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    Re: Moon altitude problems.
    From: George Huxtable
    Date: 2006 Aug 20, 19:04 -0500

    Robert Eno questioned Frank Reed's statement-

    | If the line is tilted so that the
    | > upper cusp is to the right of the lower cusp (and the Sun is to
    | > right), then the Lower Limb should be used.
    | Robert asks:
    | Frank, perhaps I am misinterpreting your description and/or
    visualizing this
    | incorrectly, but it seems to me that under such conditions, would it
    not be
    | the upper limb that should be used?

    Frank has it right.

    | Keeping along the same lines as my original posting, minus the coded
    | explicatives in deference to George Huxtable's sensibilities...

    Robert has completely misunderstood me. I am not at all bothered
    whether he uses "coded explicatives" , or even real explicatives, in
    his postings. It's nothing to do with any such prudishness. It's just
    a plea that posters choose an appropriate threadname that is relevant
    and searchable, for the benefit of those that will wish to unearth a
    topic in the future. Indeed, it seems likely that the curious
    characters in that threadname have also caused more than one such
    message to be rejected in transit as spam; that was an additional
    defect I had not foreseen.

    | ... I posed this
    | question to a good friend of mine, a Master Mariner with three
    decades of
    | seagoing experience (prior to GPS) and who taught celestial
    navigation at
    | the Nautical Institute in Nova Scotia for many years after he left
    | seagoing service.
    | My friend's opinion of moon shots contains even more colourful and
    | descriptive explicatives than those which I offered in my original
    post; as
    | can be expected from a hard-boiled merchant mariner.  Essentially
    though, he
    | told me that moon shots tend to be inaccurate and unreliable and
    that in his
    | career, he avoided them if he could. The moon, my friend says, is
    too fast
    | and too close. There are too many variables with the moon.

    Maybe he had inherited similar prejudices, from another old salt, in
    his own early days. And now, Robert is doing his best to pass those
    prejudices on to another group of navigators, us. All very well, if
    there is reasoning and hard evidence to back it up, but those so far
    have been conspicuously lacking.

    Yes, some extra care has to be taken with corrections for the Moon,
    but these are well known and well tabulated in the almanac, and
    provided that has been done correctly, there's absolutely no reason
    why a Moon observation should be less accurate than any other (in my
    own view). In Robert Eno's case, when using a bubble sextant
    attachment, errors
    are far more likely to stem from its own inherent inaccuracies, rather
    than any problems with the Moon itself. I have no experience at all
    with bubble devices, but one possible difficulty comes to mind, with
    the Moon and a bubble sextant. When observing the Sun, as I understand
    it, the aim is to match the disc of the Sun with the disc image of the
    bubble, and try to keep it there. How does that work out with the
    non-disc of the Moon, when only a part of that non-disc is the crisp
    edge of the appropriate limb? What do you centre, against what? Are
    the problems that Robert finds with a bubble sextant relevant to an
    observer who can see a horizon? Have a significant fraction of the
    many Moon observations he relates been measured against a real
    horizon, and do the same discrepancies occur then?

    If Robert is prepared to show us his own observation log, so that we
    can check over the problems he tells us about, we may be able to
    detect whether he sees observational scatter or some systematic error.
    And perhaps, some possible defect in the way that those all-important
    corrections have been made.

    Until now, all we have seen is hearsay evidence. And prejudice.

    By the way, the Moon altitude correction tables, in the back pages of
    the nautical almanac, were devised with a horizon sextant, rather than
    a bubble sextant, in mind, so they include a semidiameter correction.
    rules are given for applying them to a bubble sextant, and presumably
    these have been rigorously followed. I hope so.


    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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