Solar constant

     Solar rays falling on earth subtend an angle of only 32 minutes because, the sun is at a very large distance from the earth. Energy flux received from the sun before entering the earth's atmosphere ,can be considered as constant.

        The rate at which solar energy arrives at the top of the atmosphere is called the solar constant Isc .This is the amount of energy received in unit time on a unit area perpendicular to the sun's direction at the mean distance of the earth from the sun. Since the sun's distance and activity vary throughout the year, the rate of arrival of solar radiation also varies. Thus, the so called solar constant is an average from which the actual values vary foto about ±3%. However, this variation is not very important for practical purposes

            Based on measurements made up to 1970, a standard value of 1353 W/m2 was adopted in 1971.However, based on subsequent measurements, a revised value of 1367W/m2 (1.940 cal/cm2 min or 432Btu/ft2 h or 4.921 MJ/m2 h) has been adopted by the World Radiation Center (WRC)