The sun is class G2-V yellow dwarf star of radius 6.95508 x 107 km and surface area of 6.087 x 1022 cm2. It emits radiation produced by the internal conversion of matter into radiation into the entire 4-pi steradian solid angle (sphere), with the sun at the center. The mean radiation intensity, or radiance of the solar surface is 2.009x107 watts per square meter per steradian (Wm-2 sr-1), or a total of 2.845x1026 watts. The Earth's orbit is elliptical with an eccentricity of 0.0167 (1.4710x109 km at perihelion, 1.5210x109 km at aphelion). At the mean Earth-Sun distance, the sun subtends a solid angle of 9.24 milliradians or 0.529°.1 Thus the sun is not truly a point source, and the rays from the sun are not truly parallel, but diverge into a cone with half angle of about 0.529°. At the mean distance of the Earth from the sun of 1.495979 x 109 km, the solar radiation reaching the top of the Earth's atmosphere is 1366.1 Wm-2 ± 7.0 Wm-2 or 1.959 calories cm-2 minute-1.2
The Earth's elliptical orbit causes the distance between the Earth and the Sun (the Earth's radius vector) to vary by 3.39% from perihelion (closest) to aphelion (farthest). These variations in distance cause the intensity of solar radiation at the top of the atmosphere to vary as 1/R2, where R is the radius vector. Thus the solar input at the top of the atmosphere varies from 1414 Wm-2 (in December) to 1321 Wm-2 (in July). Additional variations in solar intensity, or brightness, result from the solar sunspot cycle, and even solar oscillations. These slight variations in the solar output are usually accounted for in the calculation of solar energy available at the top of the atmosphere, or the total extraterrestrial solar radiation, referred to as ETR. The ETR has only been monitored from space since the early 1970's, or almost three solar sunspot cycles. Excellent histories of ETR measurements and analysis are provided in Frohlich3 and Gueymard.4
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Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.