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I Calculating the incidence angle of the Sun

  1. Apr 22, 2018 #1
    Hi all,

    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!
  2. jcsd
  3. Apr 23, 2018 #2


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    Without looking very deeply into those formulae, it looks to me that they are calculating different things. The angle of incidence relates to the normal and the incident ray but the second formula gives the angle between the Sun's projection on the ground and a "plane". The only unique angle relative to a plane has to be relative to the normal. Your text book may have a diagram that it's referring to.
    Last edited: Apr 23, 2018
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