# NavList:

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

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Re: ex-meridian sights - further question
From: Barry Hudson
Date: 2001 Feb 13, 9:26 PM

```Most interesting. The Ex meridian method stems
from ancient times before the good Marc. Ste. Hilaire position line method.
However it appears
to be time honoured especially with DOT examiners. What has always
interested me is
why not simply use a sight taken 10 or so minutes
before or after noon ( up to 30 or so minutes
allowed by ex-merid tables) as a simple P/L. It
can be used with future or past P/L's to give a
position. Historically the idea of the ex-merid. was
that commercially it was most important to have
a noon position so that distance run  and distance
to go was known and course fuel consumed,
slip and the rest of it calculated. Commercially of
course the noon pos. is still important although
usually worked from GPS. I dont want to knock
the users of the ex. merid. its just that I have never
Regards
Barrie
----- Original Message -----
From  "Steven Wepster"
To:
Sent: Tuesday, February 13, 2001 6:13 AM
Subject: Re: ex-meridian sights - further question

> Hi Rusell,
>
> >   Hi Steve and thanks for the reply. That does make a bit more sense. By
the
> >   way, according to the example in Nories on ex-meridian sights, the
resultant
> >   LOP will not run due West-East but is offset by the difference LHA and
0
> >   degrees.
>
> That puzzles me. Surely, the latitude that comes out of the
> calculation is dependent on an assumed longitude, because the meridian
> angle t is. But then there is no reason why the directional offset of
> the LOP should be equal to the LHA (is that what you mean?).
> Noorduyn's "Leerboek der Zeevaartkunde" and Cotter's History of
> Astronomical Navigation" both do not mention the offset. I'll have a
> look in some other sources tonight. Can you give a reference to Norie?
>
> >   Why is this since the sight is 'back-dated' (or pre-dated if taken
> >   before LAN) so that it represents the LAN altitude. Surely then the
> >   ex-meridian LOP should be a latitude? i.e. the latitude which should
have
> >   been obtained at LAN. According to the examples the resultant latitude
is
> >   just a point on the LOP. If this is true why bother with the
ex-meridian
> >   correction??
> >
> Please note that the latitude you get is the latitude at the time
> when you took the sight, not at the time of merpass. The tables only
> correct for the apparent motion of the body, not for the motion of the
> observer.
>
> >   regards
> >   Russell
>
> So long, _Steven.
>
>
```
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