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Hi Ken,

Very interesting observations. I have long considered the feasibility of 
floating a reflective surface, however, the nicety of construction necessary 
to obtain a truly horizontal floating surface, both longitudinally and 
transversely, is somewhat daunting to say the least. It simply may not be 
practically possible to obtain such a surface, given the bulk shown in your 
photos.

Hydrostatically speaking, if we view your apparatus as a floating barge, it is 
necessary that the positions of the centers of gravity and buoyancy, both 
longitudinally and transversely, lie in the same vertical plane to produce a 
no list/no trim, or truly horizontal, attitude. It might be theoretically 
possible to construct such a floatation device, although hardly practical - 
so what do we do: possibly install indicators (bubble levels) and correctors 
(ballast) to return our mirror barge to the truly horizontal position - and 
now we are right back to where we started from, with an adjustable reflecting 
surface.

There are a number of problems associated with an adjustable reflecting 
surface, i.e., mirror, glass plate, etc., not the least of which is the 
sophistication of the level indicating devices which must be extremely 
accurate. Most of the literature I have read seems to support an opinion that 
a simple liquid or mercury filled tray provides to simplest and best 
solution.

Regards,

Henry
 


--- On Fri, 9/12/08, George Huxtable  wrote:

> From: George Huxtable 
> Subject: [NavList 6251] Re: Hybrid Artificial Horizon
> To: NavList@fer3.com
> Date: Friday, September 12, 2008, 3:18 AM
> Interesting and inventive notions, there, from Ken Muldrew.
> 
> He ended-
> 
> "What would really be nice would be some kind of
> simple calibration 
> procedure that could be used to measure the error every
> time you set it up 
> (like measuring index error) but I can�t think of anything
> off hand."
> 
> If he has something in view with an unchanging altitude,
> such as a distant 
> street-light or a culminating star, why not measure its
> altitude, then 
> rotate the whole gear, raft and tray, by 180�, and
> remeasure?
> 
> If a trick like that works, then he can correct for small
> errors of tilt, as 
> long as he always keeps the orientation of the raft aligned
> (at least 
> approximately) with the direction of the object being
> observed. Perhaps, by 
> that technique,  he could find and mark the direction on
> the raft in which 
> the tilt was zero, and always align that.
> 
> But the important aim would be to ensure stability of the
> flotation plane of 
> the raft, whatever happens. Ken has mentioned the problems
> that will ensue 
> if water drops on the top upset its balance. Any contact
> with the sides of 
> the tray would have to be avoided, or even a near-miss,
> because of 
> surface-tension meniscus effects. To that end, it might be
> helpful to gently 
> tether the raft by gluing light silk threads to hold it
> loosely in place, to 
> keep it roughly central. They would have to be long enough
> to let the raft 
> settle to the bottom when the water was emptied.
> 
> But the big problem, as I see it, is to avoid the trapping
> of air bubbles at 
> the under surface, and for that I have no solution.
> 
> George.
> 
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> 
> 
> 
> 
> 
> contact George Huxtable at george@huxtable.u-net.com
> or at +44 1865 820222 (from UK, 01865 820222)
> or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
> ----- Original Message ----- 
> From: "Ken Muldrew" 
> To: 
> Sent: Thursday, September 11, 2008 9:07 PM
> Subject: [NavList 6250] Hybrid Artificial Horizon
> 
> 
> For those of us who navigate from our back yard, for fun,
> practice, or
> just as an excuse to watch the night sky, an artificial
> horizon is a
> necessary part of our kit. Now that mercury is hard to come
> by, and a bit
> of a nuisance anyway, most of us use a tray of water to
> reflect the object
> that we are trying to get an altitude for. This works well
> for the sun and
> moon, and is OK for bright planets, but it can be a trying
> experience to
> bring down stars. As an alternative, some people of a
> metrological bent
> will level a mirror or piece of glass, but this is a real
> challenge in
> precision, and the trying leveling process has to be
> repeated every time
> the mirror is moved (some back yards have a lot of trees).
> I think that
> everyone who tries this in the dark begins to wish they had
> a jar of
> mercury that they could just pour out and start observing.
> 
> I decided to try to combine the two approaches by floating
> a mirror on
> water. I had no wish to try to build a perfectly balanced,
> hollow mirror,
> so I thought that I could just mark the surface so that I
> would always use
> it in the same orientation and the error could be
> calibrated, just as is
> done for the index error with a sextant. So I bought a
> mirror, cut it in
> half, and used silicone sealant to glue strips of 1/4"
> plexiglass around
> the sides. The mirror was 1/8" plate glass (it was
> actually a front
> surface mirror, but polished on the back, so I used it as a
> rear surface
> mirror to protect the coating). I used 3 pieces of plexi on
> each side to
> end up with a sealed box that was about 6" x 3.5"
> x 1". I built a tray to
> float it that was just a bit bigger so that I could stick
> my fingers in on
> the sides and remove the mirror from the water. The
> attached pictures show
> the whole setup.
> 
> Last night Jupiter culminated just as twilight was ending
> so I used that
> to get a calibration. Just a note on the ease of use: it
> set up in
> seconds, and stopped bobbing in about 5 seconds; a light
> breeze didn�t
> affect it at all, and you can see the whole night sky
> (piecewise ;-) ) as
> clearly as if you were looking up. The measured maximum
> altitude was
> 31�9�, the index error 8.1�, giving an apparent altitude
> of 15�30.45�.
> Refraction (10�C, 1000m above sea level) was 3.14� for a
> meridian altitude
> of 15�27.3�. The declination of Jupiter was 23�9.1� and
> my latitude was
> 51�8.8� so the altitude should have been 15�42.1�. If I
> add 29.6� to the
> doubled angle for a mirror error, then these numbers match,
> so I�ll call
> that the mirror error.
> 
> To test it out, I took altitudes of Arcturus and Alpheratz,
> both trivial
> to find in the mirror despite it being just after twilight
> and all the
> lights of a big city adding to the general lack of darkness
> (not that it
> should be difficult, but if you have ever hunted for a star
> in a water
> horizon, you�ll know why I mention it). Here is the data:
> 
> Arcturus
> 
> 9h18m46s 51�7.6'
> 9h21m27s 50�15'
> 9h22m39s 49�50'
> 9h23m24s 49�37'
> -------- ------
> 9h21m33s 50�12'24" - 8.1' + 29.6' / 2 =
> 25�17' - 1.9' refr = 25�15.1'
> 
> Almanac gives 25�28.2'for a difference of 13.1'
> 
> Alpheratz
> 
> 9h27m25s 63�19.8'
> 9h28m30s 63�40.2'
> 9h29m29s 63�57.6'
> 9h30m30s 64�15.8'
> 9h31m24s 64�32.2'
> -------- ------
> 9h29m27s 63�57.12' - 8.1' + 29.6' / 2 =
> 32�9.3' - 1.4' refr = 32�7.9'
> 
> Almanac gives 32�24.8'for a difference of 16.9'
> 
> So the results are not very good. Note that I moved the
> mirror and spilled
> water between each round of sights. Also, when I moved the
> setup after
> getting Jupiter�s altitude, I noticed that water spilled
> over the edge
> just as I touched the side of the water box (without
> actually moving the
> box) so there was a meniscus on one end. I don�t know how
> much off-level
> the water might have been due to surface forces, but it may
> have had some
> bearing on the reading. Also, I fear that I slopped some
> water drops on
> top of the mirror during the moves; that might also have
> changed the
> balance of the floating box.
> 
> I will have to do some more testing to see if better
> technique can improve
> the results. I hope so, because the ease of use makes this
> setup really
> attractive. A more careful assembly would probably help (I
> spent about an
> hour building the whole apparatus). What would really be
> nice would be
> some kind of simple calibration procedure that could be
> used to measure
> the error every time you set it up (like measuring index
> error) but I
> can�t think of anything off hand.
> 
> For those interested, the mirror I used was this one:
> http://www.surplusshed.com/pages/item/l3757.html
> everything else was scrap.
> 
>   
> \----------------------------+----------------------------+
>   o_,
>  O_/ \    Ken Muldrew, PhD      | Voice: (403) 220-5976
>      |   <\__/7
>  <\__  \  Dept. of Cell Biology | Fax:   (403)
> 270-0617      |     | /
>   "\ L  | University of Calgary |
> kmuldrew@ucalgary.ca       |   / /
>    <   
> +-----------------------+----------------------------+ / /
>                Morning coffee recapitulate phylogeny       
>   L/
> 
> 
> 
> 
> 
> 
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