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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@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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