I'd like to write a python app which allows properly setting up a parabolic dish for satellite reception.
I know the formula which give altitude and aimut given dish position, so it would be quite simple; but I'd like to add one more feature: a compass, so the program user would just need the cellphone, and not also a compass, to actually set up the dish.

But I can't figure out the algorithm to determine south direction given sun position; I found two useful sites:
http://tions.net/CA256EA900408BD5/vwWWW/outdoor~01~02
http://www.qcontinuum.org/compass/
can anybody help figuring out the formula?

I'll show on phone display a circle which shows sun in the upper part, and an arrow whic points toward north; so, the formula should be:
alfa = f(sun_elev, sun_azimut)
Where alfa is angle among north-arrow and sun on the screen.

Actually I think the formula could be a bit more complex:
alfa = f(sun_elev, sun_azimut, dish_latitude, dish_longitude)