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    Re: marine sextant on land
    From: Will Ross
    Date: 2008 Oct 29, 14:14 -0700

    Why dont you use the dip short tables?
    
    On Oct 27, 3:12�pm, Paul Hirose  wrote:
    > 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.
    >
    > --
    > I block messages that contain attachments or HTML.
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