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    Re: time of noon from the Sun.
    From: Ken Muldrew
    Date: 2006 Nov 24, 14:59 -0700

    On 24 Nov 2006 at 19:59, George Huxtable wrote:
    > 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.
    That's fair enough. I was coming at the problem from a perspective of not
    having a chronometer. I still maintain that *setting* a watch for local
    apparent time (as opposed to determining the error) is simplest at noon if
    one is taking a noon sight anyway.
    > 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.
    Here I'll repeat that I don't think there is any value in having a watch
    set to read exact local apparent time, only in knowing the offset to get
    exact LAT. If you only want your watch to show approximate local time,
    then there is no extra effort in using the noon sight if it's already
    being taken. That you need a proper time sight before you can know the
    exact locat apparent time is understood. I would liken using a time sight
    to set your watch to trying to get a sextant index error of zero (more
    trouble than it's worth).
    > | 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.
    Here we have no argument. It went without saying (but in retrospect
    probably shouldn't have) that one has to do a time sight in close
    proximity to one's lunar distance sight (depending on the rate of the
    watch, any E/W movement, and the rate of change of the EOT).
    > | 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?
    I meant that I would get my longitude from a map rather than a lunar. If I
    didn't have a map, then my longitude wouldn't do me any good anyway.
    > 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.
    Of course we are in agreement here. I just left out my unstated assumption
    that I wouldn't take a lunar without also taking a time sight. If we have
    any disagreement, it would seem to lie with whether or not a watch keeping
    local time should be kept so that it has no error, and that seems mostly a
    matter of personal preference.
    Ken Muldrew
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