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    Re: Real accuracy of the method of lunar distances
    From: George Huxtable
    Date: 2004 Jan 12, 23:23 +0000

    Richard Pisko's carousel and lamp make a really good analogy  for the
    parallax of the Moon in the Earth-Moon system, though I got a bit lost
    later on, and didn't understand the point of the three marks 120 deg apart.
    But when I got to the end of part I, I wondered if he was using a
    lunar-distance as it's intended.
    A lunar distance is intended to provide GMT ONLY, using the position of the
    Moon against the star background like the one hand of an immense clock,
    that makes only one turn per month. From the measured position of the Moon
    against the stars, having made those essential corrections, then by
    comparison with the predicted position of the Moon against the stars at
    various times, you can work out the time. Then, knowing how the rotation of
    the Earth varies with GMT (simple), you can work out what position the
    Earth has turned to at the moment of the lunar. That's the job a lunar
    It strikes me that in the paragraph-
    >Locating the Part I or "parallax scroll" position with a fixed
    >precision and accuracy of observation would give the angular position
    >of the platform to a varying degree of accuracy, greatest when nearest
    >the street lamp, and least at the far left or the far right.  Greatest
    >when the vector arrow length (arc distance divided by time) of the
    >parallax is longest.  Translating this to Celestial navigation, it
    >would seem the accuracy of position finding would be greatest when the
    >observed moon crosses the meridian.
    Richard may be trying to derive the angular position of the Earth
    (carousel) from the perturbation by parallax of the apparent position of
    the Moon. If I have understood it correctly, that isn't the way a lunar
    works. However, I should add that because it is such a big correction,
    under certain circumstances, parallax can play a large part in the
    It's also true that measuring the altitude of the Moon (which is usually
    necessary in order to deduce the parallax) can provide a useful
    position-line to use when the GMT has been obtained.
    If these comments betray my own misunderstanding of Richard's analogy, I
    hope he will put me right.
    I thought it might be helpful to get these matters straight before
    considering that picture in further detail.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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