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Re: Correction: Error in H.O. 229 Introduction/Explanation?
From: Paul Hirose
Date: 2013 Jun 16, 16:22 -0700
From: Paul Hirose
Date: 2013 Jun 16, 16:22 -0700
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. --