Solar Constant

solar constant, 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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solar constant [′sō·lər ′kän·stənt]

(meteorology)

The rate at which energy from the sun is received just outside the earth's atmosphere on a surface normal to the incident radiation and at the earth's mean distance from the sun; it is approximately 1367 watts per square meter.

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solar constant

The average rate at which radiant energy is received by the earth from the sun; equal to 430 Btu per hr per sq ft (1.94 cal per min per sq cm); a constant employed in calculating air-cooling loads due to the effects of solar radiation on buildings.

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Solar Constant 

the amount of radiant energy received from the sun in 1 min by a surface that is perpendicular to the sun’s rays, is 1 cm² in area, and is located outside the earth’s atmosphere at the earth’s mean distance from the sun. Knowledge of the exact value of the solar constant is very important for the study of heat-exchange processes in the earth’s atmosphere and for the investigation of processes occurring in the sun.

The first attempt to determine the solar constant was made by the French scientist C. S. M. Pouillet in 1837. An important contribution to early investigations of the solar constant was made by the Russian scientists R. N. Savel’ev and A. P. Ganskii. Up to the mid-20th century, the solar constant was determined from measurements of solar radiation at the earth’s surface for different altitudes of the sun. This method permits the absorption and scattering of sunlight by the earth’s atmosphere to be taken into account. The first direct determinations of the solar constant were made in the 1960’s, when it became technologically feasible to lift instruments outside the earth’s atmosphere by means of rockets and artificial earth satellites.

On the basis of analysis of a large number of research projects carried out in the USSR, the USA, and other countries, the value of 1.95 calories/cm²-min, or 136 milliwatts/cm², has been derived for the solar constant. The accuracy of this value is approximately 1 percent. The solar constant apparently varies slightly with time. Many years of painstaking measurements, however, are necessary before it can be determined how these variations occur.

REFERENCES

Kondrat’ev, K. Ia. Aktinometriia. Leningrad, 1965.
Makarova, E. A., and A. V. Kharitonov. Raspredelenie energii v speklre Solntsa i solnechnaia postoiannaia. Moscow, 1972.

M. DZH. GUSEINOV