A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2015 Mar 30, 04:04 -0700
I’m speaking from memory now from about 47 years ago, so we all know how accurate that’s likely to be, but I seem to remember being taught that the only thing wrong with using equatorial coordinates, and thus Sidereal Hour Angle (SHA) and declination, to position stars is that the little devils seem to move around because of the Earth’s Precession and Nutation (P&N). For example my 1980 Epoch edition of AP3270 Volume 1 gives Arcturus as SHA 146 19’, declination N 19 17’. If I’ve converted correctly, the letter below gives today’s values for Arcturus as SHA 149 44’, declination N 19 06’. Is it possible that using ecliptic coordinates avoids P&N?
I also seem to remember that there’s actual motion of the stars themselves, albeit very slow, which is why today’s star formations don’t always look very like the names first given them by the ancients. I’m not sure what we mean by six significant figures. For declination for example, do we mean: Degrees, Minutes, Seconds, i.e to the nearest second; to the nearest degree to four places of decimals (2.7 times larger); or to the nearest places of decimals, which is tiny. If the latter, how long would a value last until we had to correct for P&N or even the real motion of the stars? Dave