A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Fleming
Date: 2020 Sep 9, 16:00 -0700
I'm not confident this will help but let me try. If I was given sunGHA and I knew the date, which I'm going to take from the sunDec and my knowledge of the season, then I would go to the daily pages of the nautical almanac and get AriesGHA at the same time as sunGHA agrees with given value.
But i don't think that is your question. First there is no way for sunGHA to be given to you exmachina. With an almanac and a GMT watch including date then one can proceed as above.
As the earth is spinning once a day GHAs changes on average around 15 degress around per hour for the Sun. As the year determines the seasons, the sun moves along the ecliptic once around the Celestial Sphere every 365 1/4 days. Do to the elliptical orbit of the Sun, it speeds up and slows down on that orbit. The equation of time connects the average Sun position in GHA to the actual instantaneous position.
Aries is fixed (ignoring 26,000 yr cycle of the precession of the Earth's poles) on thec Celestial Sphere. Being fixed its GHA cycles 366 1/4 times in a year with no equation of time correction to its GHA.
Above is the calculation of the Almanac, to the level I understand.
If you could ask perhaps with an example we could be of more service.