NavList:

Sam,

The bottom of Grand Canyon isn't a good place for celestial navigation or for GPS. At least you have the Colorado river as a line of position to start with. The artificial horizon may not work as you have eluded to so a bubble octant will be the right tool for the job. Don't try using sextant or octant while transiting the rapids ;-)

Horizontal sextant angles can be done without a sextant. Take bearings using a hand held compass or binocular with a compass. The difference of the two bearings will be your horizontal angle to the nearest degree or two which should be good enough on the river with a topo map. Plot the bearings directly then do a horizontal angle as a check.

To plot the horizontal angle divide half the distance between mark A and B by the SIN of the horizontal angle. This is the radius of the circle of position. Plot using a compass divider set to the radius distance. Arc the intersection off mark A and B then arc the circle of position. See attached example.

Greg Rudzinski

Horizontal Sextant Angles and False Horizons etc.
From: Sam Willis
Date: 2013 Jun 6, 08:44 -0700
Hello everyone. HELP!

I am involved in a project that will involve the use of sextants in the Grand Canyon.

1. We will need to use a false horizon because we are in the middle of the desert. Am I right in thinking that a) using a false horizon makes it very difficult to shoot the stars? and that b) because of our lattitude (36) and the declination in August, I will not be able to do a Mer Pass on a 120 degree sextant because the sun will be too high to use a false horizon [and double the angle]?

2. It would be great to do some exercises with horizontal sextant angles. Can someone advise me on this because I am unfamiliar with it. Do I need a special plotting tool?

many thanks
Sam Willis
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