# NavList:

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

**Re: Compact Data for Navigation and Astronomy**

**From:**JC Sutherland

**Date:**1999 Sep 09, 1:59 PM

Hello Andres, No, I am not using the program in The 'Compact Data for Navigation and Astronomy'. However, I would recommend this book as a very good alternative to the 'Nautical Almanac'. It is cheaper and the data covers five years. Also the program and data on the CD that comes with it when loaded into a PC will provide a complete Astro sight reduction package. It will evaluate a position fix, obtained from combinations of sights, using any of the navigational bodies and give a statistically calculated Ellipse of Probability for the complete fix. Unfortunately the program on the disk cannot be used by a calculator and for this purpose it is necessary to use the data printed in the book instead. The book gives all the formulae required and the data is easy to use. A yacht navigator would not wish to rely on a single sextant observation of the Sun or other body at sea as this would give no indication of its reliability. By taking several sights through the sextant one after the other within a few minutes and by plotting the trend on graph paper, the linearity of the group can be assessed. Any blunders can be rejected and the most suitable sight selected for subsequent reduction to a P/L. This method has been in use for many years but without some calculator help the sights cannot be analysed statistically. Using the Ti-82, this plotting can be done on the calculator screen, the raw data compared with a plotted Linear(or a Quadratic)'Least Squares Fit' equation evaluated by the Ti-82 and a statistical 'Figure of Merit' calculated. See Note below. Once this procedure has been run on the Ti-82, using a sea horizon and data corrected for Index Error and Watch Error, the selected least squares fit equation can then be used in my sight reduction programs. these will first calculate the 'Most Probable Altitude' for the 'Mean Time of Sight' derived from this input data. This combination of TIME and ALTITUDE is used to calculate an INTERCEPT and AZIMUTH using the 'Compact Data' to evaluate GHA and DEC etc. Finally the 'figure of merit' of the sight group used directly to indicate the Probable Error of the plotted P/L. I have written some sight reduction programs specifically for the Ti-82,and based on the 'Compact data' above. If there is sufficient interest I will send a sample of them to the group for an opinion as soon as I have dotted the T's and crossed the I's. FOOTNOTE, For a training exercise, since it is not necessary for the sights to be 'Reduced' to a real P/L in order to find the 'Figure of Merit', IT IS NOT NECESSARY FOR A REAL HORIZON TO BE USED to gain experience in using the marine sextant. Timed sights can be taken ashore using a distant roof or hilltop, for example, as a dummy horizon. Provided that this dummy horizon is horizontal and distant and is vertically below the Sun or Moon at the time of the sight. Nor is it necessary to apply any corrections (except perhaps Index Error if a plastic sextant is used). The Figure of Merit calculated by the Ti-82 would be the measure of the observers skill. The aim of the exercise would be to gain practice by attempting to reduce its value while at the same time reducing the span of time needed to acquire a set of observations.