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    Re: Simple celestial navigation in 1897
    From: Frank Reed CT
    Date: 2006 Mar 6, 00:22 EST

    George H., you wrote:
    "Layton seems to go  about the process of interpolating for the change in
    latitude in a curious way.  "
    
    Just for background for anyone who hasn't followed the details, in  a typical
    case (e.g. May 1, 1897), the Morgan's navigator writes down the  declination
    from the almanac (which is for noon Greenwich time) and then adjusts  it for
    ship time twice, or, at least, in two interpolation steps.  Why??
    
    Actually, it makes pretty good sense when you see it in action on a  day when
    there is a latitude calculation, as well as a longitude calculation.  From
    the almanac, we take out the Sun's declination for Greenwich Noon (it's
    Greenwich Apparent Noon, by the way. Doesn't make more than a fraction of a mile
    difference in the final position, nonetheless that is the declination that the
    Morgan's navigator is using, and it's the recommended choice, too). But why
    interpolate twice? Because we want the best possible declination for our Local
    Apparent Noon sight AND the best declination for the local time of the time
    sight. So you adjust the declination from Greenwich Apparent Noon to Local
    Apparent Noon using the longitude (just a rough estimate is fine) and then you
    adjust that declination to the the Apparent Time of the time sight.
    
    Also,  it's probably worth remembering that there were tables for this. You
    could look  up in a table the change in declination for a given longitude and a
    given rate  of change of the Sun's declination and read out the adjustment to
    the  declination for noon in your longitude. Then using a separate table you
    could  adjust the declination to your local apparent time, e.g. 3:00 in the
    afternoon.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

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