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    Re: On measuring the (lunar) distance
    From: George Huxtable
    Date: 2002 Mar 23, 01:10 +0000

    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.
    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
    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.
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.

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