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    Re: An interesting question
    From: Lu Abel
    Date: 2013 Oct 10, 16:45 -0700
    The question was simply one of whether the Great Plains are flat enough to use them for a horizon.  

    As I said, my first instinct is to say no, both because of real but difficult to detect undulations in the ground and because the Great Plains are not truly horizontal. 

    But I added the bit about being lost with only a sextant and NA to give the question some flavor.

    On 10/10/2013 11:41 AM, Brad Morris wrote:

    Why no artificial horizon.  You carried the sextant, almanac and sight reduction tables.  No AH?  odd.

    On Oct 10, 2013 2:27 PM, "Lu Abel" <lu{at}abelhome.net> wrote:

    from a student in my celestial navigation class:
    If one were on the Great Plains (of the US, for our non-US members)
    could one take a sextant shot and get a LOP with reasonable accuracy?
    The Great Plains are a vast area of what appears to be totally flat land.
    My first instinct was (and still is) to say "no."   I don't believe the
    human eye could distinguish a one degree slope in the ground, and so the
    Great Plains easily could undulate.
    But I also wondered if I were lost in my covered wagon with nothing but
    a sextant and a Nautical Almanac, could I get a reasonable LOP?
    I've driven from St Louis to Denver.  You cross vast areas of what
    appears to be totally flat ground ("on a clear day you would be able to
    see Pike's Peak except for the curvature of the earth" was a saying when
    I was a student at the University of Illinois).
    Let's assume it's truly flat ground.   Back of the envelope, the
    distance is 1000 miles and the elevation gain is one mile.   That's a
    right triangle with a hypotenuse of 1000 and an opposite side of 1.
    Sine of a small angle is approximately equal to the angle itself
    expressed in radians.   So the angle is 1/1000 radians or 3.5 minutes of
    Assuming no undulation in the ground, getting a LOP that's off by 3.5
    miles ain't bad.

    : http://fer3.com/arc/m2.aspx?i=125279

    : http://fer3.com/arc/m2.aspx?i=125280

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