NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: On polar nav
From: Paul Hirose
Date: 2002 Sep 23, 16:02 -0700
From: Paul Hirose
Date: 2002 Sep 23, 16:02 -0700
George Huxtable wrote: > > If Amundsen's measurements were to determine his distance from the pole to > within a mile or so, then this levelling had to be similarly precise, to a > minute of arc. This is asking a lot of a spirit level, which would need to > be as sensitive as that found in a theodolite. The levelling process would > have to be very painstaking. However, having been levelled once, the It may not be as bad as you think, George. The level on my Wild T3 theodolite is very sensitive, 7 seconds of arc per 2 mm division. Bringing the instrument accurately level is a delicate task but not terribly difficult. I'm sure it could be done wearing heavy gloves. The quickest way, for me, is to turn the footscrews rapidly until the bubble shoots across the vial. Then I reverse the motion, but about 3x slower, and again overshoot the mark. I repeat the process until the bubble is centered within about one division. Now the fine adjustment can begin. Carefully noting the bubble position of one end of the vial, I turn the theodolite 180 degrees and observe the same end of the vial to see where the bubble settles. The midpoint of this position and the old position is my target. It will not necessarily center the bubble, but at this stage I'm only looking at one end of the bubble. At the end of fine leveling I do check for centering, just to see how much the level is out of adjustment. Of course the instrument actually has to leveled in two axes, so in practice it's being turned back and forth 90 degrees throughout the process. It has been awhile since I played with the T3, so I have just put it on a kitchen table and brought it level within about 2 seconds of arc. >From start to stop I required seven minutes. A lot of that time was spent waiting on the bubble to settle; these tenth second instruments have slow moving bubbles. It didn't help that I was quite out of practice. Robert Eno said he has 30 second levels for his artificial horizon, so their bubbles would be much livlier than the one on my T3. With two levels there would be no need to turn a single one back and forth through 90 degrees. And of course there would be no need to level within a couple arc seconds!