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    Re: Advice concerning sextants
    From: Paul Hirose
    Date: 2011 Jan 06, 15:11 -0800

    Patrick Goold wrote:
    > My boat 
    > being on the hard, I had to take the sight from my front garden and that 
    > required using an artificial horizon.  Many problems with this.  Wind 
    > disturbs the reflecting surface even with the glass covers in place.  
    
    
    A suggestion I've floated here before -- use the sextant as you would at
    sea, estimating the horizon as best you can. Of course accuracy is low.
    But for practice this method has many advantages.
    
    • It's so much quicker and easier to shoot without the bother of a
    fiddly, jiggly artificial horizon. This is especially nice when the
    weather is not!
    
    • The knack of rocking the sextant ("swinging the arc") to make it
    perpendicular to a sea horizon can be practiced. The corresponding
    movement with an artificial horizon is not the same.
    
    • Stars and planets are not a problem. You can plan a round of evening
    stars and shoot them much as you would at sea. Stars are difficult with
    an artificial horizon.
    
    • The necessary compromise between star visibility and horizon quality
    immediately becomes obvious. Too early in the evening and you can't see
    the stars. Too late and the horizon is indistinct. The books tell you 
    this, but nothing beats seeing it for yourself.
    
    • You'll also realize why many navigators precompute approximate
    azimuths and altitudes for star shots, and preset the sextant. They can
    find their stars while there's still plenty of light for a sharp horizon.
    
    • The relatively low accuracy allows many shortcuts. Time isn't very
    critical. Instead of shooting a limb, you can simply estimate the center
    of the Sun or Moon. Angles may be read to the nearest 5' or so. On the
    other hand, there's no reason you can't read the vernier to .1' and
    apply all the corrections for the sake of practice.
    
    • If desired, the scope may be removed. It doesn't get you any more
    accuracy. In fact, with no magnification and both eyes open you can
    better guess where the horizon lies. If you've never tried shooting the
    Sun or Moon that way, I think you'll be surprised at how easy and
    comfortable it is. (But I find the scope helps with stars.)
    
    
    I'm not saying you should abandon the artificial horizon. It's far more
    accurate. However, estimating a natural horizon is a better 
    approximation to using a sextant at sea.
    
    -- 
    I filter out messages with attachments or HTML.
    
    
    
    

       
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