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    Re: How Many Chronometers?
    From: Brad Morris
    Date: 2009 May 6, 19:26 -0700

    Hi Jim
    Your definitions are precise.
    Chronometer rate is the gain or loss of time per day, or whatever period you 
    may chose to define.  The assumption is that the rate is constant is 
    fallicious.  The rate of a chronometer varies with many factors.  Take a 
    close look at the chart recently provided by George.  
    If the rate was constant, the line would be straight, independent of 
    corrections at known longitudes.  In fact, we see the converse.  The line 
    changes slope from correction to correction.  We even see positive going 
    slopes change into negative going slopes.  The rate, therefore, is changing.  
    The argument will be offered that modern mechanical chronometers are superior. 
     In fact, they are.  However, they are not perfect.  The rate still changes 
    with the viscosity of the lubricant with temperature. Hence the double box, 
    to slow the transfer of thermal energy, with a glass top, such that you can 
    see the chronometer without opening the box. The quartermaster is cautioned 
    to wind the chronometer every day, at the same time, by the same amount, such 
    that the chronometer utilizes the same spring region, independent of the wind 
    down counter that provides the hours until unwound.  All this attention to 
    detail, and the rate still varies.
    If you remember other parts of the Royal Observatory, where H1 lives 
    (hooray!), then you have seen the gradual transformation to other time 
    pieces, away from the mechanical.  And each time piece has been superceded by 
    the next, each with dilemmas, problems and issues.
    Back to Greg's question though, which I understand to be the determination of 
    when a chronometer has gone bad, we have offered the solution of solving for 
    time when the longitude is known. If the longitude is not known, then you 
    suffer from not knowing which one is wrong, the longitude or the chronometer! 
    Bruce Stark, when he was contributing to the list, offered a wonderful 
    exposition on how to recover GMT via lunars.  The details are found in a 
    NavList posting whose number I cannot remember.  I did keep a file of it 
    though, and link it herein.  All the way from not knowing any time to GMT! I 
    have not exercised the method and can offer no insight as to how accurately 
    one can recover GMT from it.  Based upon the source, however, I suspect it 
    works quite well!
    Best Regards
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