Author

Topic: NAO sight vs 249 reduction

navi

posted May 30, 2017 10:15 AM
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 104106 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
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
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
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
Thanks. I'll try it!
From: San Diego


