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Bruce Stark wrote-

>George has pointed out that a phenomena he's
>discovered and given the name "Parallactic retardation" makes the measurement
>twice as critical if the moon is near the meridian.

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Reply from George-

Well, I don't claim to have "discovered" the effect, as it's been referred
to in a text over 100 years ago, though in German. The effect seems to be
little-known in the English-speaking community of navigators, though it has
big implications. It was a great surprise to me, as well as to Bruce.

It's not quite like Bruce states, though. The Moon needs to be high in the
sky, not near the meridian, for the effect to be important.

I wonder whether you all agree that "parallactic retardation" is a
horrible-sounding expression: perhaps someone can suggest a more euphonious
one.

I can only defer to Bruce's comments about observing techniques. Clearly he
has a lot more experience in these matters than I do. I don't claim to be
much of a lunar observer at all, just someone who has taken an interest in
how the thing works. Taking a good lunar, especially in a sea, must have
been the ultimate test of an observer's skill and judgment. Our newbies at
the game should not expect to reach that standard without a lot of
practice. The observational task of taking a good lunar was matched by the
mathematical task of working out the answer.

I agree with Bruce that only one altitude of each body is needed before and
after the lunar measurements. The best order of doing the job seems to me
to be-
1. Altitude of Sun or other body (because it matters least).
2. Altitude of Moon.
3. Several lunars in quick succession.
4. Altitude of Moon.
5. Altitude of Sun or other body.

This keeps the sequence symmetrical in time, and keeps together in time the
measurements that depend most on time. The aim should be for the mean time
of the two Moon altitudes to be about the same (to within a minute) as the
mean time of the lunar distances. The timing of the altitudes of the Sun
(or other body) matters little.

If you find, after taking the observations, that there's a significant
difference between the mean time of taking the two Moon altitudes and the
mean time of taking the lunars, then you can interpolate between the values
of the Moon altitudes rather than simply averaging them. I suggest this is
done best with a simple graph.

To me, it's most satisfying to see the group of lunatics collaborating
together and converging on an answer to Chuck's observation.

If anyone needs another exercise to get their teeth into, there's Steven
Wepster's Mars lunar taken at sea last year. Details can be dound in "About
lunars", toward the end of part 2. I have had no reports that any
listmember has tackled that exercise. If anyone has, it would be
interesting to learn how they succeeded.

George Huxtable.

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