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    Re: "Artificial Sights"
    From: Jared Sherman
    Date: 2003 Mar 21, 00:16 -0500

    I'm afraid I don't quite understand what the good author was up to with his 
    "This I hope will give you some idea of what "Artificial sights of the [lower 
    limb]" means".
    
    It sounds like he is saying that they pick any random flat spot--position 
    unknown and unascertained--and then use an artificial horizon to double down 
    on the sun, noting the exact time of the incidence when the two images 
    coincide. At 20 random times.
    
    But without knowing *where* they are, how does this information tell them 
    anything about how accurate their chronometer is?
    
    Or, does his account perhaps err in that they would have used a known position for their sitings?
    
    It is interesting that circa 1900 they were apparently using mechanical 
    chronometers with a "flying" dial that read in 10th's of a second. I've seen 
    small dials like that set into chronometers, but always considered that 
    information useless for the single user.
    
    I also had to stop and reread "at most they were about three tenths of a 
    second out, that is not a quarter of a mile, as 4" (seconds) wrong in 
    chronometer time means 1 mile wrong in distance " which I assume is simply a 
    matter of understatement: If four seconds error translates into one mile, 
    then one second is 1/4 mile, and 3/10 second is merely 1/12th of a 
    mile--considerably better than "not a quarter of a mile".
    
    I suppose on our returning topic of how accurate sextant positions can be, at 
    least this author presumes that a position should ordinarily be accurate to 
    better than a quarter mile. Quite a nice feat.
    
    
    

       
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