NavList:

George is certainly on the right track.  When I have a bit of time, I will tell you about the problem I ran into when I was required to shoot and reduce an ex-meridian of the sun for my Chief Mate/Master's assessment sheet.

 

This is a very difficult ex-meridian due to the height of the sun and the limitations of the tables.  Picking the correct "a" number is not straight forward, and due to the fact that it borders on the cusp of the tables, a true double interpolation is not possible.  Trying to use the formula is equally difficult as it loses accuracy at such high Ho's.  The best you can do is pick an "a" value as close as you can figure and work the sight then see how well it does against a standard sunline.  This was indeed the challenge of this exercise, and it is why I was asking people to post how they obtained their corrections.

 

Fortunately, in higher latitudes, or when the declination name is contrary to latitude name, where the differences in latitude and declination are much greater, the "a" values are significantly smaller and a more accurate ex-meridian correction can be derived.

 

My solution and comments will be forthcoming.

 

Jeremy

 

Jeremy's ex-meridian exercise turns out to be of great interest. It has
taken me into unfamiliar waters, as I have never even attempted an
ex-meridian correction before. And it's showing up some of the problems that
are involved.

Am I right in thinking that this exercise goes right up to, and perhaps even
beyond, the viable limits for which the ex-meridian procedure can be
expected to work? Perhaps that's the lesson that we are intended to draw.
With such a high Sun, it doesn't allow much difference, between observer's
long and Sun's GHA, before the Sun's direction gets way off the meridian.

The difficulties of using ex-meridian tables really show up here. In the
tables that I have, Bowditch and a modern(ish) Norie's, the quantity A, or
alpha, is tabulated only to the nearest degree of lat or dec., and changes
greatly for each such step, so getting the right number calls for a tricky
double-interpolation. Just by in-the-head estimation, I would put A at about
21 or so. And that leads me to an ex-meridian correction of about 14', but I
can see that the correction could easily be half an arc-minute off, either
way.

Am I in the right ball-park, I wonder? Normally, I would have gone straight
for a St Hilaire position-line. Being pushed by Jeremy into trying an
ex-meridian method has only reinforced that view.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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