Solar radiation

At present the Sun radiates energy at the rate of 3.9 x 1026 W. At the top of the Earth's atmosphere an average power of 1353 W m-2 is passing through a plane perpendicular to the direction of the Sun. As shown in Fig. 2.1, regular oscillations around this figure are produced by the changes in the Earth-Sun distance, as the Earth progresses in its elliptical orbit around the Sun. The average distance is 1.5 x 1011 m and the variation is ±1.7%. Further variation in the amount of solar radiation received at the top of the atmosphere is caused by slight irregularities in the solar surface, in combination with the

Sun's rotation (about one revolution per month), and by possible timevaria-tions in the surface luminosity of the Sun.

Figure 2.1. Yearly variations in the "solar constant" (the head-on solar radiation flux at the Earth's distance; NASA, 1971).
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