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    Re: On polar nav
    From: Robert Eno
    Date: 2002 Sep 23, 18:48 -0400

    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
    ----- Original Message -----
    From: George Huxtable 
    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
    > 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
    > firm, to ensure that when the (slight) weight of the spirit levels is
    > 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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