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. eter. 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 total radiated power that is received in a certain point 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 total radiated power received by the Earth 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 total radiated power received by the Earth from the sun is roughly 174 PW (1. 74 X 1017 W) . The total 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. 6 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. To conclude, the solar constant contains the visible spectrum of light and all other wavelengths of solar electromagnetic radiation. It is commonly associated to the apparent magnitude of the Sun, which is -26. 8. The magnitude of the sun and solar constant are two means of expressing the apparent brightness of the sun, even though the numbers only quantifies the visual output of the sun.