NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Simple celestial navigation in 1897
From: Frank Reed CT
Date: 2006 Mar 6, 00:22 EST
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
