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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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