NavList:

Very interesting, Peter. You wrote:
"The goal is to maximize the contrast between the limb's brightness at radius r and at radius r+dr, integrated over the 180 degrees of available limb (though with a thin crescent Moon, the horns are not contributing much and the effective limb is more like 140 degrees or 120 degrees).  The optimizer seems stable.  The points along the limb at the various radii are resampled with a cubic spline, and a dr of 0.5 pixel seems to work well."

To get down to the last few seconds of arc in this process, I think you should be looking at a model of the Moon's actual limb, with craters and depressions and mountains and highlands. Since we now have high-res DEMs (digital elevation models) of the Moon from orbiting spacecraft, it's possible to model the limb in exact detail. You should be able to get the Moon's position to the nearest second of arc with all the data available in one of your images. From that you'll get a position fix at known UT to the nearest nautical mile, no horizon required.

Frank Reed