NavList:

I've read several times about, and have attempted, a "back sight" and that you use the horizon 180 degrees out from your intended horizon. So far I've seen nothing in the mirrors. Do I understand correctly that I'm to see the Sun (for example) in the horizon mirror?

Bowditch, Chapter 16 speaks of a back sight.

“Occasionally, fog, haze, or other ships in a formation

may obscure the horizon directly below a body which the

navigator wishes to observe. If the arc of the sextant is

sufficiently long, a back sight might be obtained, using the

opposite point of the horizon as the reference. For this the

observer faces away from the body and observes the

supplement of the altitude. If the Sun or Moon is observed

in this manner, what appears in the horizon glass to be the

lower limb is in fact the upper limb, and vice versa. In the

case of the Sun, it is usually preferable to observe what

appears to be the upper limb. The arc that appears when

rocking the sextant for a back sight is inverted; that is, the

highest point indicates the position of perpendicularity.”