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Frank,

This is an interesting thread and an detail I have never considered. But I'm not following it very well. My trouble is trying to link three thread-topics: star-to-star distance, refraction
and using a sextant with no index correction together and what this means in practice or sextant calibration.

Are we:

1-  understanding why a sextant (with "no index error" as stated in last paragraph) will measure someting less than calculated geo-centric distance because "Atmospheric refraction decreases the apparent angles between the stars." ?

2- understanding how much the refraction value will be? A rule of thumb and one that avoids messey temperature and pressure?

3- noting that corrected sextant star-to-star distances will be MORE than what is found by the Law of Cosines ... "That would be 0.1' greater than expected, as calculated above..."

Do I conclude that you can not determine a sextant's Index Error reliably by using star-to-star distances because of refraction if you are using just Law of Cosines to find geo-centric distances and not considering the influence of refraction. But the rule of thumb gets you closer???

I guess I'm not sure how to appreciate or use what is evolving in the thread despite the refraction calculation.

Ed