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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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