NavList:

Hi David,

Thank you.  I must get back to your other puzzles when time permits.

I've never tried this with the sun, and have never tried with the scope reversed.  I have set up morning star sights with the scope out altogether.  On a small boat in a seaway it is easier to positively identify a chosen star without the scope and to bring it down to the horizon, then to screw in the scope to complete the sight.  Without the scope, two eyes working allows reference to the rest of the sky.

There are many references to the use of a sight tube (or equivalent names) in older literature and online (Admiralty = “plane tube”, meaning in the plane of the sextant.  Bowditch = “blank tube”).  Greenwich has one dating from the 1880s but not on display.  None of my sextants has one and I’ve never used one, but if the opportunity ever arises, I will procure one if it will fit any of my sextants.

In addition to training and as a step during star sights in rough weather, a sight tube (without glass) can be used when it is very wet on deck to avoid the issue of fogging or salt spray on the lens.  Again, I’ve never tried this but have certainly had to deal with salt water on my glasses while trying to take a sight in boisterous weather. Finally – for horizontal angles, though I have always used the low powered scope for these.

Murray.