NavList:

Brad,

If you want the cleared star-star distance for any star-pair of your choice, you can use the equation

cos(CD) = sin(H1) sin(H2) +
cos(H1)cos(H2)*( cos(D) - sin(Hr1)sin(Hr2) )/( cos(Hr1) cos(Hr2) )

where D is the observed distance, and H1 and H2 are the two observed altitudes. The cleared distance is CD, and Hr1 and Hr2 are the refraction corrected altitudes. If you want to automate the whole computation, use the equation for refraction correction given in the almanac.

And if you want to measure a sextant’s error (added to operator error), calculate the expected star-star distance from:

Cos CD = sin(dec1) sin(dec2) + cos(dec1) cos(dec2) cos (dGHA)

Where CD is that distance, dec is the declination of each star, and dGHA is the difference in their GHAs. Of course, get the declinations and GHAs from the almanac on the date of observation. In that case, aberration is included in the calculation.

I’m not clear what your objective is, but in any case, the end result of this last calculation should have a maximum error of 0.2’, which is due to the limitation of the almanac’s tabulated data.

Hope this is useful,
John

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