Bird Sky Clear Model
This function computes the apparent shift in the solar elevation angle caused by atmospheric refraction.
Note that the output is a positive number; i.e.: near the horizon, the solar disk appears higher in the sky than its true position. The apparent elevation is the true (geometrical) elevation plus the correction factor.
The current implementation assumes default values for the following parameters:
Bird, R., Hulstrom, R., A Simplified Clear Sky Model for Direct and Diffuse Insolation on Horizontal Surfaces, 1981 NREL SERI/TP-642-761