# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Use for Prime Vertical (PV)
From: Gary LaPook
Date: 2016 Jun 7, 18:10 +0000
Of course but the opposite in the southern hemisphere.
These are the rules for my flat Bygrave. In he first two cases the body never crosses the prime vertical.
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EXPLANATION OF AZIMUTH RULES

`In most situations there is no ambiguity as to which quarter the Zn lies since you know the approximate direction you are looking when you take the sight. The problem arises because the azimuth angle is limited to the range of zero to 90 degrees and when the Zn is near east or west the correct Zn might fall either side of the line so there is an ambiguity in converting from azimuth angle to Zn.One easy rule to apply first is that if the declination is greater than the latitude then the azimuth can never be in the opposite semicircle. To generalize this rule, if the declination has the same name as the latitude and the declination is greater than the latitude, then you start with the direction of the elevated pole (the nearer pole) when converting from azimuth angle to azimuth (Zn.)The second rule to apply is that if the declination is contrary then the Zn must be in the opposite semicircle. To generalize this rule, if the declination and the latitude have contrary names then you start with the direction of the depressed pole (the further pole) when converting from azimuth angle to Zn.These two rules take care of most of the cases, especially for navigators in low latitudes.The remaining ambiguity concerns situations in which the declination is the same name as the latitude but is less than the latitude. In this situation the azimuth of the body will be both north and south of the east - west line during part of each day. The rules compare "W" with latitude to resolve this remaining ambiguity.`

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`Of course "W" only is useful with the Bygrave.`

`gl`

To: garylapook@pacbell.net
Sent: Sunday, June 5, 2016 6:38 PM
Subject: [NavList] Use for Prime Vertical (PV)

Hello:
I have not done any CN for at least  6 months.So a couple of days ago I went to a spot with my Astra and took some shots. I had made up my mind I would reduce the sight by HO 249, Weens 1927, and HO 208...see how much I'd forgotten. Because I was sighting to the west, I even had my "trusty" WW ll army surplus compass (probably used too close to car). The compass results indicated the sun at 270 +/- (true). I've always liked Weens for Hc and calculated the Z with his tables. BUT I could not quickly make sense of his quadrant rules. HO 249 works "like a charm". The bearing was about 260 true. HO 208 also was no problem with quadrant identification.  I then realized that by calculating the PV  angle it would immediately be obvious if I was sighting to SW or NW,ie,Ho greater than PV then sighting to SW; Ho less than PV sighting to NW for this time of year at this latitude.
Any other uses for PV?
Bruce

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