# NavList:

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

**Re: Sextant Arc Calibration : FIRST LIGHT**

**From:**Frank Reed

**Date:**2014 Mar 21, 14:14 -0700

Hi Brad,

Looks really interesting. I hope you can get it working.

As with any of these means of estimating arc error, you have a problem to consider: how will you prove that you're measuring something? How will you test your accuracy? Repeatability is probably the place to start. Try this with one sextant on two days and see if you get the same results --or at least close. Oddly enough, you really need a sextant with some substantial arc error so that you will have some guaranteed signal above the noise. After repeatability, you'll need to find way of proving that the system itself isn't producing some sort of bias. For example, a (hypothetical) misalignment of some sort might lead to an apparent arc error from the system rather than the individual instruments. For example, suppose the measuring system introduces a bias of 0.5'*(angle/90°)^2. That is, it's proportional to the square of the measured angle, and at 90° the bias is half a minute of arc. As long as any such bias is stable, you would probably be able to detect it after accumulating data on something like a dozen different instruments. And of course that complicates matters quite a bit... You would almost need to open a laboratory to test sextants just to get the data to prove that you CAN open a laboratory to test sextants.

-FER

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