NavList:

   

Reply

Sean C wrote:
> "...the introduction to H.O. 229 is in error. The author claims that a body 
with a Declination of 35°S at an LHA of 240°, when viewed from a location at 
45°S, will have a true azimuth of 45.6°. This is simply wrong." -Me

Due to the pressure to finish my lunar distance program I did not have
time to look into this matter until now.

I don't see anything wrong with the explanation in HO 229. The problem
seems to be that Sean doesn't understand the difference between
azimuth and azimuth angle. Compare his paraphrase of HO 229 (quoted
above), to the second boldface sentence in his original message:

http://fer3.com/arc/m2.aspx/Error-HO-229-IntroductionExplanation-SeanC-apr-2013-g23600

His original message shows what the book really says: "actual azimuth
angle" not "true azimuth."

"azimuth angle. Azimuth measured from 0° at the north or south reference
direction clockwise or counterclockwise through 90° or 180°. It is
labeled with the reference direction as a prefix and the direction of
measurement from the reference direction as a suffix." (Bowditch
Glossary of Marine Navigation, 1981)

HO 229 does not fully conform to that definition. But the missing
letters cause no ambiguity. The latitude and LHA make it obvious azimuth
angle is measured from south toward east.


> Okay, maybe 'wrong' is too strong a word. Perhaps, 'too vague'? I think at 
the very least, the author should include the cardinal designators, i.e. 
S45.6°E. But why even reference this type of azimuth at all? And why call it 
the "actual" azimuth? It is neither the type of answer one needs for a plot, 
nor the answer one would arrive at when following the rules on each opening. 
This introduces much more possibility for confusion among sophomores such as 
myself, as you can plainly see. ;D

There he goes again. HO 229 calls the angle "actual azimuth angle," not
"actual azimuth". The word "angle" completely changes the meaning.

There's nothing hard about plotting azimuth angle. I believe Sean is so
fixated on true azimuth (probably the fault of the books he has read)
that he cannot see an elegant solution: use the azimuth angle directly
to plot the sight. For years I've advocated that method. It eliminates
an arithmetic operation and you can forget the azimuth rules.

First set an edge of the protractor perpendicular to a meridian near the
assumed position. That's the orientation of the LOP if azimuth angle is
zero. Then rotate the protractor CW or CCW, as the case may be, by the
azimuth angle. Ignore the numbers on the protractor. Just count tens and
single degrees by eye.

Now the edge is set to the orientation of the LOP. Maintaining that
orientation, slide the protractor so the edge coincides with the AP.
Finally, slide it toward or away from the body the appropriate distance.

That's the method I use with a square protractor and straightedge. I
never draw a line in the direction of the body. It's superfluous with
this method.

--

   

Reply