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    Re: Making an artificial horizon, and leveling thereof
    From: Jeremy C
    Date: 2011 Jan 22, 11:02 EST
    I would think that it would be a nice experiment for someone to build a rafted mirror and float it in a artificial horizon and see what kind of errors result since they can be compared to the known position.  We can then get a good set of numbers that indicate the real result of errors that George pointed out.
     
    I suspect that standing close to a artificial horizon would require you to be quite low to the ground for lower altitudes.  Standing further back would allow for you to stand erect and still see the sun.  This is assuming that you don't put it on a reasonably level table.
     
    Jeremy
     
    In a message dated 1/22/2011 5:29:46 A.M. Central Asia Standard Time, george{at}hux.me.uk writes:
    Alan wrote-

    "In any event, re "leveling", if I remember correctly, I read somewhere
    that this was NOT critical barring setting the thing up on a steep
    hillside, as "water, I suppose ditto for oil and or mercury, seeks it's own
    level". Is this, or is this not the case re using an artificial horizon?"

    Yes, that's correct.

    In guessing at how little Mercury one might be able to get away with, I
    wrote, on 20 Jan-
    "Mercury is VERY dense (over 13) so an ounce of the stuff won't go far;
    occupying about 2 millilitres. My guess is that  around 10-15 ml would be
    required in the trough of a sensibly-sized art. horizon, to make it easy to
    use without having to be over-careful about levelling. That would
    correspond to 5 to 8 ounces. Maybe it would be possible to penny-pinch and
    get way with somewhat less."

    Since then, Bill Morris has actually tried it out, to see how much Mercury
    is required to get uniform coverage over the floor of an artificial
    horizon, without the liquid gathering into blobs, and has assessed it as
    750 grams. This is about 55ml, which is very much more than my own guess
    that 10-15 ml might suffice. I've no doubt that he is right, and accept
    that judgment. There's nothing like practical trial, to get a reliable
    result.

    And normally, the levelling of such an artificial horizon is very
    non-critical, just as Alan says. It's only if skimping on the Mercury, that
    any tilt might result in the liquid gathering in one part of the trough,
    leaving another bare, or affected by meniscus.

    Alan continued-

    "I've done sun shots with mine, in the spring and summer, standing in a
    reasonably level parking area at our apartment complex, taking sun sights
    several hours apart, that when plotted show quite small displacement
    between my calculated fix and  GPS coordinates."

    This list thrives on numbers, Alan. Without numbers, even approximate ones,
    a statement such as "show quite small displacement" has no meaning to
    anyone other than you.

    "Seems that orienting the ah properly is an important factor, as is being
    able to stand far enough away from the ah so as to be able to see the
    reflected and sextant suns."

    That's a surprise, to me. On what basis do you deduce that any such
    discrepancy is the result of mis-orientation? And why do you need to stand
    back to see the two views? Surely, the closer you can get, the larger is
    the solid angle that's available in the liquid reflector. I see no such
    advantage in standing back, as long as the wind-shield isn't interfering
    with the direct view.

    George.

    contact George Huxtable, at  george{at}hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.




       
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