This algorithm calculates the solar zenith and azimuth angles in the period from the year -2000 to 6000, with uncertainties of +/- 0.0003 degrees based on the date, time, and location on Earth. (Reference: Reda, I.; Andreas, A., Solar Position Algorithm for Solar Radiation Applications, Solar Energy. Vol. 76(5), 2004; pp. 577-589). The software has not been tested on a variety of platforms and is not guaranteed to work on yours. It is provided here as a convenience.
This algorithm is available in ANSI C and CRBasic; how to use this algorithm and a description of the variables is included in the
spa.h header file (for C) and spa_tester.cr1x (for CRBasic). Further information on this algorithm is available in the following
NREL technical report:
Reda, I.; Andreas, A. (2003). Solar Position Algorithm for Solar Radiation Applications. 55 pp.; NREL Report No. TP-560-34302, Revised January 2008. (1.9 MB PDF Document)
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Note: An online SPA calculator that uses the C source code is also available.