the average amount of radiant energy received by the earth's atmosphere from the sun; its value is about 2 calories per min incident on each square centimeter of the upper atmosphere. The actual value of the energy varies with several factors; the most important factor is the earth's distance from the sun, which changes because of the earth's elliptical orbit. For computing the value of the solar constant, the astronomical unit
, or average earth-sun distance, is used.
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