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The libraries account for the differences between geographic and geocentric coordinates. This difference is only for latitude and reaches its maximum of 0.2 degree (approx. 20 km) at 45 degrees. Coordinates in equations for sun geometry are geocentric, while latitudes used by customers are geographic.
These software codes are available under a Creative Commons license (CC-BY).
Solar Geometry is the first generation of library for computing the relative position of the sun and the earth. It has been used to develop many tools relating to the method Heliosat-2. Formulas are from the European Solar Radiation Atlas of the European Commission. Download solar geometry in C. For the compilation, refer to the files README and INSTALL. There is no example of use of the library, but the codes provide a satisfying number of comments to help using this library.
Solar Geometry 2 (SG2) is the second generation of library for computing the relative position of the sun and the earth. Valid over the time period 1980-2030, the algorithm is 20 times faster than the well-know SPA algorithm, with an accuracy order of approx. 0.005°. Reference article: Blanc P. and L. Wald, The SG2 algorithm for a fast and accurate computation of the position of the sun for multi-decadal time period. Solar Energy 88, 3072-3083, 2012, doi: 10.1016/j.solener.2012.07.018.