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I have some difficulties understanding a formula for calculating the incidence angle for incoming solar radiation on a sloping surface. I found that:

angle of incidence calculated as cos(j) = sin((pi/2)-surface slope)*sin(altitude angle) + cos((pi/2)-surface slope)*cos(altitude angle)*(cos(surface aspect - azimuth angle))

However, in my text book it says "The angle between the sun’s projection on the horizontal plane and the plane of the gridcell’s slope is calculated as: azimuth angle * tan(surface slope + cos(azimuth angle - surface aspect))"

Can anyone explain me what the difference between these two formulas is and how they are related?

Thanks in advance!

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# I Calculating the incidence angle of the Sun

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