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Peter Fogg wrote-
|
| The correction for parallax is positive and that for refraction is
| negative. There will be an infinite number of positions where the
two
| corrections can cancel one another, ie; the distances will be the
| same.

I think Peter has misunderstood here. Yes, it's true that parallax and
refraction work opposite ways. But parallax will always be the greater
of the two, so they never actally cancel out. But, for the purpose of
Frank Reed's rather unrealistic exercise, it wouldn't matter if they
did. As long as the discrepancy between calculated altitude and
measured altitude varies in some way with altitude, that discrepancy
can, at least in theory, be used to determine the Moon's altitude.

But I suggest that Frank Reed is guilty of rather over-egging his
pudding here, in writing- "The results are accurate to +/-6 miles in
the positional fix (less accurate in one dimension as the Moon falls
lower in the sky)." Come off it, Frank! That method would, at the very
best, be half as accurate (or twice as inaccurate) as
longitude-by-lunar. So if he could achieve an accuracy of 6 miles
either way, a devout lunarian could correspondingly claim 3 minutes
for a longitude. Both would be related to the unrealistic claim of
angular-distance measurement to 0.1 arc-minutes. Just because some
observations on one occasion, from on land, fell within that bracket
does not imply any such accuracy; as Frank, with a scientific
background, should be well aware.

And three of us have asked Frank to show how a position is to be
deduced from a pair of such observations, showing the working. I have
presumed that any delay in responding was because Frank was working on
the details of how to formalise it. We were offered a nice graphical
picture of intersecting cones, but that was not what had been asked
for. Is that all we're going to get?

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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