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If the diameter of the sun dS is assumed up to the photosphere (approximately 1.39 109 m) and a mean distance between the sun and the earth (LES) of approximately 1.5 1011 m is taken into consideration, a radiant flux density of approximately 1,370 W/m2 can be calculated at the top rim of the earth atmosphere (see /2-3/). This mean value is called the solar constant. Over several years it varies less than 0.1 % due to a fluctuation in solar activity.

The solar radiation incident on the atmospheric rim throughout the course of the year is nevertheless characterised by seasonal variations. They are caused by the elliptical orbit, where the earth moves around the sun during the course of one year (Fig. 2.2). This changes the distance between the two celestial bodies. And this distance variation leads to a fluctuation in the radiation incident on the atmospheric rim; this results in the course of the solar constants shown in Fig. 2.3.

March 21 /

C ^ Ecliptic

June 21 / December 21

March 21 /

! September 23

Fig. 2.2 Elliptical orbit of the earth around the sun (see /2-2/)

Thus, the solar constant reaches its maximum in January at almost 1,420 W/m2, due to reaching the shortest distance between the sun and the earth (Perihelion) on January, 2nd. The opposite takes place on June, 2nd, when it reaches its minimum with approximately 1,330 W/m2 (Aphelion).

1,420

I2 1,400

1,380

8 1,360

1,340

1,320

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