Regression of the sun declination and the equation of time for processing observed data of daylight illuminance and solar radiation

Many measurements of daylight and solar radiation have been carried out at many stations in the world since 1991. For the speedy processing of a large volume of observed data, it is essential to establish a moderate method to calculate the sun position. In order to identify the accurate position of the moving sun, it is necessary to refer to the values of the sun declination and the equation of time. In this paper, regression equations which derive the sun declination and the equation of time as a function of mean solar time are developed according to the principle of the sun movement on a celestial sphere. They include two astronomical constants; the one revolution year ( = 365.2422 days) and the inclination of the axis of the earth (= 23.439 degree).