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    Re: Making an artificial horizon, and leveling thereof
    From: Gary LaPook
    Date: 2011 Jan 24, 21:55 -0800
    "When life hands you lemons, make lemonaid."

    I just had a brilliant idea for you guys on the east coast that want to make a better artificial horizon. Put a bowl of water outside with some cover to keep the wind from rippling the surface. After it freezes the surface should be level. Remove the cover and use the frozen surface as your artificial horizon or place a mirror on top of the ice and it should also be level.

    An easy way to practice with an artificial horizon is to take a number of
    observations to allow you to determine your accuracy is to use the
    meridian transit of Polaris. When Polaris is crossing the meridian it
    is moving horizontally and its altitude doesn't change. For the period
    of more than 15 minutes both before and after meridian passage the
    altitude of Polaris changes less than one - tenth of a minute (0.1').
    For a period of 34 minutes before and after passage the altitude of
    Polaris changes less than one half minute (0.5'.) For 48 minutes
    before and after its altitude changes less than 1 minute.

    Calculate the time that Polaris is crossing your meridian and get out
    early and start shooting. It is about 6:30 pm and am now. Calculate
    the altitude by adding your latitude to the polar distance of Polaris,
    now 40.1 minutes, and then ADD the refraction correction (yes ADD) which
    will then give Hp (precomputed altitude, this is how it is done by
    flight navigators) because this procedure allows you to compare your
    Hs directly with Hp to determine the intercept immediately, or the
    error in the observation if taken from a known point. If shooting a
    lower transit subtract the polar distance from your latitude. You only
    have to update this Hp from time as the declination of Polaris changes
    slowly.



    Now is a good time to shoot Polaris with an artificial horizon as it is crossing your meridian about at a bout 6:30 p.m. so its altitude doesn't change even by one tenth of a minute for about a half hour so you can take many shots and don't have to compute a different Hc for each shot. Here are some links to my prior posts about this technique.

    http://www.fer3.com/arc/m2.aspx?i=111363&y=200912

    http://www.fer3.com/arc/m2.aspx?i=111364&y=200912

    This is a link that provides the time that Polaris is crossing your meridian.



    http://www.cadastral.com/2011jan.htm

    gl

    ---------------------------------------------------------------------------------------------------

    --- On Mon, 1/24/11, Fred Hebard <mbiew---.net> wrote:

    From: Fred Hebard <mbiew---.net>
    Subject: [NavList] Re: Making an artificial horizon, and leveling thereof
    To: NavList@fer3.com
    Date: Monday, January 24, 2011, 2:12 PM

    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 <george{at}hux.me.uk> wrote:
    >
    > From: George Huxtable <george{at}hux.me.uk>
    > 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: <Anabasis75---.com>
    > To: <NavList@fer3.com>
    > 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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