Fig. 3.22 Useful angles
The zenith angle, 0z, is the angle between the vertical and the line connecting to the sun (the angle of incidence of beam radiation on a horizontal surface). Likewise, the angle between the horizontal and the line to the sun is the solar altitude angle, (the complement angle of the zenith angle), hence a a + 0z = 90°.
The angle between the earth-sun line and the equatorial plane is called the declination angle, 5, which changes with the date and it is independent of the location (see Fig. 3.22). The declination is maximum 23°45' on the summer/winter solstice and 0° on the equinoxes (Fig. 3.24).
The following accurate expression is considered for declination angle, 5, in radians and the eccentricity correction factor of the earth's orbit as defined above
5 = 0.006918 - 0.399912cosT + 0.07257sinT - 0.006758cos2T
+ 0.000907sinT -0.002697cos3T + 0.00148sin3T . (3.31)
It is also possible to consider the following expressions for the approximate calculations of 5 and E0 (Iqbal 1986) as
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