NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: 1769 navigation.
From: Paul Hirose
Date: 2012 Jun 14, 21:44 -0700
From: Paul Hirose
Date: 2012 Jun 14, 21:44 -0700
A 2102-D exactly correct in 2012 is about 3.4° in error in 1769 due to precession (about 50" per year). My figure is for the worst case: a star on the ecliptic. The greater the distance from the ecliptic, the less the error. There are two points where it's zero. The total error is a combination of SHA and declination errors. The error in each coordinate depends on the star's position on the disc. In other words, there's no simple correction. On the other hand, the Sun, Moon, and planets have to be plotted on the disc with the coordinates "of date", which automatically eliminates the above error. For these bodies the 2102-D accuracy does not degrade over time. All you need is a source for the coordinates. For star finder purposes (not sight reduction!) it's sufficient to assume the Sun follows an identical cycle of GHA and dec. every year. (The motivation behind the adoption of our Gregorian calendar was that the old Julian calendar wasn't accurately synchronized to that cycle.) For instance, here are the Sun coordinates at Greenwich noon, Oct 15, in several years in the 18th and 21st centuries: GHA dec. 3.6° -8.7° 1769 3.5° -8.6° 1770 3.5° -8.6° 1771 3.6° -8.8° 1772 3.6° -8.8° 2012 3.6° -8.7° 2013 3.6° -8.6° 2014 3.5° -8.5° 2015 Originally I used today's date, but by coincidence the equation of time on June 15 is so close to zero that the GHAs were either 0.0 or 359.9. I thought this might lead to suspicion that I specially picked the date to get "nice" data. So I changed the month! --