NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: sextant for use on land
From: Jean-Philippe Planas
Date: 2006 Aug 12, 03:12 -0500
"James R. Van Zandt" <jrvz@comcast.net> wrote:
From: Jean-Philippe Planas
Date: 2006 Aug 12, 03:12 -0500
Hello James,
I own 3 A12s and they are perfectly suited for sun and moon shots. They require batteries and working llighting assemblies for night shots to illuminate the bubble. The star sights are more tricky with those sextants, specifically with faint stars because
the stars appears over the very dark background of the bubble that has to be kept very faintly illuminated with a rheostat. Therefore the relative position of the star with respect to the bubble is sometimes more a guess than a positive measurement.
The A12 is the lightest of the bubble sextant you quote and is very ergonomic and easy to handle and maintain. It uses 2 C size 1.5v batteries that have to be pealed off of their external metallic skin to fit in the battery assembly. The altiude is read out of a vernier scale that can be interpolated to 1 minute.
The Bendix AN5851 is far more complex and heavier
than the A12. Therefore it is much more difficult to put back in working order when something goes wrong. Its main advantage is that it also features a direct view to the natural horizon which enables this instrument to be used at sea like a regular naval sextant. The altitude is directly read out of counters.
The Kollmans are periscopic instrument that required a specific mounting to fit into the aircraft astrodome. It is therefore very clumsy to use them on land. Furthermore they need a bubble illumination day and night out of the 28V AC/DC power source. The altitude is directly read out of counters.
As far as precision is concerned, aviation sextant can hardly give altitude at less than 1 or 2 minutes. This was enough considering the speed of the aircraft and the visibility around of about 10 NM or more. They are all fitted with averager devices to average out the aircraft accelerations
induced on the bubble and other vibrations or air bumps. They are of little use for backyard practice on only add weight.
At last they are about 10 time less precise than the best naval sextant that can provide altitude to 0.2 or 0.1 minute.
If you want an aircraft bubble sextant/octant, I definitly recommand the A12 as a training tool for backyard practice.
Good luck
JPP
"James R. Van Zandt" <jrvz@comcast.net> wrote:
I have gotten interested in celestial navigation, and would like some
advice in choosing a sextant.
I would like to be able to take sightings from my New Hampshire home,
which is some distance from the water. That pretty much rules out a
regular sextant.
We are also surrounded on most sides by tall trees, which would make
it hard to use an artificial horizon (limited to 60 degrees
elevation). Besides, I want to make star sightings, which seem pretty
difficult with reflections off water. (I could try to get hold of
enough mercury to fill an artificial horizon, but it would probably
not be worth the hassle.) [1]
My first feasible alternative would be a bubble horizon for a regular sextant:
http://www.celestaire.com/catalog/Marine_Sextants/Cheap_Sextant/ $27
http://www.celestaire.com/catalog/products/0511.html practice bubble horizon $49
http://www.celestaire.com/catalog/Marine_Sextants/Cassens_and_Plath/ $900
The last one is definitely out of my price range.
I have a couple of questions:
1) I'd like to know if these levels are coordinated, in the following
sense: suppose I start with both a star and the bubble centered in
the image, then I lower the sextant so the star rises half way to the
top of the image. Does the bubble rise to the same height? That
would greatly simplify the measurement. Of course, the bubble would
still be sensitive to linear acceleration, which would make no
difference to a regular sextant.
2) Does Celestaire's "practice" bubble horizon fit on any of the Davis
sextants?
My other alternative is a bubble sextant, e.g. one of those at eBay
designed for aircraft:
Kollsman MS28011-S
Kollsman 1471-01
Bendix AN-5851-1
Link A-12
3) Are these set up for star sights, as well as sun and moon?
4) At least some of these call for 28 VDC. Is that only required for
the averager, or also to illuminate the bubble or for some other
function?
5) I can't judge size very well from the pictures. Are these sextants
too heavy or bulky to be hand held? (Although I think it would be
cool to mount one in a sun roof :-)
I'd appreciate any pointers.
- Jim Van Zandt
[1] I see Lewis and Clark sometimes used a carefully leveled mirror
http://www.davidcortner.com/2822/mandan_moon.html
I suppose I could try that.
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