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I had written-

"Here is a simple case, with lunar distance at 90 degrees. The Sun is at
the zenith, the Moon is on the horizon. How sensitive is the cleared
distance to the altitude of the Moon? And that of the Sun?"

and Frank replied-

George, first of all you've picked a case that would never come up in the
real world of lunars. Today no one would shoot lunars with altitudes below
three to five degrees, and historically anything below ten degrees was
considered off-limits. Second, as I've said many times, the formulas for
altitude accuracy which I've given apply when the refraction is
small --above ten degrees in the altitudes. The behavior when either body
is below ten degrees is qualitatively similar but there are moderate
quantitative changes. When both bodies are above the thick air near the
horizon, it is the parallax of the Moon that drives the whole
problem --EVEN with respect to the required accuracy in the other body's
altitude.

==========

As I understand it, the only reason for not taking lunars at such low
altitudes is because the refractions are unpredictable, on account of
fluctuations in the atmosphere. There's nothing fundamental to disallow
such lunars.

So let's imagine that we happen to know that the atmosphere is in such a
state that atmospheric refraction follows the tables given in the almanac,
precisely. Under those imaginary conditions, am I then allowed to repeat my
question? What goes wrong? I often find that it's instructive to push
situations to the physical limit, to see what happens.

George.

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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