NavList:

Here's a cheap "fixed-angle sextant" that can get longitude by equal altitudes as in that video. It's nothing more than two notches cut into a little board with two small square mirrors held in place in the notches. You don't need to measure anything, and there are no significant adjustments. It's a sextant distilled down until there's nothing left but the principle of double reflection itself. You'll need to figure out some way of shading the Sun in front of the index mirror. Got any eclipse glasses lying around?? I still have about fifty from the 8 April 2024 total eclipse. Just clip a filter onto the upper mirror with a paper clip.

A trivial instrument like this can get you a complete running fix during the day. One could make a half-dozen of these for various altitudes for very little money. Like a Bris sextant, you calibrate by observing. That's easy enough, right? And you're not limited to longitude. You can get excellent running fixes, good to a few miles in both longitude and latitude all day long with a set of six of these "fixed-angle sextants". 

The biggest risk with this very cheap example in my photo is that the mirrors could easily shift, and if they do, the calibration is gone. But with some small modifications that "design flaw" could be fixed. And it wouldn't be terribly hard to make the frames from some more stable material.

Why doesn't anyone do this? Why don't we all have a set of "fixed-angle sextants"? Probably because you can get a lightly used Davis Mk 3 for less than $40 on a good day. And an instrument with a movable index arm feels more like navigation and looks more like navigation... The task of manual adjustment screams out Look, Ma! I'm a navigator!

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA

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