# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Time Sight Problem: Joshua Slocum Lunar
From: David Fleming
Date: 2011 Mar 25, 16:22 -0700

Now that I have asked the question I believe I can answer it.

For the afternoon sun sight, we have
hs = 38d 39m
-3.9m ic and 4 ft ht of eye
+14.8 main ( ref & semidiameter)
ho = 38d 49.9m = 38.83166...

and NA gives Sun GHA(t) = (t-21)14.963 + 134.79
( ie this equation gives values of sun GHA around time of interest )

with DR Lat of 11d 4m S and above ho

by solving Law of Cosines for LHA we find LHA = 38d 38.62m = 38.64366...

thus if DR Lo were = (t-21)14.963 +134.79 -38.64366.. then we would have our sun sight anytime throughout that period.

Now we need a similar equation for the lunar observation. Getting a similar equation for DR Lo (t) which would give the lunar observation. This could be a little harder due to variation of lunar dec but it is probably sufficient to ignore that and get a linear expression.

Equate the two expressions and the value for t is GMT for the pair of observations.

Dave Fleming
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