The Physics
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Power Density of Solar Radiation

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Bibliographic Entry Result
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Tipler, Paul A. College Physics. New York: Worth, 1987: 316. "The average energy flux at this distance on a surface perpendicular to the sun's rays is about 1353 W/m2 1.353 kW/m2
"The Sun." Encyclopedia Britannica. vol. 27. Chicago: Encyclopedia Britannica, 1998: 456. "… it's radiative output, called the solar constant, is 137 ergs/m2/sec, or 1.98 cal/cm2/min" 1.38 kW/m2
Cowen, R. Science News. 152 (27 September 1997): 197. "… the sun's output had climbed from 1367.0 to 1367.5 watts per square meter" 1.367–1.3675 kW/m2
Brooks, William O. and George R. Tracy. Modern Physical Science.New York: Holt, 1957: 566. "We get energy from the sun at the rate of five million horsepower per square mile." 1.44 kW/m2
Rosner, Robert. MacMillan Encyclopedia of Physics.vol. 4. New York: Simon & Schuster, 1996: 1545. "The most obvious solar effect on the earth is radiation, roughly 1.4 kW/m2 (the so called solar constant)" 1.4 kW/m2

The sun is the source of heat and energy for the earth. The solar output on the earth is called the power density. The power density of the sun's radiation on the surface of the earth is approximately 1.4 kW/m2. This value varies slightly throughout the year but by no more than 0.1 percent. One reason for this variation is the changing earth-sun distance. This distance varies by about six percent throughout the year, causing the power density to range from about 1.308 kW/m2 to 1.398 kW/m2. The power density also varies with the 11-year cycle of sunspots. In the 1980s, scientists discovered that the total amount of solar radiation ebbs or rises in synch with the increase or decrease of sunspots during this cycle. During the peak of the cycle, hundreds of dark spots cover the surface of the sun with bright regions giving off extra radiation. During the minimum, the sunspots disappear, causing the sun's energy to decrease by about 0.1 percent. Furthermore, the energy the sun gives off, and hence the power density on the earth, will keep on changing with time because, as the sun evolves, its total radiation output varies. The power density 4.5 billion years ago would be smaller than that today since the sun's brightness has increased by roughly 30 percent.

Today, by measuring the power density, there is evidence that suggests that the sunlight hitting the earth is slightly brighter than that ten years ago. It has been reported that, between 1986 and 1996, the intensity of solar radiation increased by 0.036 percent. If such speculation is true, it raises the possibility that the global warming experienced over the past decade can be attributed to the variances in solar output.

Manica Piputbundit -- 1998

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