Solar Constant

 

The sun is a large sphere of very hot gases, the heat being generated by various kinds of fusion reactions. Although the sun is large, it subtends an angle of only 32 minutes at the earth’s surface. This is because it is also a very large distance. Thus the beam radiation received from the sun on the earth is almost parallel. The brightness of the sun varies from its center to its edge. However for engineering calculations, it is customary to assume that the brightness all over the solar disc in uniform. As viewed from the earth, the radiation coming from the sun appears to be essentially equivalent to that coming from a back surface at 5762 ^O k.

“The rate at which solar energy arrives at the top of the atmosphere is called solar constant”. 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. Because of the sun’s distance and activity vary throughout the year, the rate of arrival of solar constant is thus an average from which the actual values vary up to 3 percent in either direction.

The National Aeronautics and Space Administration’s (NASA) standard value the solar constant, expressed in three common units, is as follows:

(i) 1.353 kilowatts per square meter

(ii) 116.5 Langleys per hour (1 langely being equal to 1cal/cm² of solar radiation received in one day)

(iii) 429.2 Btu per Sqr.ft. per hour.

The distance between the earth and the sun varies a little through the year. Because of this variation, the extra – terrestrial flux also varies. The earth is closest to the sun in the summer and farthest away in the winter.