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Go out and give your dog some water. Then use the reflection of the sun in the water in the dog's water dish with your sextant to measure twice the actual altitude then do the normal site reduction.

gl

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From: Brad Morris <bradley.r.morris{at}gmail.com>
To: garylapook{at}pacbell.net
Sent: Thursday, October 10, 2013 5:09 PM
Subject: [NavList] Re: An interesting question

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I believe the answer to your specific question to be no, the surface of the great plains are not spherical to the degree of sphericity as the oceans.  Large errors will be introduced because of the reference horizon.

And that, Lu, is why I suggested the artificial horizon.  It does conform to a horizontal surface!

Brad

On Oct 10, 2013 7:54 PM, "Lu Abel" <lu{at}abelhome.net> wrote:

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The question was simply one of whether the Great Plains are flat enough to use them for a horizon.  

As I said, my first instinct is to say no, both because of real but difficult to detect undulations in the ground and because the Great Plains are not truly horizontal. 

But I added the bit about being lost with only a sextant and NA to give the question some flavor.

On 10/10/2013 11:41 AM, Brad Morris wrote:

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Why no artificial horizon.  You carried the sextant, almanac and sight reduction tables.  No AH?  odd.

On Oct 10, 2013 2:27 PM, "Lu Abel" <lu{at}abelhome.net> wrote:

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from a student in my celestial navigation class:

If one were on the Great Plains (of the US, for our non-US members)
could one take a sextant shot and get a LOP with reasonable accuracy?
The Great Plains are a vast area of what appears to be totally flat land.

My first instinct was (and still is) to say "no."   I don't believe the
human eye could distinguish a one degree slope in the ground, and so the
Great Plains easily could undulate.

But I also wondered if I were lost in my covered wagon with nothing but
a sextant and a Nautical Almanac, could I get a reasonable LOP?

I've driven from St Louis to Denver.  You cross vast areas of what
appears to be totally flat ground ("on a clear day you would be able to
see Pike's Peak except for the curvature of the earth" was a saying when
I was a student at the University of Illinois).

Let's assume it's truly flat ground.   Back of the envelope, the
distance is 1000 miles and the elevation gain is one mile.   That's a
right triangle with a hypotenuse of 1000 and an opposite side of 1.
Sine of a small angle is approximately equal to the angle itself
expressed in radians.   So the angle is 1/1000 radians or 3.5 minutes of
arc.

Assuming no undulation in the ground, getting a LOP that's off by 3.5
miles ain't bad.

Thoughts/comments?

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