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    Re: Finding the true horizon
    From: David Cortes
    Date: 2013 Mar 26, 20:40 -0400

    Thank you, thank you, thank you.
    For many years I've been away from my sextant because I can't shoot stars
    method of without a very difficult to use and inaccurate artificial horizon
    constructed out of a mirror sitting on adjustable screws.  Your idea seems
    to be quite workable.
    Thanks yet again.
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Frank Reed
    Sent: Tuesday, March 26, 2013 4:57 PM
    To: dcortes@rwlw.com
    Subject: [NavList] Re: Finding the true horizon
    David, you wrote:
    "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? "
    It can be. If you're in a city, you can find the true horizon this way
    (probably easiest to connect up the perspective lines by taking a photo).
    Then, if you're lucky, you'll find that you have some horizontal feature,
    like a building roof line, sufficiently far away that can serve as a
    surrogate horizon, close to the true horizon that you've determined by
    crossing perspective lines. If it's a little above or below the true
    horizon, then you can just work out some effective dip by measuring between
    them. In fact, you could also do this by direct altitude observations
    treating dip as an unknown. The distance "sufficiently far" should be far
    enough away so that a small change in your height of eye does not
    significantly move the line. If you always take your sights while standing
    and from a fixed location (e.g., always on the same floor of your home or
    maybe always from your back deck), then your height of eye will surely vary
    by less than six inches. A ratio of 3438-to-1 is a minute of arc, so if the
    surrogate horizon line is 1700 feet away or more, then it will move by less
    than a minute of arc. If it's over three miles away, then a six inch change
    in height of eye would affect the location of that surrogate horizon by only
    a tenth of a minute of arc.
    Speaking of treating dip as an unknown, for any of you who were following
    the thread about measuring dip, you can measure it with an ordinary properly
    adjusted sextant as long as your goal is just collection of data for
    statistical analysis (as opposed to live navigation). You measure altitudes
    from a known location and clear them treating dip as unknown (set to zero).
    Then the error in the sights is equal to your dip.
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