NavList:

Don,

You wrote:
"Of course there is some parallax error associated with sighting a cliff top a few hundred meters away, but the collective wisdom of this forum should be able to work out the necessary correction."

I would say that the parallax correction cancels out as long as you don't move much. If we maintain the sextant within 10cm of the same position between observations and if the surrogate horizon is 500m away, then the change in its angular altitude would be less than 0.7 minutes of arc. Another problem would be making sure that the sextant is vertical and that the angles are measured in a common vertical arc. If the canyon edge is sloping in angle relative to the true horizon, then "swinging the arc" relative to it might lead to some error.

It's a great idea though, and it's got me thinking of a variant for sextant enthusiasts who do not happen to find themselves at the bottoms of canyons and instead are stuck in suburbia on an early summer day at local noon. If you have a good, straight roofline to your south it would be analogous to the top edge of the canyon wall (and also straight and possibly very close to horizontal so verticality might be less of a problem). In this case, it might make more sense to measure the angle from the reflected Sun in the A.H. to the roofline and then from the roofline to the true Sun. Around noon, the "hang" at constant altitude is long enough that you could alternate a couple of sights like this and add up to get the Noon altitude. This might work really well.

-FER

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