Solar Geometry

These libraries in C and Matlab provide means to compute the position of the Sun and related astronomical quantities

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Two libraries are available: "solar geometry" and the more recent and accurate "solar geometry 2". They comprises the following items:

  • geometry of the solar beam,
  • position of the sun in the sky,
  • and for "solar geometry" only, sunrise, sunset and astronomical daylength, changing the time system,  the extraterrestrial irradiation, and procedures providing the above parameters for a given day, or on monthly or yearly averages.

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).  CC-by icon

Solar Geometry

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

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.

  • The code is available in C langage. The zip compressed file contains the library, a makefile for the Unix environment and an example of use of the library. This is still a code for research, so it contains quite a limited number of comments. As well, no recommendation is given to create the makefile in the Windows environment.
  • The code is also available in Matlab.
  • A Web processing Service is also available. It executes Solar Geometry 2 with geographical coordinates and time period as inputs and returns topocentric declination and hour angle, topocentric Sun azimuth angle and elevation angle without refraction correction.

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