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From: Frank Reed (no email)
Date: Mon Mar 06 2006 - 00:22:18 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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