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Because how do you tell whether you see a star
reflected in the oil or some other star?
And because several stars are usually reflected in the oil.
And they are hard to distinguish.
This is my experience (which is small).
Have you actually tried the method you describe with stars?
A


On Sun, 3 Oct 2004, Robert Gainer wrote:

> Why not just turn the sextant upside down and sight the star in the scope
> and when you are in the ballpark turn the sextant over and finish the job
> with the sextant right side up. It?s easy to bring the oil up to the star
> instead of trying to bring the star down to the oil.
> All the best,
> Robert Gainer
>
>
> >From: Alexandre Eremenko 
> >Reply-To: Navigation Mailing List 
> >To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
> >Subject: Russian trick
> >Date: Sun, 3 Oct 2004 18:30:01 -0500
> >
> >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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