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

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Re: Practical difficulties
From: Gary LaPook
Date: 2011 May 15, 23:20 -0700

And you can pick any place you want to use as the AP, the end of the channel or the destination. If you are using a calculator this is trivial. If you are using tables then there are ways to accomplish the same. The first instinct is to pick even times such as on the hour, 20 minutes after, 40 minutes after but you don't need to. You can get the GHA to equal whole degrees by picking the right time for the first data point. If using the N.A. look at the GHA of the sun when it is on the whole hour just to the east of your chosen A.P. Find the difference between the GHA and the assumed longitude and convert to time. Let's say at 1800 Z the sun's GHA is 117° 55.6' and you want to use the south end of the Channel Islands breakwater as the A.P. which is at 119° 13.8' W, a difference of 1° 18.2' so the sun will be at that longitude at 1805:13 Z. So you do the calculation for that time and plot on the graph and then at 1825:13 and 1845:13, etc each time is exactly 5° of GHA so you use the same A.P. Then you use the standard delta "d" correction and then also make the adjustment for the minutes of latitude of the A.P by using the correction table from H.O. 214 that I posted before. http://fer3.com/arc/img/105707.ho214-delta%20lat.pdf

see:

http://fer3.com/arc/m2.aspx?i=105707

gl

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