A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tony Oz
Date: 2018 Jan 15, 02:52 -0800
I programmed my TI-83 to do the computations as per Chapter 15 of Henning Umland's excellent "Short Guide to Celestial Navigation", where the author warns on the accuracy:
The maximum error of GHA and Dec is about ±0.6'. Results have been cross-checked with Interactive Computer Ephemeris 0.51 (accurate to approx. 0.1'). Between the years 1900 and 2049, the error was smaller than ±0.3' in most cases (100 dates chosen at random). EoT was accurate to approx. ±2s. In comparison, the maximum error of GHA and Dec extracted from the Nautical Almanac is approx. ±0.25' (for the sun) when using the interpolation tables. The error of SD is smaller than ±0.1'.
Indeed, I can see the difference between the above formulas output and the printed/official NA data, usually the difference is within the advertised ±0.3' but at times it does reach almost 1'.
Yes, a ±1' data is better than no data - in case of a disaster, but could I get a better formulas/coefficients (for a programmable calculator like TI-83) to have a more accurate ephemerides?