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    Re: Parallactic Retardation
    From: George Huxtable
    Date: 2004 Jan 29, 14:40 +0000

    Frank Reed wrote-
    If you measure the Moon's altitude, it's simple enough: you
    >need more accurate observations than you would lower down. And high altitudes
    >are always dificult to measure so this is a real concern --measure the
    >altitude of the Moon wrong by a quarter of a degree and you'll throw off your
    >longitude by as much as an eight of degree.
    I agree with Frank's numbers here. However...
    There are two main contributions to the true, cleared, lunar distance. One
    is the error in the clearing process, and Frank has assessed the main
    component of that correctly.
    The other is the contribution from the measurement of the lunar distance
    itself. If we were aiming for a contribution to the overall error in
    longitude from that source to be no more than an eighth of a degree, then
    the angle between the Moon and the other body would have to be measured to
    a quarter of a minute (this factor of 30 arises because the Moon moves past
    the stars at only 1/30 of the speed that the Earth spins).
    So there we have the comparison: for the same accuracy in the result, the
    lunar distance must be measured to a quarter of a minute (very difficult),
    whereas the Moon altitude only needs to be measured to a quarter of a
    degree (very easy). That's why, in my simplified picture, I have completely
    ignored any such inaccuracy in the clearing process itself, and presumed
    that all the error resides in the lunar-distance measurement. It's not
    exactly true (approximations never are) but it's quite good enough for the
    purposes of our discussion.
    It's a poor navigator who could measure Moon altitudes only to a
    quarter-degree, though special circumstances may arise, such as measuring a
    Moon's altitude at night above a shimmering horizon that's lit only by that
    So when Frank says-
    >The rapidly changing parallax of the Moon at high altitudes does have an
    >effect on the accuracy of lunars. It's not a huge issue, but there's
    >something to
    I can't disagree. It isn't a huge issue, at all. It's a VERY small one. But
    as Frank says, there is something in it, if not much. It's pleasing to note
    that Frank's views and mine appear to me converging, about the general
    principles involved.
    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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