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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Compass correction at sea using HO 229

   
Author Topic: Compass correction at sea using HO 229
Bumper


 - posted July 10, 2011 04:00 PM      Profile for Bumper           Edit/Delete Post 
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?

From: upper right
HHEW


 - posted July 11, 2011 08:53 AM      Profile for HHEW           Edit/Delete Post 
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

David Burch


 - posted July 11, 2011 09:02 AM      Profile for David Burch           Edit/Delete Post 
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.

From: Starpath, Seattle, WA
Bumper


 - posted July 11, 2011 07:54 PM      Profile for Bumper           Edit/Delete Post 
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.

From: upper right
HHEW


 - posted July 12, 2011 07:27 AM      Profile for HHEW           Edit/Delete Post 
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

David Burch


 - posted July 12, 2011 11:44 AM      Profile for David Burch           Edit/Delete Post 
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


From: Starpath, Seattle, WA
Bumper


 - posted July 19, 2011 04:02 PM      Profile for Bumper           Edit/Delete Post 
To all respondants:

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

From: upper right
David Burch


 - posted July 19, 2011 04:07 PM      Profile for David Burch           Edit/Delete Post 
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.

From: Starpath, Seattle, WA
David Burch


 - posted July 19, 2011 04:23 PM      Profile for David Burch           Edit/Delete Post 
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

From: Starpath, Seattle, WA
Bumper


 - posted July 23, 2011 05:26 PM      Profile for Bumper           Edit/Delete Post 
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.
From: upper right
David Burch


 - posted July 23, 2011 08:56 PM      Profile for David Burch           Edit/Delete Post 
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.

From: Starpath, Seattle, WA


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