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Subject: Re: ex-meridian sights - further question
From: Steven Wepster (wepster@XXX.XXX)
Date: Tue Feb 13 2001 - 09:13:28 EST
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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