NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: An interesting question
From: Gary LaPook
Date: 2013 Oct 12, 12:39 -0700
From: Gary LaPook
Date: 2013 Oct 12, 12:39 -0700
And I posted this in 2007:
From: Lu Abel <lu@abelhome.net>
To: garylapook@pacbell.net
Sent: Friday, October 11, 2013 10:02 PM
Subject: [NavList] Re: An interesting question
You can also take sights using an artificial horizon. To make one of these simply put water in a saucer (or, much better if you can get it, use murcury) and the surface of the liquid will be perfectly horizontal. Measure the altitude of the star using its reflection in the saucer as the reference point instead of the horizon. Due to the geometry of the situation (angle of incidence equals angle of reflection) you end up measuring exactly twice the altitude. Then simply apply the index correction and divide the remainder by 2, then apply refraction but not dip corrections. You need a still night so that the wind doesn't ripple the surface of the water. I have been using a small bottle of murcury for over forty years for this and because of its reflective quality I can take second magnitude stars such as polaris, with water you can shoot first magnitude stars easily. If shooting the sun make sure you use both sets of shades and line up the lower limb with its reflection in the liquid which will be the top of the sun's reflection. I remember one night in 1990 we were anchored in a long fjord on the east coast of Tahaa (an island about 20 nm east of Bora Bora) and it was so still that I could see the stars reflected in the ocean around the boat. I got out my Tamaya and took a round of sight and got a fix that crossed on our anchorage.http://fer3.com/arc/m2.aspx/First-sights-LaPook-jan-2007-g2125
From: Lu Abel <lu@abelhome.net>
To: garylapook@pacbell.net
Sent: Friday, October 11, 2013 10:02 PM
Subject: [NavList] Re: An interesting question
Gary, I respect you and your contributions to this list a lot. But
have you ever tried a casual AH site such as you suggest?
I remember being tied up at the Royal Nova Scotia Yacht Club in Halifax a couple of decades ago (VERY nice folks) and just before hitting my berth noticed that the waters were totally flat and there was a great image of the moon directly aft of my stern. So I whipped out my sextant and used the waters as an AH. Reduced my sight and was disappointed to come out about 20 miles from my KP.
I remember being tied up at the Royal Nova Scotia Yacht Club in Halifax a couple of decades ago (VERY nice folks) and just before hitting my berth noticed that the waters were totally flat and there was a great image of the moon directly aft of my stern. So I whipped out my sextant and used the waters as an AH. Reduced my sight and was disappointed to come out about 20 miles from my KP.
On 10/10/2013 9:05 PM, Gary LaPook
wrote:
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
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
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!BradOn Oct 10, 2013 7:54 PM, "Lu Abel" <lu{at}abelhome.net> wrote:
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:
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:
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?View and reply to this message: http://fer3.com/arc/m2.aspx?i=125279View and reply to this message: http://fer3.com/arc/m2.aspx?i=125280
View and reply to this message: http://fer3.com/arc/m2.aspx?i=125284View and reply to this message: http://fer3.com/arc/m2.aspx?i=125286
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