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    Re: Finding the true horizon
    From: David Cortes
    Date: 2013 Mar 26, 12:34 -0400

    So is this of some help to us land-locked navigators who want to practice
    with our sextants without the hassle of an artificial horizon? 
    
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
    Sent: Tuesday, March 26, 2013 12:25 PM
    To: dcortes{at}rwlw.com
    Subject: [NavList] Finding the true horizon
    
    The true horizon is invisible but easily defined. You find the vertical
    (gravitationally, with a level or a plumb line or similar) at some point on
    the Earth's surface and then you locate all points that are 90� from that
    vertical line. That's the true horizon. It's important to note that this
    horizon and the vertical it's derived from are locally variable. It's only
    good luck for us earthling navigators that the Earth's gravity field is
    rather smooth, closely approximating a spheroid. If we lived on a
    gravitationally lumpy world, celestial navigation would still be possible,
    but the connection between coordinates drawn on the sphere and the readings
    of celestial navigation would be much more complicated. 
    
    There's a trick for finding the true horizon that works extremely well in
    some cities. Buildings are constructed "plumb". The floors are supposed to
    be level. If you live in a city with modern buildings, and if those
    buildings aren't in danger of collapse or settling over many decades, then
    you can count on floors being level. If you look into the distance and sight
    down the side of a building, perspective will lead you right to the true
    horizon. This is especially easy in photographs. I've attached a photo
    trimmed from a random photo looking down an avenue in New York City. The
    lines drawn from floors on the building on the left converge in the
    distance. That crossing point is located right on the true horizon for this
    observer's location. Note that a building at great distance wouldn't work
    since there would be some rotation due to the curvature of the Earth.
    
    You can use this trick in photographic experiments in astronomical
    position-finding. Get the horizon from the lines of neighboring buildings.
    Note that this technique finds the true horizon so, of course, there's no
    correction for dip, refracted or otherwise.
    
    -FER
    
    
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    Attached File: http://fer3.com/arc/imgx/true-horizon.jpg
    
    
    View and reply to this message: http://fer3.com/arc/m2.aspx?i=123101
    
    
    

       
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