Glenn Elert
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The Physics Factbook™
Edited by Glenn Elert -- Written by his students
An educational, Fair Use website
topic index | author index | special index
| Bibliographic Entry | Result (w/surrounding text) | Standardized Result |
|---|---|---|
| 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 onthe earth is called the power density. The power density of the sun's radiationon the surface of the earth is approximately 1.4 kW/m2. This valuevaries slightly throughout the year but by no more than 0.1 percent. One reasonfor this variation is the changing earth-sun distance. This distance varies byabout six percent throughout the year, causing the power density to range fromabout 1.308 kW/m2 to 1.398 kW/m2. The power densityalso varies with the 11-year cycle of sunspots. In the 1980s, scientists discoveredthat the total amount of solar radiation ebbs or rises in synch with the increaseor decrease of sunspots during this cycle. During the peak of the cycle, hundredsof dark spots cover the surface of the sun with bright regions giving off extraradiation. During the minimum, the sunspots disappear, causing the sun's energyto decrease by about 0.1 percent. Furthermore, the energy the sun gives off, andhence 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 billionyears ago would be smaller than that today since the sun's brightness has increasedby roughly 30 percent.
Today, by measuring the power density, there is evidence that suggests thatthe sunlight hitting the earth is slightly brighter than that ten years ago. Ithas been reported that, between 1986 and 1996, the intensity of solar radiationincreased by 0.036 percent. If such speculation is true, it raises the possibilitythat the global warming experienced over the past decade can be attributed tothe variances in solar output.
Manica Piputbundit -- 1998
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