A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brian Walton
Date: 2019 Mar 12, 21:15 -0700
Dave et al,
You were asking what use could be made of sunrise/set tables and corrections to them for latitude.
Fact: the rise and set time of the Sun at a place 90° different in latitude to your latitude, is also the time the Sun is on an azimuth of 090 or 270 at your position, that is, on the Prime Vertical. To see the Sun on the PV it must be summer.
Look up your correction table, and find the correction to be made to your sunrise, to get the sunrise time at a place 90° different in latitude. Check out the azimuth of the Sun at your position, at that time, using the NavList Data app. If it is not 090 or 270, it is because your correction table is rather crude. It doesn’t matter much, but it tells you when to get your Sungun out.
What use is a shot on the PV? The trigonometry now becomes sin Alt=sin Dec/sin Lat. Set your Sungun up to that figure, with normal corrections reversed, and go outside just before the time of PV. Need I go on?
.I will. If you were doing a time sight 150 years ago, you could enter your epitome and take logsinlat away from logsindec, and look up that figure in the same logsin table and take out an angle(Hc) and time(LHA) You have just precalculated your shot just as well as you did with sandwich fixes. When you take the real shot time, you compare your LHA to Greenwich ( maybe need equation of time in an old style almanac) and get Long.
Those old guys were smart.