 09.0’. GHA 118 26.8’ a-Lon -119 26.8 W, LHA-359, Dec S 26 28.5’, a-Lat-39N. Pub 249 yields Hc 24 31.0 yet NAO yields an Hc 26 28.0’ a difference of 117’?
From: Earth
David Burch


 - posted December 02, 2020 08:40 PM      Profile for David Burch           Edit/Delete Post 
I apologize, compressed photo in above post did not show. So we are looking at a DR 39 21.8’ N, -119 45.7 W. GMT 19h36m02s on December 2nd 2020. Sight Antares Hs 24 09.0’. GHA 118 26.8’ a-Lon -119 26.8 W, LHA-359, Dec S 26 28.5’, a-Lat-39N. Pub 249 yields Hc 24 31.0 yet NAO yields an Hc 26 28.0’ a difference of 117’?

Here is the data as i see it for the sight reduction:

LHA-359, Dec S 26 28.5’, a-Lat-39N.

For this data Hc = 24º 31.1', Zn = 179.0

see
https://www.starpath.com/calc/Celestial%20Navigation/srt1-2.html

so it looks like there is some error in the NAO form. Please see if you can find a PC and then run the NAO check in celestial tools so you can find your error. Or start from scratch on a blank form and do it again, rather than look for errors in one already filled out.

From: Starpath, Seattle, WA
David Burch


 - posted December 02, 2020 08:48 PM      Profile for David Burch           Edit/Delete Post 
Actually we will just do this. Here is the answer from CelestialTools: NAO gets 24º 31' with Zn = 179, which is the same.



We strongly recommend, tracking down a PC and using CelestialTools to find sight reduction errors. Use www.starpath.com/calc to find out what is the right answer to use as the reference, as done here.

From: Starpath, Seattle, WA
Soulstice


 - posted December 03, 2020 10:26 AM      Profile for Soulstice           Edit/Delete Post 
Thank you kindly, that was a great help. I was able to locate a small rounding error with Dec effecting F. I do appreciate the assistance and your time.
From: Earth


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