NavList:

Robin,

You are partially right.

If your corrective lenses (astigmatism or other) have an axis, that axis must be parallel to a line between your two eyes no matter how you tilt or swivel your head.

Astigmatic corrective lenses do have an axis. Opticians mount them in your eyeglass frames so as to keep them parallel to your eyes. Moving your head moves the frame and keeps the lens-to-eye-axis relationship intact.

When you use a sextant in normal mode, its frame is held vertically.  Here, I just taped the eyecup with its lens axis horizontal to the natural horizon.

But lunars calls for something else. In lunar's, you often tilt the sextant at some angle but you may still keep your head upright or near so. Because the sextant is now rotated with respect to your eye, you now have to rotate the lens (rubber scope cup in my case) to bring the lens axis back to parallel with your two eyes however you are tilting your head.

Ed