NavList:

Suppose I'm using a common marine sextant on land to make artificial horizon sights of the Sun at regular intervals, maybe every 5° change in altitude, during the morning hours as the Sun climbs from 5° altitude up to a noon peak of 60°. My artificial horizon is a bowl of water. It's a wind-less day so water is fine. I aim my sextant down so that the horizon view through the sextant points at the Sun's reflection in the bowl, and I adjust the index arm to point to my best guess of double the altitude, and then I wobble around a bit until I'm able to line up the direct sun image from the index mirror with the reflected image in the bowl of water. Got it? I'm just shooting the Sun in a simple A.H., and the angle I'm measuring is double the actual altitude. 

When I start my observations in the morning, I take note of the tilt of the index arm relative to the local vertical. Picture a little plumb-bob, like a washer on a thread hanging down from the middle of the index mirror. The main axis of the index arm makes an angle with that thread. The index arm is inclined back about 40° when the Sun is low. As the day continues, and I rotate my sextant through a wide range, eventually reaching all the way out to the 120° limit of the instrument, it seems that the index arm maintains a constant angle with the local vertical. Am I imagining that constancy, or am I right? Is this really a property of artificial horizon sights? Is the index arm --and therefore also the index mirror--always at one contant angle relative to the vertical no matter what altitude I'm shooting? Can you prove or disprove this claim? And if it's true, does this do me any good?? Is there any practical advantage to this fixed orientation of the index arm?? Could I somehow clamp the index arm onto a fixed support of some sort and let the instrument rotate around that known fixed orientation? Or is it just a form of amusing geometric trivia about these sights? Or is this contancy of orientation wrong, or merely approximate?

Just thinking "out loud" here. Maybe it's real. Maybe not. :)

Frank Reed