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    Time of noon from the Sun.
    From: George Huxtable
    Date: 2006 Nov 24, 11:40 -0000

    I have renamed this thread, because this aspect has diverged
    significantly from the original topic.
    
    I had written, in NavList 1724, Re: lunars with and without altitudes
    
    | "I am aware that Frank Reed has been advocating the use of
    observations
    | around noon to determine the moment of noon for some time, and  have
    no
    | wish now to rehearse once again the weaknesses in that procedure.
    As
    | he says, it's an inferior way to get local time."
    
    and Frank replied, in Navlist 1728
    
    | Well, let's not confuse matters by misquoting me, George.
    Observations
    | *around* noon are a very good way to determine the moment  of noon,
    as I have
    | described at some length in the past (and probably will again
    soon!).
    
    What I had added, but Frank didn't quote, was-
    
    "To achieve any
    precision, it needs a wide bracket of equal altitudes, before and
    after noon.", and I hope that Frank will agree that is the case.
    
    | What I
    | referred to as an "inferior" way to get local time was the late
    18th/early 19th
    | century practice of "calling out" noon based on the *single*
    observation of the
    | Sun's maximum altitude.
    |
    | And you wrote:
    | "What, then, would be the procedure for discovering the moment of
    noon
    | AT noon, in such a way that the moment of noon can be "called
    out"?"
    |
    | I was referring to the "common practice" for setting local time on
    pocket
    | watches carried by the ship's officers.
    
    and added
    
    I believe you are under the impression
    | that ordinary watches were rare c.1800. There's good evidence that
    they were
    | not. Most officers aboard ship seem to have carried them by this
    date.
    
    I don't see what the rarity or otherwise of such watches has to do
    with this discussion.
    
    | They
    | set  them at noon to local apparent time as determined by "calling
    out" noon
    | during  the Noon sight. This was a common practice, whether we like
    it from a
    | modern  theoretical standpoint or not.
    
    What evidence can Frank offer to back that statement? It seems an
    absurd thing to do, if later that day they were to reset them as he
    describes later.
    
    Navigators later in the day "regulated the
    | watch"  by doing a time sight. This would lead to an adjustment (or
    | correction) of the  watch by a few minutes. That "regulated" local
    time from the time
    | sight  would then be compared with the Greenwich time from a lunar
    distance
    | sight or the chronometer.
    
    That is the right time for correcting the watch from an observation;
    either earlier in the day, when the Sun was rising, or later, when it
    was falling. Any time, indeed, other than noon, the worst possible
    moment. If the watch had been set correctly earlier, by a previous
    time-sight, , what on Earth would be the point of deranging it, to an
    unknown extent, by making an imprecise observation, only to reset it
    again, correctly, later?
    
    Of course, corrections to the watch reading, in order to know local
    time, have to be kept in mind, and carefully noted, following the most
    recent time-sight determination. These corrections are due to any
    uncorrected watch error determined then, any known watch-rate error,
    any change in equation of time (easily determined), and most
    important, any longitude change, estimated by dead-reckoning.
    
    I think there may be confusion between a call of "make that noon", or
    some similar words, for any others, out on the bridge with their own
    sextants, to freeze the measured altitude at that maximum value. That
    would not imply a resetting of watches.
    
    I ended up by asking, about the notion that watches would be reset to
    local time at noon by calling out noon-
    
    "How would that
    procedure cope with any North-South component of the vessel's speed,
    and the changing declination of the Sun? And what would be the
    expected level of precision in the result?"
    
    Any attempt to determine the moment of  noon from the moment of
    maximum Sun altitude is, as Frank well knows, very susceptible to
    large errors caused by any North-South motion of the vessel (or even
    the changing Sun declination, to a lesser extent). But that question
    wasn't answered.
    
    =======================
    
    Of course, it never matters exactly what a watch-dial actually reads,
    as long as any error on that reading is known.
    
    I think that's what Ken Muldrew was getting at, when he wrote, in
    [NavList 1729]-
    
    "When keeping apparent time on a watch, it seems to me that using the
    approximate time of noon through a sextant altitude observation is
    perfectly satisfactory. In my own attempts to keep apparent time on a
    mechanical pocket watch, I found it impossible to set accurately from
    a
    time sight. It was much easier to simply set the approximate time
    (taking
    care to match the position of the minute hand between tick marks to
    the
    position of the second hand) and then use a time sight to get the
    error.
    Between the equation of time and any East/West motion, the watch will
    be
    off so quickly that there is really no point in taking pains to set it
    accurately."
    
    He seems to accept, then, that the setting at noon was to no more than
    an approximate time, to be put right at a later time sight. After
    having set it approximately, around noon, would he then be prepared to
    use that approximate time for navigational purposes, prior to
    determining those errors with a proper time-sight?
    
    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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