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    Re: Winter Sextant Sight Accuracy?
    From: Jared Sherman
    Date: 2002 Jan 11, 3:09 PM

    George, for the sake of brevity I omitted the things that I was not seeking 
    discussion of. But since you asked...
    
    <1. I am puzzled by what Jared says about checking index error. >
     I did check the index error before taking sights. Perhaps you are familiar 
    with the Cassens & Palth design which puts a large index error adjustment 
    screw at the end of the vernier? That's what I was using, and in the cold 
    with gloves I *suspect* that I loosened the screw and tightened it again as I 
    went back and forth, so that I unlocked the wheels and the zero slipped. This 
    *after* I had checked that it was zeroed, during the sightings.
    
    I have now tightened that screw strongly enough so that it will not accidentally come undone.
    
    
     Why? If it is the same temperature at both times, what should change? And 
    given that a high sighting of a pinpoint object is going to be sharper than a 
    low sighting of a somewhat large object in disturbed air...Why not?
     Note that I am not relying on the zero from the previous night--I am checking 
    it against the horizon on the following day anyway. But I have found that in 
    the same temperature, a zero from a pinpoint light source will be easier and 
    more accurate than from any other source.
     One of the other things that I didn't mention for brevity is that I have some 
    sight damage from LASIK. The pinpoint light source from a star is also easier 
    for me to use than a horizon or sun disk.
    
    2.  Nope, didn't say they were. The 
    winter means that I will be dealing with a lower sun though. I said late 
    winter afternoon, and around here that means the sun is near 28 degrees at 
    high noon, 14 degrees at mid afternoon, and somthing like ten degrees or less 
    in the late afternoon.
    
    Compare the atmospheric effects for a sighting at ten degrees to a typical 
    summer midday sighting at altitudes that neer come near ten degrees. THAT's 
    the effect of winter.
    
    <>
     Thank you George! That's the point that I was asking for a response on. 
    Although, you still haven't responded on the main point: What kind of 
    position accuracy do YOU get with a sextant under similar circumstances? How 
    tight a cluster and what absolute position accuracy? Can you get 2/10th's of 
    a mile with a rough horizon and an Ho on the order of 10 degrees? 1/2 mile? 1 
    mile?
    
     Yes, and that is accounted for.
    

       
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