Because the earth moves around the sun in an elliptical orbit, the earth-sun distance, R, varies throughout the year, by about 3.4% from its minimum value on about
January 3 to its maximum value on about July 3. Thus, the variation in R and therefore, in the extraterrestrial solar irradiance, is about 6.9%, which implies that the radiation incident at the top of the atmosphere is almost 7% larger in January (Southern hemisphere) than in July (Northern hemisphere).
The illumination of the earth's surface depends on the solar zenith angle, d0, which is the angle between the local zenith and the direction of the center of the solar disk. The solar zenith angle depends on the time of the day, the time of the year, and geographic location.
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