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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Help! Computed Solutions

Author Topic: Help! Computed Solutions

 - posted December 31, 2019 01:26 PM      Profile for Aonghus           Edit/Delete Post 
Fellow Cel Nav Enthusiasts,

Greetings from Galway on west coast of Ireland!

I'm a beginner and need some help with computed solutions. I have completed DB's home study course, have also read Emergency Navigation and have now begun HBS.

I found Hewitt Schlereth's book Commonsense Cel Nav really worked for me especially Ch 15 Solving Nav Triangle Directly where he gives formulae and rules for calculation of Hc and Z.

In direct calculation of Hc using formula

Sin Hc = sinLsind + cosLcosdcost

(where t = internal angle at apex of triangle at pole regardless of LHA) I am getting answers very close to Hc in Workform Solutions. Am I right in thinking this difference is due to use of DR Lat/Long in direct calculation as opposed to AP Lat/Long in Workform Solution. For example;

Direct Calculation vs Workform Solution

P1 Venus 12 28 vs. 12 08.4
P1 Sun 27 42 vs. 27 46
P2 Sun 47 4 vs. 46 47
P2 Sun 60 46 vs. 60 56
P3 Sun 22 08 vs. 22 08
P3 Sun 70 53 vs. 71 29

This may appear to be a ridiculous question but I'll take the embarrassment rather than pretend that everything is OK and carry on regardless.

So, if one of you has nothing better to do please clarify for me.

Happy New Year!


From: Galway, Ireland
Capt Steve Miller

 - posted December 31, 2019 06:29 PM      Profile for Capt Steve Miller           Edit/Delete Post 
When using the 'direct calculation' method you are not using rounded off data that you find using the sight reduction tables. The values in the Pub 249 tables are are rounded to the minute. The values in Pub 229 are rounded to tenths of a minute.
As you have found there are differences between the methods, but are usually very close, certainly close enough for work at sea where if your results are within a half a mile you are doing an excellent job.

From: Starpath
David Burch

 - posted January 01, 2020 12:17 PM      Profile for David Burch           Edit/Delete Post 
To compare computations with tables you also need to use the AP values of aLat and aLon for both solutions. then they will agree to within a tenth or two with 249 and with in 0.1' with 229.... which i believe is what you realize with your notes.

there is little value in comparing them with one using AP and one using DR... since these positions differ anywhere from 0' to 50 some minutes.

PS. thanks for the snacks offer!

From: Starpath, Seattle, WA

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