NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2013 Feb 20, 12:11 -0500
Hi John
The 1.2 minutes is incorrect and is entirely my mistake. The delta between the calculator solution and the tabular solution is actually less than 0.1'.
You wrote:
The C correction is just a tool to avoid the plotting of a running fix, which mainly depends for its accuracy on the course and distance run between sights.
I agree completely. The course and distance cannot be so precisely known, otherwise we would not have required a solution to the longitude problem.
The very example in Burton shows a 0.23' correction in longitude for every 1' error in latitude. Burton then illustrates an 8' error in Latitude, with a resultant 2' correction.
Based on Frank's estimation of a 3' error window for celestial (I do hope I haven't miss stated this!), the correction shown by Burton is lower than our margin of error. AND it assumes that we know course and distance to such precision that this correction will actually have a positive, instead of random, affect on our position.
There are two other uses for the ABC tables. Shall we investigate?
Regards
Brad
Thanks Brad, for that interesting exercise. I used a calculator to check the azimuth, but otherwise didn't look into the accuracy of the result.
You have identified me as ‘defender and advocate’ of Burton’s tables so I had better make my closing speech, even if it means re-stating some basic, obvious stuff. ;-)
The C correction is just a tool to avoid the plotting of a running fix, which mainly depends for its accuracy on the course and distance run between sights. After several days of sledging over drifting sea ice, with overcast skies, these things can not be well known.
In your exercise the difference between the longitude ‘correction’ obtained by direct trig and the tables, is 1.2 minutes, equal to just a quarter of a mile in 78 degrees of latitude.
Precision. Burton was ahead of his time when he published his Four-Figure Tables, a slimmed down version of the standard tables. In the preface to the 4th edition he argues for ‘greatly increased ease and speed’ of sight reduction and a ‘consequent reduction in the chances of making arithmetical mistakes’. He goes on to say that the use of the tables will allow for solutions of the astronomical triangle correct to within one minute of arc. This was in 1963, so perhaps he had got wind of the new kid on the block, HO 249.
I like the ABC tables. But then I was once a merchant seaman and practical navigator, just like Stephen Burton.
Cheers
John
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------
