ABSTRACT
The sun is the nearest star to the earth, and its radiant energy is practically the only energy source to the earth. Very small and insignificant quantities of energy are available from other sources such as the interior of the earth, the moon, and other stars. The mean sun-earth distance, also known as one astronomical unit (1 AU), is 1.496 x 108 km or, more accurately, 149, 597, 890 ± 500 km. The earth revolves round the sun in an elliptical orbit. The minimum sun-earth distance is about 0.983 AU and the maximum approxi mately 1.017 AU. The earth is at its closest point to the sun (perihelion) on approximately January 3 and at its farthest point (aphelion) on approxi mately July 4. The visible disk or photosphere has a radius of 6.599 x 105 km, and the solar mass is 1.989 x 1030 kg (Goody and Yung, 1989; Iqbal, 1983).
