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    Re: time of noon from the Sun.
    From: George Huxtable
    Date: 2006 Nov 24, 19:59 -0000

    Ken Muldrew wrote, (in [NavList 1739] Re: time of noon from the Sun)
    about using time from a pocket watch that had been set approximately
    from the Sun at noon, and not yet corrected by a later time-sight-
    
    
    | Let's agree that if you're using a mechanical pocket watch to
    navigate,
    | then you're also stuck somewhere about 200 years ago.
    
    I'm puzzled why Ken says that. Even navigating with a chronometer, you
    would still need some measure of local time, whether it's used to set
    a pocket watch or to compare with a chronometer reading, to get
    longitude. And the worst moment of the day to make that measurement is
    the moment of noon. Only if it's widely bracketed on either side of
    noon, and suitable corrections are made for North-South speed and
    declination change, will that setting of noon, at noon, be accurate.
    
    | This means that you
    | are not doing position line navigation but independent latitude and
    | longitude measurements.
    
    Agreed.
    
    | Since the noon sight has given you a good
    | latitude, and since the best you can hope for with longitude will
    come
    | from a lunar distance (at least as far as celestial observations
    go),
    | which is pretty innaccurate even with good sights, then I would say
    that I
    | would be perfectly happy to navigate using the watch that shows only
    | approximate time.
    |
    | My watch is showing local apparent time, so it is being used to
    measure
    | intervals for updating the dead reckoning; I wouldn't be trying to
    infer
    | longitude from the time.
    
    But if you HAVE taken a lunar distance, inaccurate though that may be
    in itself, to get Greenwich time to within a minute or two , then it's
    necessary to use a measure of local apparent time to infer the
    longitude. And even if you have a precise chronometer, giving you
    Greenwich time directly, again, you need local time for longitude.
    That's the demanding application for which a rather precise
    determination of local time (within a minute or so) is needed. For
    other purposes, such as ringing the ship's bell to end a watch,
    precise time isn't needed, of course.
    
    | If land is nearby, then surely I would be
    | navigating by latitude + observation + soundings and not by anything
    that
    | requires knowing the precise local apparent time. I'm not sure what
    I
    | might miss by having a watch that is, say, 10 minutes off.
    
    Well, if you don't care about the longitude, then of course, you don't
    need precise local time. But why would you wish to know latitude and
    not wish to know about longitude, with land nearby?
    
    If you were navigating with a watch that was 10 minutes off from local
    time, it would mean that any lunar-distance or chronometer
    determination of longitude would, quite unnecessarily, have incurred
    an error of 10 minutes of time, or 2.5 degrees of longitude, just from
    that error in the clock setting. That would far outweigh the errors in
    any lunar-distance measurement itself, and would ruin it; just as it
    would ruin a longitude-by-chronometer.
    
    For measurement of longitude, reasonably-precise local time is
    essential.
    
    George.
    
    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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