# NavList:

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

**George's test**

**From:**Alexandre Eremenko

**Date:**2004 Nov 12, 18:06 -0500

About 2 years ago, (Sat Apr 21 2001 - 07:38:22 EDT) George Huxtable proposed the following test for the sextant rigidity. Measure some vertical angle which does not change much with time. Then take your sextant upside down and measure the same angle. (With a good sextant the result should be the same). George proposed to measure parallax of a neighbor's roof (it does not really matter what you measure, it's the precision of your measurement that does. He proposed to take all possible precautions, like rotating your screw in the same direction etc.) I think this is a good test indeed for checking the RIGIDITY of your sextant. On my opinion, this is a really crucial property of a sextant. (For everything else, you can determine "corrections" by all sort of experiments and then apply them). This is also a very important property for the Lunars, where your sextant is frequently in an unusual position. I did this test today with my usual test for the index error. Here are the results. Sextant: SNO-T, made 1990, scope "inverting". Time: about noon, Lat approx 40, AltSun approx 32d, SD=16.2, (sun semidiameter), 4SD=64.8' Sextant upright position: Up Low Sum IndEr 32.5 32.5 65 0.0 32.5 32.4 64.9 -0.05 32.4 32.4 64.8 0.0 Sextant upside down: Up Low Sum IndEr 32.4 32.4 64.8 0.0 32.5 32.6 65.1 +0.05 32.7 32.3 65 -0.2 Now, if we average all 6 observations (Low and UP) we get 64.9 for upright position and 65.0 for upside down. Which gives 0.025' for the "non-rigidity error". Conclusions. 1. My SNO-T is pretty rigid. 2. Index error is probably 0. 3. My random human error is about 0.3 for the Sun. In fact, this 0.3 (human?) error causes a lot of trouble for me. a) I almost always tend to overshoot. So the correction (when it is substantial) is always negative. b) The error varies from 0 to -0.6' in my observations, and it is most frequently about -0.3' (if exists), but c) for very long distances (more than 120 d) it is sometimes -0.6' What is the resaon I cannot understand. Either it is a sextant instrumental error, or really some human error. All collimation tests I could do show 0. Alex.