It can be calculated from pi/2-cal_solarinclination.
solar_declination
is solar declination anger. It can be calculated from cal_solardeclination()
Details
The minus and positive signs are taken before and after solar noon,
receptively. The reason for having the positive-and-negative signs is
merely an artificial convention so that we are able to distinguish between
the sun lying westwards (posite angles and after solar noon) and eastwards
(negative angles and before solar noon).
References
Teh CBS.Introduction to mathematical modeling of crop growth:
How the equations are derived and assembled into a computer model.
Brown Walker Press, 2006.