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    Re: An interesting question
    From: Brad Morris
    Date: 2013 Oct 13, 01:26 -0400

    Hi Lu

    The question from your student should really be "Is there an area flat enough for celestial navigation on land without the use of an artificial horizon?"

    One obvious candidate is the Bonneville Salt Flat.  Herein find the description of the surface

    The surface of the Bonneville salt crust north of Interstate Highway 80 had 1.3 feet of relief in 1976. The mean altitude of the salt crust was 4,214 feet, and it is the lowest area in the western Great Salt Lake Desert. The surface of the Bonneville playa generally slopes upward in all directions from the edge of the salt crust, and it attains a maximum altitude of about 4,230 feet along its western edge. East of the Bonneville salt crust and north of Interstate Highway 80, the playa surface slopes upward to an altitude of 4,217 feet in a distance of about 10 miles and then gradually drops off to the east. The eastward slope is part of the Great Salt Lake drainage basin.

    I read that to be an elevation change of (at most) 16 feet over 10 miles, which works out to just over 1 arc minute. This spot would work nicely.


    On Oct 12, 2013 3:23 PM, "Frank Reed" <FrankReed@historicalatlas.com> wrote:

    Lu, you wrote:
    "there was a great image of the moon directly aft of my stern. So I whipped out my sextant and used the waters as an AH. Reduced my sight and was disappointed to come out about 20 miles from my KP."

    I would bet you had some other coincidental error. This is a familiar trap: we try something tricky, make some error, then conclude the "tricky thing" doesn't work or is very difficult. I've done Jupiter sights off of still water in Fishers Island Sound with excellent results --less than 2' error.

    And I do agree with Gary that the quick solution if you have a sextant is a makeshift artificial horizon. There are other simple tricks on land, of course, which would even allow you to live without a sextant, especially at night when there are lots of stars near the zenith.

    Back to your original question, if you look at topo maps of the Great Plains, there's plenty of local relief. There are certainly areas where it would work, but the problem of determining which area is flat enough is probably not much different from the process of setting up an artificial horizon. Of course there are loads of lakes out there, the majority artificial. Get down low enough, and you have a good horizon on any but the smallest. And then there are dry lakes...


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