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    Re: Corrections for speed and bearing
    From: George Huxtable
    Date: 2005 Jun 7, 15:09 +0100

    Robert Eno asked, on 5 June-
    "But this also begs the question: does this correction really matter for
    the surface navigator who is not likely to be moving faster than 7 - 10
    knots? Having myself, taken numerous sights at sea, including noon sights
    (just in case George questions my experience!) it seems to me that this
    factor is not terribly significant. Or is it?"
    Here, I understand that Robert is questioning the effects of North-south
    speed of the vessel on a noon observation.
    And the answer to his question is "No, it doesn't matter (much), and is
    usually neglected."
    But let's be clear about it. Usually a noon sight is used only for
    measuring latitude, and not for longitude at all, for reasons we have
    discussed at somewhat painful length.
    If you observe for latitude, to make the calculations simple, the Sun's
    altitude should be measured at the moment of meridian passage. You can
    estimate your longitude, look up the almanac, and work out when that moment
    will be, and measure the Sun's altitude then, using your chronometer.
    That's the number you need. It won't in general be the Sun's MAXIMUM
    altitude, though, because you may be moving toward the Sun, in which case
    it will still be rising then, to peak later. And vice versa.
    Or you can do it differently, in a way that doesn't need any knowledge of
    time. You can simply watch the Sun and measure its MAXIMUM altitude. For
    the reasons given above, it won't be the MERIDIAN altitude, which is what
    you really need. But the Sun's altitude is changing only slowly around
    noon, so for a vessel travelling slowly, as ours all do, it won't be far
    off the meridian altitude. That gives rise to a small error of no more than
    a few minutes in calculated latitude, which is often simply neglected.
    That's the situation that Robert Eno correctly remembers.
    But it's quite different when you try to extract longitude, from the TIMING
    of the observed maximum altitude. That timing is MUCH more sensitive to
    North-South velocity. It's one of the (many) reasons why longitudes at sea
    have been measured at any time EXCEPT near noon, at times when the rise and
    fall of the Sun, due to its motion in the sky, quite overwhelms any effect
    of the speed of the vessel.
    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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