// Astronomical computations (sun direction)
// Last edited on 2005-08-28 14:47:50 by stolfi


#macro compute_sun_direction(latitude, season, hour)
  // Direction of sun in heliocentric system:
  #local d0 = vrotate(<1,0,0>,season*360*z);

  // Direction of sun in global geocentric system (Z = Earth axis)
  #local d1 = vrotate(d0, -23*x);

  // Direction of sun at noon in geocentric system of local meridian:
  #local d2 = vrotate(d1, -season*360*z);
  
  // Direction of sun at the given hour, same system:
  #local d3 = vrotate(d2, -360*(hour-12)/24*z);
  
  // Ditto, local geographic system:
  #local d4 = vrotate(d3, latitude*y);
  
  // Change axes (Z = up, Y = North, X = East):
  < d4.y, d4.z, d4.x >
#end