Hi dacruick,
Thanks for your quick reply. What I'm trying to calculate is the time that twilight (sun = 18 degr below horizon) starts, or more generally speaking; I'm trying to calculate the time of day by a given altitude of the sun.
I've been browsing the web and found this...
At time T(sunrise) the SHA = -81.361029147899785, which makes frac. hours to noon (SHA / 15.0).
Is calculating the time of astronomical twilight to noon as simple as (SHA - 18) / 15.0?