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    Re: Mid XIX century Nav
    From: George Huxtable
    Date: 2005 Nov 15, 11:28 -0000

    Alex wrote
    
    >I am reading a book by a famous Russian XiX century
    > author, Goncharov, on his voyage to Japan in 1853/4.
    > ...The Russians asked for a
    > permission to land on some rock between their ship place
    > and the land. Apparently a small uninhabited island.
    > The reason they clamed for this permission was the
    > "Necessity to check chronometers".
    > Apparently they thought that this was a good reason
    > for landing on a rock).
    >
    > Unfortunately the author of the book was not interested
    > in navigation and did not want to explain to the readers
    > this point. WHY did the Russians think that this was a
    > legitimate reason?
    >
    > Why exactly did they need land to check their chronometers?
    > Or, more precisely, why did they think that "checking chronometers"
    > is a legitimate reason for a landing?
    >
    > What was the proper procedure of "checking chronometers" they had in mind?
    > Was this by the Lunars?
    > Did they mean that a firm ground
    > is needed for the Lunars? Or Jupiter satellites? to install a powerful
    > telescope?
    
    ================================
    
    Response from George.
    
    There were things you could do on firm ground that couldn't be done from a
    ship, even at anchor.
    
    I will presume, for short, that Russians had then adopted Greenwich Time,
    without being certain that they had.
    
    In general, even the best chronometer will not keep perfect Greenwich Time.
    There are two important things that need to be known. Unfortunately, there's
    often confusion between them, because the words 'fast' and 'slow' are used
    for both.
    
    First, what is its error on GMT at the moment? It will be leading  or
    lagging on GMT by an amount, which needs to be known to within a second or
    so for good navigation.
    
    Second, what is its rate of gaining or losing on GMT? Usually, it will not
    run exactly 24 hours in a (mean Greenwich) day, and the mariner was not
    expected to adjust it to do so; that was discouraged, even forbidden. The
    rate will have been previously set, with some precision, by its maker or
    adjuster, on land, as near to correct as possible. The mark of a good
    chronometer was not that it kept perfect time, but that it had a CONSTANT
    rate, whatever the weather, which would apply over a whole voyage. The
    navigator's duty was to 'rate' it, to measure how much it was diverging from
    GMT, whenever he could. Then he could apply that rate, over the weeks or
    months of a forthcoming voyage, to predict what the time error on GMT would
    be from  one day to the next. If the vessel would be returning to St
    Petersburg, an immensely long voyage lay ahead. So it was necessary to
    establish the daily rate. That rate will have to be multiplied by the number
    of days into a voyage, so the best possible precision must be applied in
    rating a chronometer, to measure its daily rate to within a tenth of a
    second, or perhaps to a few hundredths. How is the mariner to do that job?
    Deducing the moment of noon, from Sun altitudes measured during a day, isn't
    precise enough to rate a chronometer over a period of a few days; it would
    need a long stay.
    
    The answer lies in the stars. If the navigator goes ashore, and sets up a
    post with a clear view of the Southern sky behind it, and some sort of
    peep-hole to position his eye well in front of it, he can time low-altitude
    stars as they disappear, instantaneously, behind that post. The interval
    between passages of the same star is exactly one sidereal day, which is just
    3 minutes 55.91 seconds (and umpteen decimal places) short of a day by GMT.
    The sightline doesn't need to point due South, the post doesn't need to be
    precisely vertical. The chronometer doesn't need to be taken ashore, if some
    signalling procedure, by sight or sound (gunshot?) can be devised. The main
    error is in relating the observed star-time to the dial, and the ticks, of
    the chronometer..
    
    It was by such methods (using a window-frame and a nearby chimney) that
    Harrison would test his clocks and chronometers, and how Cook would re-rate
    his, during a stay in Queen Charlotte Sound, New Zealand.
    
    Note that this technique finds the rate precisely, but doesn't determine the
    chronometer's error, for that day, on Greenwich Mean Time. That error needs
    to be known, but not to such high accuracy as the daily rate; to within a
    second or two, ideally. However, such precision did not become possible over
    great distances until the advent of cable, and later radio.
    
    If that sightline had been set up to be precisely South (which cam be done
    by methods that have already been discussed on this list) then that would
    allow an accurate time-sight on the Sun, or better, a star. But that would
    do no more than determine local time, not Greenwich Time. If the longitude
    of the spot was already precisely known (but likely, it wasn't), that would
    allow Greenwich Time to be determined.
    
    Otherwise, the navigator had to determine GMT, as accurately as he could.
    That could be done by lunars, averaging a long sequence of observations,
    which could be taken on board ship. But the best and quickest method was to
    time, by the chronometer, the disappearance of one of Jupiter's inner
    satellites into the shadow of the planet. That requires a high-magnification
    telescope, which had to be mounted on firm ground. It was not possible to
    observe on-board, even at anchor. There remained a few seconds of
    uncertainty, because the moment when the last glim disappeared depended
    somewhat on the light collection of the telescope, and on the observer's
    eye. It was the best method at times when Jupiter was being cooperative.
    However, there were whole months when Jupiter was too near the Sun to be
    seen, and even when it could, many of the satellite immersions occured in an
    observer's daylight.
    
    ===========================
    
    Alex adds-
    
    > P.S. I am well aware that the first underwater transoceanic cable layings
    > were made at about the same time. So the Lunars were doomed, already then,
    > even as a
    > mean
    > of checking chronometers:-(
    
    ===========================
    
    Not really, no. You couldn't pick up timing from a cable when at sea! Only a
    limited number of ports became linked to the cable network, and others had
    to wait for radio to come. It depended on the potential for lucrative
    traffic. Lunars were doomed, when chronometers became cheap enough for
    vessels to own a set of three, so if one went wrong it was obvious, and you
    could still get the time from the other two. Before the days of radio, that
    was an important matter.
    
    Until the days of extensive cable networks, longitudes were compared by
    sending a batch (sometimes as many as 20) of precise chronometers,
    accurately rated and timed. They went to and fro between measurement
    stations, often on many repeated trips, to compare with local time at each
    end.
    
    Fred mentioned cable-laying vessels. These needed their chronometry to be
    particularly accurate, because they wanted to know exactly where their
    cables would lie, in case (as often happened) they needed to be fished for
    later if they failed.
    
    George.
    
    contact George Huxtable at george---.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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