The SolarPos package is an implementation of NREL's Solar Position Algorithm for Solar Radiation Applications. As noted in the paper, the algorithm is intended "to calculate solar zenith and azimuth angles in the period from the year -2000 to 6000, with uncertainties of ±0.0003°."
For most uses the main entity will be SolarPos, which calculates geocentric and topocentric angles for a given date/time and location on earth.
