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Re: Newbie Question - Parallel LOPs
From: Gary LaPook
Date: 2009 Jan 30, 08:10 -0800

Most formulas used to compute azimuth (Az or Z) produce a result between 0º and 180º (though some produce results in the range of 0º to 90º.) Prior to plotting the LOP the Az are usually converted to Zn in the range of 0º to 360º (but it could be done differently.) For your situation you must subtract your Az from 360º to find the Zn for plotting the afternoon LOPs. ( I assume you are in the northern hemisphere, add Az to 180º if you are in the southern.)

If you used either H.O. 229 or H.O 249 you will find the rules for the conversion from Az to Zn in fine print on each page of the tables.

gl

--- On Thu, 1/29/09, Lu Abel <lunav@abelhome.net> wrote:

From: Lu Abel <lunav@abelhome.net>
Subject: [NavList 7197] Re: Newbie Question - Parallel LOPs
To: NavList@fer3.com
Date: Thursday, January 29, 2009, 9:44 PM

Mark:

I assume the times you gave were local times.  And I'm going to assume 
you are north of the equator.   What I would guess is that you made a 
mistake in your Zn calculations.   Your 1000 Zn should have been to the 
southeast and your 1400 Zn should have been to the southwest.   Don't 
know how you did your sight reductions, but if you were using the 
trigonometric equations and solving them on a calculator, remember that 
arccos (Zn)  produces two possible angles.

Welcome to the list!

Lu Abel

Watermark wrote:
> I've recently taught myself Celestial Navigation from library books
> but I still have some newbie questions. I thought I was doing fairly
> well using an artificial horizon - accuracy to within 6NM last week -
> but today after
 taking sun sights at 10.00 and 14.00 all my LOPs ran
> parallel. I reduced 8 different sights and double checked everything
> but the same result. I had read that a 4 hour spread would be enough
> but today my LHAs and Zn's all came out similar. My DR Long is 121W. I
> can only deduce that taking sights of the sun at a similar altitude at
> 10.00 and 14.00 caused the LHA to be too close despite 4 hours time
> difference? Is this correct and will this always occur taking sights
> at this time if not moving? Excuse the naivety but like I said I am
> still a beginner.
> Many thanks,
> Mark
>
> >
>

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