The amount of radiant energy has been found to be constant. There is little variation is the quantum of solar energy received at the outer margin of our atmosphere, a fact verified by observations made at various places on different occasions.
The Smithsonian Institute, U.S.A., made observations for several years from the observatories of Mount Wilson, California and the Table Mountain and came to the conclusion that the standard value of solar constant is 1.94 gram calories per square centimeter per minute.
Solar constant is defined as the rate at which solar radiation is received outside the earth’s atmosphere on a surface perpendicular to the sun’s rays when the earth is at an average distance from the sun.
Since there is a fluctuation in the amount of radiant energy emitted by sun due to periodic disturbances on the solar surface, the amount of solar constant, therefore, also registers a slight increase or decrease.
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However, on an average this variation hardly exceeds 2 to 3 per cent and, therefore, hardly matters so far as the receipt of solar energy to the earth or the atmosphere is concerned.