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To all practicing land (or ice:-) navigation:

On taking altitudes with artificial
horizon based on horizontal liquid level
(like Davis horizon, or a plate of oil/water/sugar sirup:-).

Here is the trick which permits to take altitudes
of stars and in general simplifies the use of art.
horizon very much.
(The problem with taking star altitudes with art. horizon
is that it is very hard to get both images of the SAME
star in your field of view. Just because it is
hard to tell which star is which when looking through
your sextant). But the trick also greatly simplifies ALL
observations with artificial horizon.

I learned the trick from Chauvenet (vol. 2) and he refers
on a Russian astronomer Knorre. It is so nice that
I want to share it with everyone who practices
land navigation. (Besides, Chauvenet is out of print,
and I'm afraid that not everyone has access to this
great book). The trick is based
on the following

Theorem. When the two imnages of the star in your
field of view coinside, the angle of inclination
of your index mirror (and thus the index arm) to
the horizon is constant, INDEPENDENT of the star.
This angle is thus a characteristic of your sextant.

Proof. It is a simple exercise in high school geometry.
Just make a picture and see what this angle is:-)

Conclusion: attach to the index arm a little bubble
level in such a way that you can see it during the
observation, without having to move your sextant.
The angle of the level attachment can be easily found
by experimentation or theoretically.

Point your sextant so that you see the image
reflected in water through the transparent part of
your horizon glass (and the plane of the sextant is vertical).
Then (holding your sextant firmly)
move the index arm until the level
begins to play.
In this arm position, both images will be in your filed of view!

Alex.

   

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