void Update_SUN()
{
KPP_REAL SunRise, SunSet;
KPP_REAL Thour, Tlocal, Ttmp; 
const KPP_REAL PI = 3.14159265358979;  

  SunRise = 4.5;
  SunSet  = 19.5;
  Thour = TIME/3600.0;
  Tlocal = Thour - ((int)Thour/24)*24;

  if ( (Tlocal >= SunRise) && (Tlocal <= SunSet) ) {
    Ttmp = (2.0*Tlocal-SunRise-SunSet)/(SunSet-SunRise);
    if (Ttmp > 0) Ttmp =  Ttmp*Ttmp;
             else Ttmp = -Ttmp*Ttmp;
    SUN = ( 1.0 + cos(PI*Ttmp) )/2.0; 
  } else {
    SUN=0.0;
  }
}