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    Re: Dip by sextant and systematic error
    From: Brad Morris
    Date: 2013 Apr 2, 16:34 -0400

    Hi Frank

    If the NavList is "A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding" then you must agree that pursuit of a 200 year old problem in celestial navigation is worthy of discussion here. So even if we are on a wild goose chase, I'm still having fun (yes, I'll say it again).  Not everything will have a solution in equations.  The Soviet Dip Meter solves the problem  I can do the same, obviously, with the Prismatic Circle of Reflection.  Both afford a direct measurement of dip.  So instead of modeling the dip, it can be measured.  It works at sea, so the common critique that "its not practical & doesn't work at sea" is voided.  The only real solution (from the cheap seats over here) is to measure it contemporaniously with the altitudes.

    As you, Frank, have said many times lately, navigation is a solved problem.  I agree. Celestial Navigation has little place in that solution.  Navigation is now position finding via GPS.  The back up for that is another GPS.  Celestial Navigation is now only distantly considered a viable solution.  It's really just a quaint practice, like knowledge of slide rule manipulations.

    So forgive us our wild goose chase.  So what if we're tilting at windmills.  Its a real celestial navigation problem.   This is the NavList, were we discuss such things.

    Brad



    On Apr 2, 2013 3:18 PM, "Frank Reed" <FrankReed{at}historicalatlas.com> wrote:

    Marcel, you wrote:
    "However, I think that the availability of environmental (atmospheric) data and of analytical tools have improved over the past 200 years. One may therefore hope to derive from a sufficiently large set of observations some improved estimation methods. "

    I've considered this, too. Certainly today we can do highly detailed numerical simulations based on much finer resolution models of atmospheric temperature variations. The environmental data, though, is not really suited to our task. To predict refraction between an observer 25 feet high and the ocean horizon "zone" centered some five miles away, we would need access to detailed temperature and pressure data for that path. There are definitely some basic cases that we can handle and that we can measure like the case of constant lapse rate that I mentioned previously. It doesn't even have to be purely linear as long as it the profile all along the path is reasonably similar. I do agree that we could generate "on-the-fly" dip tables for situations where we have an extraordinary amount of environmental data. I can't imagine any circumstance where this would have navigational value. It's interesting physics, but ever more removed from the topic of navigation.

    You added:
    "Even if one would not arrive at an improved estimation one could at least *show* how it scatters."

    I fully agree that measuring the scatter is useful. The observations ARE worth doing.

    By the nature of the experiments, every set of data would unfortunately be limited to a particular locale. The horizon phenomena that I see from my favorite observing locations here on Conanicut Island are distinctly different from the phenomena that I see from Stonington Point. The two places are separated by only 24 nautical miles. The former faces open ocean with moderately high land to the immediate west while the latter faces a nearly enclosed sound with land to the north. I would guess that the different geography of these places influences the layering of air over the water, but this isn't really any help in generalizing the results from one observing location to another.

    -FER


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