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    Re: 1769 navigation.
    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!
    
    -- 
    I filter out messages with attachments or HTML.
    
    
    
    

       
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