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    Re: Finding the true horizon
    From: Frank Reed
    Date: 2013 Mar 26, 13:46 -0700

    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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