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    Re: Early Land Navigation.
    From: Don Seltzer
    Date: 2015 Feb 10, 09:19 -0500

    Two years ago, historian Sam Willis stopped by this forum for advice
    on land navigation.  He was about to participate in a re-enactment of
    the 1869 Powell Expedition down the Colorado River, through the Grand
    Canyon.  He was hoping to use a sextant to take some readings, but was
    faced with the same problem of a high summer sun and an artificial
    horizon.
    
    The expedition was co-sponsored by the Discovery Channel and BBC, but
    I don't think the program ever aired in the US.  I don't know if he
    succeeded in taking any sightings.
    
    Don Seltzer
    
    On Mon, Feb 9, 2015 at 5:31 PM, Frank Reed  wrote:
    > David Pike, you wondered:
    > "What did you mean the sun is so low it makes the use of an artificial
    > horizon impractical?"
    >
    > I believe he said that the Sun is so far south that it makes the use of an
    > artificial horizon impractical. But you see... he is in the southern
    > hemisphere, so a far southerly declination for the Sun actually implies that
    > the Sun is too high for an artificial horizon. Assuming a sextant has a
    > maximum practical angular measurement of 120� (some manage a bit more but
    > we'll ignore that), that implies a maximum altitude of 60�. How did 19th
    > century explorers/surveyors in Africa and Australia, for example, measure
    > latitudes on land with a sextant when the Sun at noon was higher than 60
    > degrees?
    >
    > Frank Reed
    > ReedNavigation.com
    > Conanicut Island  USA
    >
    >
    > View and reply to this message
    

       
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