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    Using a sextant from the bottom of a canyon
    From: Don Seltzer
    Date: 2013 Jul 1, 20:05 -0700

    Last month, Sam Willis was wondering about using a sextant for terrestrial navigation while travelling down the Colorado River through the Grand Canyon this summer. Clearly there would be no useful natural horizon, suggesting the need of an artificial horizon. However, the doubling effect would put the altitude of the noon sun beyond the range of a standard sextant.

    It occurred to me today that it might be possible to shoot the sun under such conditions by two separate sightings. The first would be of the fixed height of the canyon walls. Google Earth suggests that in many spots, the wall is about 400 meters above the river. Pick a convenient spot on the river bank with a good view of the cliffs to the south, a few hundred meters away. Using the artificial horizon, measure the relative cliff height directly to the south. Let's say that the distance to the base of the wall is roughly 400 meters also, not an unrealistic situation for that locale. The doubling of the artificial horizon results in a measurement of about 90°, well within the range of a sextant. Now with the sextant alone, bring the limb of the sun down to the cliff top. The sum of the two angles is the altitude of the sun.

    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.

    Don Seltzer
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