numeric vector Angle in degrees for the
sun position. An argument should be passed to one and only one of
elevation_angle or zenith_angle.
occluded.value
numeric Value to return when elevation angle is
negative (sun below the horizon).
Details
This is an implementation of equation (3) in Kasten and Young
(1989). This equation is only an approximation to the tabulated values in
the same paper. Returned values are rounded to three significant digits.
References
F. Kasten, A. T. Young (1989) Revised optical air mass tables and
approximation formula. Applied Optics, 28, 4735-. doi:10.1364/ao.28.004735.