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Agreed "altitudes" .. not declination for same refraction component. Don't 
know why I said declinations - brain not in gear whilst typing. I think it is 
called senility.

And agreed refraction is not completely negligable for rigorous calculation.

I also take the point the errors (being in height - altitude) are going to be 
greatest when the two stars are opposite each other towards 180 degree 
separation, and less so when the hour angle between then is smaller:  my 
point is the difference in refractions is going to be much reduced when of 
comparable altitudes - and I should now say less than 90 degree sepatation in 
hour angle -  and of a value not entirely way out and comparable to the 
accuracy of the measurement itself.

If errors in measuring; and the errors inherent in the sextant itself are 
comparable with the errors in refraction then it is not worth worrying too 
much about it, as it is more a theoretical problem.  Interesting to 
contemplate and correct for, but I would query the practicality anyway for 
serious measurement - unless lunars are your interest when refraction is 
incorporated in the procedure anyway.

Errors in calibration of the scale on a sextant can sometimes exceed a minute 
of arc for example if poorly divided, but that can only be determined on a 
proper dividing circle table or optical dividing head.  
Attempting to use star separations to try to determine scale accuracy for 
example would not be possible due to the variables in the measurements 
themselves - including the refraction component even if calculated.

Checking Polaris altitude (Northern hemisphere) is much simpler for a casual, 
simple, practical check of the sextant index error if that is all that is 
wanted.

Douglas Denny.
Chcihester. England.


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