NavList:

Geoffrey, you wrote:
"This means that the background light around the moon  will be stongly polarised when the moon is 90 degrees from the sun.
It follows that the use of a polarising element can almost completely eliminate the background light around the moon, but will only decrease the moon's light by half. It will thus be much easier to determine when the now dark gap between moon and sun (as viewed through the sextant) closes and the two bodies have 'made contact'."

Yes! This does work. I experimented with this trick a couple of years ago. The visual effect is impressive: the sky is noticeably darker. I was surprised that this didn't improve my own observations significantly, but perhaps that's because I've already "trained" myself to get the timing right. For me, the visual task when aligning a lunar is to make two circular edges match up as if to form a segment of a "figure eight" rather than eliminating the gap between the limbs of the two bodies. But using a polarizer in this fashion is certainly worth more experimentation, and it may well be that my lack of improvement was unique to me or a statistical fluke from a small number of trials. 

Where would be the best location for a polarizer in the sextant's light path? Assuming my polarizer is of average optical quality, maybe plastic, if I want to minimize blurring and other distortions, where would be the best place for the polarizing filter? At the telescope eyepiece? In from of the telescope objective? Somewhere else? By the way, this process is unrelated to the polarizing filters found on some sextants which use crossed polaroids to "dial in" a darkness level, replacing some of the standard shades on a sextant. To remove the brightness of the sky in the polarization band (a zone roughly 75° - 105° from the Sun in any direction), we want a single polarizing filter in the light path, and the orientation can be fixed. When I experimented with this, I taped a polarizer directly to the objective end of the sextant telescope.

Frank Reed