A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Tony Oz
Date: 2020 Mar 26, 15:54 -0700
Did you get the 79°30' by multiplying the 17h18m by 15° and subtracting 180°? That's incorrect.
You should have consulted the NA for the 12th of May (presumably - year 2005) at 17UT and add the GHA increment for the remaining 18 minutes. In the illustrative table you see 75°51,1' - that is the GHASun@2005-05-12UT17. Then you see the 4°30' being added - that is the GHA increment for the 18 minutes, thus the GHASun@2005-05-12UT17:18 = 80°25,1'.
If the obtained GHASun value exceeds 180° - you are in the eastern longitudes, you should subtract GHA from 360° to get your longitude. So far, with GHASun = 80°25,1' < 180° you are in western hemisphere, no subtractions needed.