NavList:

I would concur with Jean-Philippe Planas regarding the Link A-12 bubble
sextant (or rather, octant). Like him, I have three A-12's and in my
opinion, this is the most ergonomic, versatile and user friendly sextant
ever made for use on land.

I would also agree with George Huxtable's comments on the practicalities of
using bubble sextants and artificial horizons.

The problem with using a marine sextant for taking star sights at night is
acquiring the star. First is the problem of sorting out which star you are
after from all the other stars in the sky. (Not a problem during the day
when there is only one star...) This can be difficult enough using just the
Mk I eyeball, but becomes a problem with the limited field of view in the
sextant - even if you replace the telescope with a plain sighting tube.
This problem is compounded when trying to use an artificial horizon so that
you and your sextant have to be in just the right place to see the star of
interest reflected in the limited field of view of the artificial horizon
mirror.

When using bubble attachments for marine sextants, the sight picture is
usually no longer invariant with forward tilt, as it is when using a
natural horizon. This means it is necessary to hold the sextant absolutely
still to line up the star with the bubble. Getting yourself all lined up
for one star is an evening's entertainment all in itself, but taking
sightings on multiple stars - particularly when you are alone and having to
write down the measured altitudes - becomes a bit of a chore. (Remember,
you need light to see the paper you are writing on, so you have to put the
sextant down to hold the flashlight while writing down the result, or go
into the house or  some place where there is light to write down the result.)

I think this is the reason why the instrument of choice for desert
navigators in the past was the theodolite. Once the thing was set up and
levelled on its tripod, it was then relatively easy to take sightings on
multiple stars.

The great beauty of the A-12 is that it is equally as good for star sights
at night as for sun sights during the day. Acquiring the star of interest
is not a problem as there is no telescope to narrow the field of view. The
sextant is designed so that the sight picture is tilt invariant. If you
tilt the instrument slightly in any direction, the bubble follows the star.
The sextant is light and handy, so taking multiple sights on a number of
different stars  - where you are having to put the sextant down between
sightings to write down the result - is not a real handicap.

On the down side, a bubble sextant is not as precise as a marine sextant.
The vernier on the A-12 is only marked in two minute intervals - though
interpolating for one minute precision is not that difficult. A theodolite
will give much better precision, equivalent to a marine sextant or better,
but it needs a tripod to stand it on so it cannot be described as "light"
or "handy". Determining the index error on a bubble sextant is not the
simple procedure that it is with a marine sextant. Index error can and does
change with temperature, so when using a bubble sextant it is a good idea
to adopt a strategy of choosing celestial bodies such that the index error
is nullified when fixing your position.

As for accuracy, when taking sightings from the comfort of my home in
Scotland I can usually get a sextant altitude with an A-12  that is within
a minute or so of the calculated altitude. Things were rather different
when taking sightings from a tent in the middle of the Sahara desert, (see
http://www.pisces-press.com/C-Nav for an account of this trip), but even so
I was usually able to get a position fix that was accurate to about two
minutes or so on average. This is pretty much on par with what would be
expected for a small boat at sea using a marine sextant, so I don't think
the A-12 is actually that much disadvantaged by its relatively low precision.

The A-12 sextant is (I think) still available from Celestaire for about
$650. For this, you get a fully overhauled and tested instrument with full
warranty. On the other hand, A-12 sextants are very common and you will
usually see one for sale on ebay where a bid in the region of $150 will
secure the item. Of course, you have to remember that A-12 sextants are now
over 60 years old  and with that length of history behind them, there is no
guarantee that the ebay acquisition will be in good or even usable
condition. Taking an A-12 apart and putting it back together again is not
difficult, but I would say that some engineering experience is definitely
necessary before embarking on such a project.

One problem to which Jean-Philippe Planas alluded is that the C size 1.5
volt battery seems to have grown slightly over the past 60 years and modern
batteries will not fit into the cap of the bubble illumination assembly. My
solution was to mount the cap in a lathe and enlarge the battery recess in
the cap by about 0.01". I would recommend that you find someone with a
manual lathe and have him (it is bound to be a him) undertake this simple
little upgrade.

Geoffrey Kolbe




At 01:10 12/08/2006, you 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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