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    Re: Making an artificial horizon, and leveling thereof
    From: George Huxtable
    Date: 2011 Jan 22, 20:05 -0000

    Pictures I've seen of using an artificial horizon show the observer
    squatting cross-legged with the trough placed close in front on the ground,
    or else the trough placed on some sort of stool or table or tripod to bring
    it nearly against the sextant of an observer who is standing or perhaps
    stooping close by. Such closeness does not affect the reading, and would
    allow a smaller trough to be employed.
    I agree with Jeremy that it would be interesting to see how good such a
    rafted mirror could be, and I would not wish to put anyone off from trying
    it out. My intention was just to point out the problems that might arise,
    which would need to be overcome.
    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.
    ----- Original Message -----
    Sent: Saturday, January 22, 2011 4:02 PM
    Subject: [NavList] Re: Making an artificial horizon, and leveling thereof
    |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
    | 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
    | pointed  out.
    | I suspect that standing close to a artificial horizon would require you
    | 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
    | 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@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
    | 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
    | required in  the trough of a sensibly-sized art. horizon, to make it easy
    | 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
    | get way with somewhat  less."
    | Since then, Bill Morris has actually tried it out, to see how  much
    | 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
    | 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
    | 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
    | 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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