# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Index error by a star-star distance
From: Andrés Ruiz
Date: 2008 Sep 23, 09:49 +0200

Yes Gary,

I say in [NavList 6290]:

The calculated star-star distance is compared with the observed one to obtain the sextant error, that is the index error plus the instrumental error inherent to the sextant.

and as Frank says in [NavList 6302], the objective is to build a table for the sextant error in all the arc from 0º to 120/130º

In a museum I have seen a XIX century sextant with two table, one for the index error and another for the excentricity.

Also to practice with the sextant for fun.

-------------

Bill, [NavList 6295], the program runs only under windows XP, 98, 98, 200, NT, sorry.

Frank: [NavList 6303] “I think a title that would make more sense (at least in English) would be 'sextant calibration by star-star distance' or 'arc error by star-star distance'.” – Yes, I think is better.

Thanks,

Andrés Ruiz

De: NavList@fer3.com [mailto:NavList@fer3.com] En nombre de Gary LaPook
Enviado el: lunes, 22 de septiembre de 2008 19:52
Para: NavList@fer3.com
Asunto: [NavList 6294] Re: Index error by a star-star distance

O.........K.............   I guess I don't get it. Why not just sight on one star which should have a zero altitude difference between the two images of the same star. Where is the advantage in using two stars?  In addition you would not be determining "index error" since  the measured error would be the total of index error and "arc error" and you could not determine the contributions of each of these components. Star to star distance can be used to check for arc error but only after index error has been determined by some other method since this value must be subtracted from the measured star to star distance before comparing the measured distance to the computed distance thereby determining  the arc error at that point of the arc.

gl

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