# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Plexiglass horizon & siphon level**

**From:**Jared Sherman

**Date:**2003 Jul 15, 23:54 -0400

Bill- I confess I haven't reexamined the numbers in any great depth, but I did take one start. You're right, my initial numbers were badly off, I slipped something for sure. Bear in mind, I am not suggesting that one sight across a 15" gap allowing one millimeter of tilt across the gap. I'm suggesting "threading the eye of the needle", i.e. lining up the heights of two columns, to an accuracy better than that one millimeter. How accurately can the eye line up those two columns? Ask me in a day or two with rested eyes and some trials.But for the simple triangle and tangent, where the tangent of our angle of tilt =Opposite/adjacent, and the adjacent is a 15" long base (let's say 370mm) and the opposite is a full millimeter of unequal height. The tangent is about 0.0027, which indicates the angle itself is well under one degree, which is 0.017. Roughly speaking, the angle is 0.15 degrees if the difference in height of the two water columns is one full millimeter across 15". Well, that's fifteen minutes not fifteen seconds, so I guess the question is just how eenly can those two columns be compared. Or else....there's an inherent problem in the entire approach. On the other hand, one internet vendor did suggest I could buy circular levels about 2-3/4" in size for $165 each, with an accuracy of 30 seconds per 1/10th inch of bubble position. That should mean a 10 second or less error assuming the user set that bubble carefully into a hairline circle, which I think is readily possible. He's also got " a straight glass bubble level which is 6 3/8" long that has a sensitivity of 6sec./2mm " which would indicate a 3 second error could be obtained, although I didn't get a price on that one I suspect it will be similar to a hand surveyor's level at $75-100 each, two needed. So commercial levels can provide a 3-10 second accuracy for about $200, which I suppose is not unreasonable if someone really wanted a flat artificial horizon without fluids. And here I was hoping there'd be a pony in the pile.