Solar Constant

We all know that the energy coming from the sun is in the form of radiation . This radiation is released by the sun fro a nuclear reaction that produces electromagnetic energy . The so called Solar Constant pertains to the quantity of electromagnetic radiation per unit area coming from the sun . It is usually measured in a plane orthogonal to the sun 's rays , on the outer layer of the Earth 's atmosphere . The solar constant consists of all types of solar radiation in its spectrum . A satellite measured it to be approximately 1366

Watts per sq . meter . But this value fluctuates from 1321 to 1412 W /m2 due to the fact that the distance of the Earth from the sun varies through out the year , because of the elliptical path or orbit of the revolution of the Earth around the sun . To be more technical , any electromagnetic radiation suffers from the consequence of the Friis ' free space propagation path loss . It states that the is inversely proportional to the distance from the source of the receiving point . This is the main reason why the value of the solar constant actually varies with the relative distance of the Earth from the sun . To get the value of from the sun , the cross sectional area of the Earth must be multiplied to the solar constant . Knowing that the Earth 's cross sectional area is approximately 127 ,400 ,000 sq . km , the the Earth from the sun is roughly 174 PW (1 .74 X 1017 W . The amount of radiation received by the Earth maybe determined by the Earth 's cross sectional area (approx ? r2 , but since the Earth rotates on its axis , the solar energy is distributed across the entire surface area of the Earth (approx . approx ? r2 . Therefore , the average solar radiation absorbed on the surface of the Earth is only ? of the solar constant or approximately 342 W /m2 . The amount of solar radiation received at the surface is dependent on the exact coordinate (latitude and longitude ) and the atmospheric condition . The solar constant value of 1366 W /m2 is equal to 1 .96 Cal per minute per sq . centimeter that is also commonly expressed as 1 .96 Ly (Langleys , named after Samuel Pierpont Langley

Langley tried to approximate the value of the solar constant from Mt Whitney in California in 1884 . He also tried to eliminate the effect of atmospheric absorption by obtaining several reading at various times of the day . But disappointingly , he attained an erroneous value of 2903 W /m2 , probably because of mathematical errors . Charles Greenley Abbot and other scientists did some measurements of the solar constant during the period from 1902 and 1957 , acquiring measurements from different high altitude locations . The values they obtained were between 1322 and 1465 W /m2 , proving that the corrections made by Langley to his computations were misapplied . He also found out that the variation in the values of the solar constant was solar in nature and not terrestrial

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