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    Re: Making an artificial horizon, and leveling thereof
    From: Fred Hebard
    Date: 2011 Jan 24, 17:12 -0500

    The advantage of a mirror over oil is finding fainter stars.  Mercury
    metal has no such disadvantage.  In general, putting a mirror on a
    stout table and leveling the mirror or table with machine screws and
    a good spirit level is much easier than floating the mirror.
    
    On Jan 24, 2011, at 4:00 PM, hch wrote:
    
    > George,
    >
    > You are, of course aware that this subject was rather exhaustively
    > explored on this List some time ago, proving the old adage to the
    > effect that ... "what goes around, comes around". It is my
    > recollection that the consensus of opinion at that time favored a
    > screw leveled dark colored glass or Plexiglas ah, assuming that
    > sufficiently accurate spirit levels could be obtained - as I
    > recall, several excellent designs were appended to posts then
    > submitted. This was, of course, intended as a substitute for the
    > liquid, or mercury, ah both of which appear superior to any leveled
    > reflective surface device.
    >
    > I recall posting at the time the significant, though not
    > insurmountable, problem of keeping the centers of buoyancy and
    > gravity aligned exactly both transversely and longitudinally in a
    > floating arrangement of composite construction, so as to insure
    > perfect equilibrium of floatation. It does seem rather extraneous
    > to me to float a reflective device when the medium of floatation
    > itself provides an adequate horizontal reflective surface to begin
    > with.
    >
    > I have searched in vain to find these postings.
    >
    > Regards,
    >
    > Henry
    >
    > --- On Sat, 1/22/11, George Huxtable  wrote:
    >
    > From: George Huxtable 
    > Subject: [NavList] Re: Making an artificial horizon, and leveling
    > thereof
    > To: NavList@fer3.com
    > Date: Saturday, January 22, 2011, 3:05 PM
    >
    > 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.
    >
    > 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.
    > ----- Original Message -----
    > From: 
    > To: 
    > 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
    > 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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