NavList:

Looking at your spreadsheet, I see that you apply altitude corrections sequentially, showing the result at each step.  When you reach semi-diameter, you subtract it from the previous value.  In a sextant telescope, we essentially treat all planets as point sources, with semi-diameters of zero, by "splitting" the planet with the horizon.  But to allow semi-diameter to be considered in an observed altitude calculation, as if we had a much more powerful telescope, we would have to consider whether we are observing the upper of lower limb.  You subtract, implying the upper limb.  Why do you do that?  I note that the USNO Celestial Navigation Data site adds semi-diameter (to refraction and parallax) to get the sum, implying the lower limb, with no further explanation.  That is also a source of confusion for me.

Stan