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    Re: Appeal to forum members to collect data
    From: Jim Hickey
    Date: 2007 Oct 25, 17:04 -0700

    On Oct 25, 5:50 pm, "George Huxtable" 
    > Jim wrote-
    > | I would like to collect some data from a variety of observers using a
    > | variety of marine sextants taken in a uniform manner.
    > I have no wish to pour cold water on such an admirable aim, but I think
    > Jim's attention should be drawn to a serious difficulty.
    > The biggest error in measuring altitudes above the horizon is in
    > establishing the direction of that horizon. The dip of the horizon can often
    > differ by a significant fraction of a minute, and occasionally by much more
    > than that, from its "book" value. This is the effect known, when it becomes
    > serious, as "anomalous dip". It can occur at its worst in coastal waters,
    > particularly semi-enclosed waters, as Alex can testify from recent
    > experiences in Kiel. It can't be predicted, and its effects can be of the
    > same order as any errors that are likely to be found in a sextant, and will
    > mask or confuse those errors. How will Jim become aware of any dip errors,
    > and disentangle them?
    I guess I am missing something! All I am looking for is the measurment
    of the lunar distance. Knowing the actual position we are taking the
    shot from, Frank's program will calculate the height of the moon and
    other body and then spit out how close the shot was.
    > By the way, Jim didn't ask for an estimate of the observer's height of eye,
    > and that can be a significant matter.
    Again, not an issue if we let the program calculate the heights. I'm
    trying to make it easy to collect the data! Get your lat and long,
    take a distance, take the time punch up the results send in the
    results. No horizon needed.
    > ==============
    > But I would like to ask a slightly-cynical question. Unless you are going to
    > use your sextant to measure lunar distances, does the possibility that it
    > might be out by a minute or two matter a scrap to your navigation, in a
    > small craft? Just thought I'd ask...
    Well, actually, I do use my sextant to measure lunar distances! As far
    as fixing a position al la regular LOP, I hope it is doing better than
    a couple of minutes. Seems to have so far.
    George, the point of the investigation is being missed.
    The intent of the investigation is to procure a set of data collected
    in a uniform way. Putting the data in an easily manipulated DB would
    allow a variety of questions to be answered. For example:
    Do the different observers get better results with sun, stars or
    What kind of spread do we see between observers?
    Do we get more precise results rather than accurate results or the
    How much better do we do with different power scopes?
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