ABSTRACT
The sun is a sphere of intensely hot gaseous matter with a diameter of 1.39 × 109 m and is, on the average, 1.5 × 1011 m from the earth. This distance compares to about 12,000 times the earth’s diameter. The eccentricity of the earth’s orbit is such that the distance between the sun and the earth varies by 1.7%. The sun has an effective blackbody temperature of 5777 K. The radiation emitted by the sun and its spatial relationship to the earth result in a nearly fixed intensity of solar radiation outside the earth’s atmosphere, often referred to as extraterrestrial radiation. The values for this solar constant found in the literature vary slightly due to the measurement techniques or assumptions for necessary estimations. The World Radiation Center (WRC) has adopted a value of 1367 W/m2 , with an uncertainty in the order of 1%.
