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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Setting up Fit Slope graph by hand

Author Topic: Setting up Fit Slope graph by hand

 - posted January 24, 2021 10:23 AM      Profile for Soulstice           Edit/Delete Post 
Hello all,

I am looking for verification in the procedure for setting up Fit Slope procedure by hand. Referencing the example in Cel Nav A Home Study (Earlier edition) Chapter 11 pp 121 last paragraph. There is mention that "if using software, it is trivial" however, if using tables, the book mentions "We can't just choose two arbitrary times, since we won't be able to get a whole-degrees value of LHA for use in the tables. So we simply do it once properly from a time early in the sights or just before it (in this case 1444), and then get Hc from that, and then look up the Hc for the same declination and latitude but for an LHA which is 1 degree higher. This will give the Hc for 4 minute later (1448)". The question here is if using NAO tables for sight reduction, would one reduce to Hc using the NAO table for a time early in the sight series and then perform another Hc reduction using NAO but only changing the LHA by one degree higher; thus achieving an exact 4 minutes later?

Question 2) I am trying to understand when setting the time interval on the X-axis of the Fit Slope table to the boxes on the graph how is this interval determined? In figure 11.4-2 on the same page ~43s is between 5 small graph boxes or between 1 set of tic marks along that axis. How was 43s of time determined for use in the graph? I have attempted to divide the number of boxes small and large by both the total number of seconds and vice a versa and the math isn't working out. Also, I have done this between the 1st and 2nd Hc times for the determined slope. I understand this may be elementary but for some reason my morning coffee has not elevated my cognition to such an advanced (or to the contrary) level.

In advance, thank you all for your feedback.

From: Earth
David Burch

 - posted January 24, 2021 03:19 PM      Profile for David Burch           Edit/Delete Post 
Can you please post a specific numerical problem you wish to be solved giving all needed input data and we will make a video on the solution showing all steps.

It is very much easier and indeed more informative to include a specific example, then we can solve it however you like or by multiple methods.

Please just post one question per topic, with a numerical example you want to resolve. Also on the fit


From: Starpath, Seattle, WA
David Burch

 - posted January 24, 2021 04:13 PM      Profile for David Burch           Edit/Delete Post 
Doing the SR for the Hc by tables is complicated as noted when doing by hand for the reasons given. To achieve this extra accuracy in the analysis you are better off computing the solution so you can do it from the times and DR you care about, which won't be the same if you are moving.

If you want to see one brute forced by hand, then just give us the data you want to analyze and we will do it... however, we would do this with Pub 229 not the NAO tables.

It does not make sense to use NAO tables for this. we are looking for corrections of possibly just a few tenths and the NAO tables are ± 0.5' at best, so using those tables the slope itself (two separate computations) could be notably more in error than those we are trying to eliminate with this method—not guaranteed to be wrong that much, but could be. In short it is just not consistent, unless the sight data is really scattered and the NAO is all you have.

From: Starpath, Seattle, WA

 - posted January 24, 2021 06:18 PM      Profile for Soulstice           Edit/Delete Post 
Thank you kindly. That answers the question of not being able to perform Fit Slope analysis with the NAO tables.
From: Earth

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