A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2016 Mar 25, 23:01 -0700
The plastic Davis Mk 3 sextant is an incredibly basic, yet fully-functional, navigation-quality sextant. You could cross oceans with one of these things. They look flimsy, distractingly cheap, but as proper instruments of reflection they do work, and they are capable of measuring altitudes within three or four minutes of arc with just a little care in the observation process.
The Davis Mk. 3 sextant has one basic functional problem. The index arm works with a pure vernier scale, and it depends on friction between the arm and the arc. There is no adjustment for this. The user slides the arm along as best as possible until the angle is right. It catches and "bounces" with static friction along the arc. Does anyone have any advice on how to "nudge" the arm to get it right? One good option is to the nudge it up above the altitude with a rising sun and then wait a minute or two until the altitude catches up with the adjusted position of the arm. It's always a game with these things trading static friction for free motion.