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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
>
>
> 

   

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