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Mr. Huxtable,

You give me far more credit than I deserve. My "expertise" in this obscure
area is merely the result of lots of practice and hard-won experience.  For
example, there are a lot of people out there who can tell you how much error
would be imparted into a sight if the artificial horizon is off-kilter by 2
seconds of arc. I cannot do that. I can only offer my two bits' worth on the
practical side of things.

You have brought up a good point regarding the practice of reversing the
spirit levels end for end as a means of ensuring that the levels themselves
have not been deranged during travel. Yes, this is a standard practice
although it does not have to be done all of the time unless you believe that
your equipment has suffered some grievous indignity. I have had my own
equipment for about 15 years and as yet, have not found the levels to be off
kilter. The adjusting screws on the spirit levels are held firmly in place
with a set screw and with a little common sense and care for your equipment,
they should stay intact indefinitely

Leaving the spirit levels on the artificial horizon is an interesting idea,
however, because of the small size of the artificial horizon, the spirit
levels end up blocking the view, thus they must be removed.  A friend of
mine has a huge artificial horizon (I don't know who manufactured it, I can
ask him and get back to the list) that will allow one to leave the levels in
place whilst taking observations.

I generally replace the levels after a round of sights just to double check
that everything is still ok. More often than not, I find that they will
almost never be perfectly level but will be off by a "hair". For practical
purposes, this "hair" can be ignored. Keep in mind, that practical
navigation, especially in a polar environment is an inexact science. Even if
you do get things set up exactly so, there are still problems with wacky
refraction errors, ice fog and all kinds of wonky things that the navigator
will never encounter in the more temperate and tropical zones of the earth.

As for a base, yes, it is vital that the base be a solid surface. Concrete
or rock is best, but a snow or ice block, in conjunction with the box which
contains the artificial horizon will do the trick.

Several years ago, I submitted a piece to the Navigation Foundation's
"Navigator's Newsletter" (Issue 33, Fall 1991) on celestial navigation in
the polar regions. I have not read it for years but I did cover some of this
stuff. Also, there are some excellent articles by some REAL authorities on
artificial horizons. These guys know more about artificial horizons than I
will ever know. I suggest that you try to get a hold of some of the back
issue of the Navigator's Newsletter. It is an excellent source of
information.

Cheers

----- Original Message -----
From: George Huxtable 
To: 
Sent: Monday, September 23, 2002 2:53 AM
Subject: Re: On polar nav


> What a privilege  it is to have on this list expertise such as Robert
Eno's
> to call on.
>
> Thanks, Robert, for a well-argued (and completely convincing) comparison
> between the relative merits of a theodolite and a sextant with artificial
> horizon, for a polar explorer.
>
> Further questions arise.
>
> Is it part of the levelling procedure for a glass artificial horizon to
> reverse the spirit levels, end-for-end, on the glass plate, in case the
> levels themselves have become somehow deranged in the knockabout of arctic
> travel?
>
> Presumably, whatever the glass horizon is mounted on must be VERY rigid
and
> firm, to ensure that when the (slight) weight of the spirit levels is
added
> and then removed, its horizontality is affected by no more than a fraction
> of a minute. Is that rigity easy to achieve? Any such difficulty might be
> overcome, perhaps, by leaving the levels in place while an altitude is
> taken, if room can be found between the levels on the glass surface for a
> Sun reflection to be seen, but I have doubts whether this would be
> possible.
>
> George Huxtable.

   

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