A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Antoine Couëtte
Date: 2018 Jan 24, 03:37 -0800
You wrote, "Curiously, his Sun GHA is as good as any method despite the greater error in GHAA."
I had long observed this.
Although I am far from any document to elaborate in further details, and to the best of my memories, the Sun Longitude is initially computed in Ecliptic 2000.0 . To get it into Ecliptic of date, you simply add "+ Delta Psi".
Then you transform the Ecliptic Longitude into Right Ascension in the Equator of date.
The precession effect in Equatorial coordinates has then become equal to "+ Delta Psi Cos (Obliquity)".
The Calculation of GHA Aries also involves the very same term, i.e. "+ Delta Psi Cos (Obliquity)".
When you compute GHA Body = GHA ARIES - Body RA, any error on the precession term then cancels out.
Hope it may help,