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    Marine sextant on land
    From: Paul Hirose
    Date: 2008 Oct 27, 12:12 -0700

    I want to use my Astra Mark III at home, but don't want to bother with
    an artificial horizon. My solution is to estimate the horizon by
    eye.
    
    In some directions I have a reasonable horizon -- mountains in the
    background but flat land nearby. But mostly the view is obstructed by
    buildings, trees, and a fence. That's all right. I already know my
    position, so the altitude intercept directly indicates the error in my
    horizon estimate. From that I'm learning where the horizon lies on these
    obstructions.
    
    I often pre-compute the azimuth and elevation, especially for second
    magnitude navigational stars. They can be hard to find in twilight
    unless the sextant is preset and I know the direction to look.
    
    Lights on the horizon can make it hard to find the star when the sextant
    is preset. It helps to use a horizon shade or simply cover the front of
    the horizon glass with my left hand. But once found, there's no problem
    keeping track of the star.
    
    Manmade lighting isn't entirely a bad thing. The additional illumination
    provides a horizon reference when it would be be too dark to shoot
    otherwise. From a high building in a big city it may be possible to
    shoot all night.
    
    Any method can be used to reduce the sights. I plot the LOPs on an 8.5
    by 11 inch sheet of paper. It helps refine my technique: I can see that
    in some directions I have a tendency to shoot a bit high or low.
    Currently I'm plotting a scale of 10 millimeters per degree, but that's
    too small. For the next sheet I'm going to expand the scale to 20 mm per
    degree.
    
    Much simplification is possible because the observations are of low
    accuracy. Time within half a minute is good enough. Refraction may be
    ignored except for very low bodies. The center of the Sun or Moon may be
    observed to eliminate semidiameter correction. Index correction is
    negligible if the sextant is decently adjusted. Dip is negligible if
    you're standing on the ground. Angles may be read to only the nearest
    tenth degree.
    
    Sun or Moon shots are best with the scope removed. With the wide field
    of view and both eyes I can better estimate the horizon. The scope helps
    for star shots in twilight, but due to the narrower field of view, it
    helps to take the sextant away from my eye for a moment to check that
    the spot where I'm putting the star looks right. Still, I'm sometimes
    way off. One predawn Aldebaran shot during the weekend missed by 2.2
    degrees! But that's part of the fun. The sights have low accuracy
    compared to what possible at sea, but that doesn't mean they're easy.
    
    Anyone who wants to practice celestial navigation with a marine sextant
    should try this. You're not actually navigating, so there's no need for
    a wide spread of azimuth. Just shoot what's visible from your porch or
    balcony. Even one window with a good view should give you some bodies.
    
    --
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