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Gary, you wrote:
"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."

I think he meant arc error. Of course, arc error and index error are
separated traditionally, but they don't have to be in a stable instrument.
Suppose I have an instrument with an IC of -1.0' and a calibration table
with values as follow 30d: +0.2, 60d: +0.4, 90d: +0.2, 120d: 0.0. Now when I
shoot a sight of, let's say, 95 degrees. I could first subtract 1.0 and then
add 0.2, or I could save myself one step (admittedly a short and easy step)
by preparing a table as follows: 0d: -1.0, 30d: -0.8, 60d: -0.6, 90d: -0.8,
120d: -1.0. Then on any sight, I look up this net correction. It doesn't
matter whether you think of it as index or arc error. It functions as if
it's an "index correction" which depends on the measured angle. Of course,
index error would change if we adjust the instrument while arc error
wouldn't usually, and that's a good reason for keeping them separate,
especially in sextants that are not so stable.

 -FER



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