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Subject: Re: On polar nav
From: Paul Hirose (paulhirose@XXX.XXX)
Date: Mon Sep 23 2002 - 19:02:37 EDT
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!
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