Online Classroom: Compass correction at sea using HO 229

This is topic Compass correction at sea using HO 229 in forum Public Discussion of Cel Nav at Online Classroom.

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Posted by Bumper (Member # 2106) on July 10, 2011, 04:00 PM:

I was working on compass error determination at sea using the three step interpolation method as described in HO 229 page XXIV. I have used a book of test problems by Leonard Gray to get time, observations and dates. Things were going fine and then all of a sudden I hit a brick wall. Here are the details:

Nautical Almanac used: 2002
Date of observation: September 10
Time of observation GMT: 07:59:00
DR: 39:36:00 S, 30:55:00 W

GHA of Sun: 07 hours 285:43.6
59 minutes 14:45.0
00 seconds 0:00.0
-----------------------------
300:28.6 degrees

Dec of Sun: 07 hours 4:58.5 N d1.0
59 minutes(d 1.0) -1.0
----------------------------------
4:57.5 N

LHA 300:28.6 GHA of Sun
-30:55.0 W Log of DR
-------------------------------
269:33.6 degrees

Rounded values to enter HO 229 on the high end

Declination: 5 degrees N
Latitude: 40 degrees S
LHA: 270 degrees Contrary

Rounded values to enter HO 229 on the low end

Declination: 4 degrees N
Latitude: 39 degrees S
LHA: 269 degrees Contrary

And there’s the problem. No LHA of 269 degrees contrary. So what is one to do in a case like this?

Posted by HHEW (Member # 1214) on July 11, 2011, 08:53 AM:

It's not LHA that is Contrary or Same, it's Declination that is Contrary or Same (to Latitude).

In this case, your have declination N and Latitude S, so try looking at the columns for Dec 4° or 5° Contrary, to see if that helps.

-Hewitt Schlereth

Posted by Instructor • (Member # 1) on July 11, 2011, 09:02 AM:

also, keep in mind you can go to this page

www.starpath.com/usno

and enter your time, date, and DR and get out the Hc and Zn for most dates

this way you check to see if you have some typo and the body is below the horizon.... that too would make it impossible to find.

Posted by Bumper (Member # 2106) on July 11, 2011, 07:54 PM:

To instructor:

The problem I was working on was taken from a reconized text of problems and the author said the observation was reasonable and reliable. So he observed the sun. I have checked the math serveral times and so far it checks.

Posted by HHEW (Member # 1214) on July 12, 2011, 07:27 AM:

Good Morning -

Well, something's out of whack for sure.

Let's take your case of Dec 4° N, Lat 39° S, LHA 269°

Now, the initial entry for HO 229 is LHA:

-You should be opening to page 363 in Vol. 3 229.

The top of this page reads "LATITUDE CONTRARY NAME TO DECLINATION L.H.A. 89°, 271°".

The bottom of the page is labeled " LATITUDE SAME NAME AS DECLINATION L..H.A. 91°, 269°"

Confusing? Could be, but this is the page you want.

-Look toward the top of the page and you will see a black line running all the way across the page under the 1° row. HO229 calls this line the "Contrary/Same" line, but you can also think of it as the horizon line.

Anyway, the way to read this page for Contrary Name Declinations is from the top down. So, since the Hc for LHA 269°, Lat 39° S, Dec 4° N is below the line, it is negative.

-Hewitt Schlereth

Posted by Instructor • (Member # 1) on July 12, 2011, 11:44 AM:

unless i have some input wrong here, the sun is below the horizon at this time and place. do ctrl+ a few times to make the text larger... then ctrl+0 to go back to normal.

--david
======================================

code:

Celestial Navigation Data for 2002 Sep 10 at  7:59:00 UT           
                                                                               
                 For Assumed Position:  Latitude    S  39 36.0                 
                                        Longitude   W  30 55.0                 
                                                                               
                        Almanac Data                |    Altitude Corrections  
 Object       GHA        Dec         Hc       Zn    |   Refr   SD    PA    Sum 
              o   '      o   '      o   '       o   |     '     '     '     '  
 SUN        300 28.7   N 4 57.6   - 3 29.8    86.5  |    ---   ---   ---   --- 
 JUPITER    338 02.0   N18 34.0   +13 45.4    51.1  |   -3.9   0.3   0.0  -3.6 
 SATURN      20 52.2   N22 08.4   +27 33.1    10.5  |   -1.9   0.2   0.0  -1.7 
 ACAMAR      64 20.4   S40 17.4   +64 31.1   257.6  |   -0.5   0.0   0.0  -0.5 
 ACHERNAR    84 28.4   S57 13.2   +51 36.2   224.5  |   -0.8   0.0   0.0  -0.8

Posted by Bumper (Member # 2106) on July 19, 2011, 04:02 PM:

To all respondants:

Thanks for your input and wisdom. All I can say is the promlem book has to be wrong.

Posted by Instructor • (Member # 1) on July 19, 2011, 04:07 PM:

Leonard Gray's book has been out a long time and is fairly well known. it is possible there is a list of errata that you can find.

best place to look would be to post a question at the NavL discussion group. Many of its active members are familiar with the book.

or just go to that page and search for the title or author etc.

Posted by Instructor • (Member # 1) on July 19, 2011, 04:23 PM:

after writing that i got up to check our library and found this tucked in our copy, printed in 1992:

gray_errata.pdf

is your problem on that list?

--david

Posted by Bumper (Member # 2106) on July 23, 2011, 05:26 PM:

After reading your reply about possible errors I went to my book, which was a 1999 copy, and found the problem that I had copied the date and DR from and I read the entire problem and my BAD. The boat traveled for another 3.25 hours before taking a sun shot. The sun was shot at 11 hours 25 minutes not 7 hours 59 minutes.

Posted by Instructor • (Member # 1) on July 23, 2011, 08:56 PM:

the beauty of cel nav is its transparency. that is, if we make a mistake it will become apparent.... however, it may not show up for quite a bit of paper work.

that is why we teach that in all work on cel nav you much check and double check your work at each step of the process so you do not make an error that takes a long time to track down.

this applies to practice as well. thus in all practice problems it is crucial to double check every step and every number as you write it down.

that is the main lesson of this exercise. it is a very important one to learn.