Determination of time and sun position system

• Historical development of related time and solar coordinate definitions.
• Several approaches to determine solar time and angle, with solar altitude and azimuth angle.
• Equations for solar altitude and azimuth are recommended for calculation purposes.
• Unification of solar position calculations based on preferred orientation and time is proposed.

Accurate calculations and recording of time either in local clock time or true solar time in regular daytime measurements of sun and sky radiation and light, in computer programs evaluating solar irradiance or sunlight and skylight illuminance currently apply differently defined solar hour angles as well as angularly determined solar altitude and azimuth coordinates. The historical basis of solar geometry used to construct sundials for the measurements of true solar time were gradually replaced during Middle Ages by spherical trigonometry relations of solar altitude and azimuth angles including solar declination, local geographical latitude and time changes in better precision. However, in different scientific and practical applications like in solar energy or daylight calculations are currently used several systems and formulae defining sun position and time which need to be unified and standardised in computer programs or measurement evaluations. Potential confusion and inconsistent results should be avoided, possible mistakes have to be checked and corrected after the valid ISO international standard. This paper discusses various approaches considering time and sun coordinates in different systems.