# NavList:

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

**Re: Instumental error?**

**From:**Alexandre Eremenko

**Date:**2005 Apr 20, 00:55 -0500

Fred, > I wonder if you could clarify how the standard deviation, > average error > and maximal span were computed. I measure some distance several times, say 7 times in 10 minutes. A star-to star distance can be considered constant for this period. The average is the arithmetic average. (x_1+x_2+x_3+x_4+x_5+x_6+x_7)/7 in this case. Here x_1...x_7 are my sextant readings. The standard deviation. I subtract the average from each x_j, square the difference add the results, divide by 6 and extract the square root. (Actually the calculator does it all for me). Maximal span. I take the maximal value of x_j and subtract the minimum value. For the Lunars (where the distance changes more quickly, I feed all 7 distances to Fred's program, and look at the errors this program produces. Then I do the above manupulations on these errors. > I would prefer that the standard deviations be given > to an extra digit. The purpose was to detect and estimate the systematic error. It is clearly much larger than the standard deviation. > the development of > proficiency is an important component in minimizing error. Sure. I was shooting distances since October 2004 almost every evening when weather permitted. Trying to develop this proficiency. But when I made almost simultaneous shots of the same distance with two sextants (The other was Bill's new Astra) and obtained a correct result with the Astra, and +0.4 error with my sextant, I finally concluded that there is arc error. > I was very accurate and precise in altitude shots (with a > Cassens + Plath, not the Husun), My Sun shots with art horizon were always much better... until I realized that I compare the final result, not the sextant reading, so I divided my error by 2:-) With actual sextant reading the systematic error is about the same. > The curves can take > surprising twists. I regret I don't have one to share You can see many on the web. Sometimes the pictures are good enough to read the certificate. > described calibrating an Astra IIIB using equipment to which he had > access. He mentioned that he fit the "observations" using Fourier I know a very clever method of calibrating your sextant "at home" that is without stars, but you need a theodolite or another instrument more precise than the sextant, and with precise level. Unfortunately I don't have a theodolite. Can describe the method to the list if anyone is interested. (I recently read about it in one XIX century paper on the Internet). Alex.