Question

Give the definition of solar constant and its value.

Answer 1

Hint: From the very name we know that the given constant is related to the sun. Find out what characteristic of the sun is defined by this constant. Also, give a brief description of how the measurement of solar constant is made. Also, find the approximate value of it. Also, find out all the related factors that affect the given constant.

Complete step-by-step solution
The solar constant which is represented by ${{G}_{SC}}$ is defined as the total radiation energy that is received from the sun per unit time per unit area on a theoretical surface perpendicular to the Sun’s rays and at Earth’s mean distance from the sun, or in other words, the solar constant is the flux density that measures mean solar electromagnetic radiation or solar irradiance per unit area. The surface that is perpendicular to the rays at which the measurement is made is one astronomical unit (AU) from the sun. The solar constant also includes all types of solar radiation along with the visible light. The measurement of this constant is found to be approximately 1.366kilowatts per square meter. That is,
${{G}_{SC}}=1.366kw/{{m}^{2}}$
However, the solar constant is not an absolute constant like the Planck constant or the speed of light. It is more like an average of varying value. It shows an increase of 0.2 percent at the peak of every 11-year solar cycle. So, over the course of the past 400 years it has only shown a variation that is less than .2 percent.

Note: The atmospheric conditions have great significance in the measurement of this constant. About 75% of solar energy reaches the earth’s surface after being reflected and absorbed by the atmosphere on a clear sky. But even light cirrus clouds have the capability to reduce it to 50% and strong cirrus clouds to 40%.