NASA Ocean Color

ocssw V2022

 #include "sun2000.h"
 #include "nav.h"
 extern double dpsi, eps;
 extern int nutime;
 void gha2000(int iyr, double day, double* gha) {
     const static int imon = 1;
     //  Compute days since J2000
     int iday = (int)day;
     double fday = day - iday;
     int jday = jd(iyr, imon, iday);
     double t = jday - 2451545.5e0 + fday;
     //  Compute Greenwich Mean Sidereal Time (degrees)
     double gmst = 100.4606184e0 + 0.9856473663e0 * t + 2.908e-13 * t * t;
     // Check if need to compute nutation correction for this day
     int nt = (int)t;
     if (nt != nutime) {
         nutime = nt;
         double xls, gs, xlm, omega;
         ephparms(t, &xls, &gs, &xlm, &omega);
         nutate(t, xls, gs, xlm, omega, &dpsi, &eps);
     }
     //  Include apparent time correction and time-of-day
     *gha = gmst + dpsi * cos(eps / radeg) + fday * 360.e0;
     *gha = dmod(*gha, 360.e0);
     if (*gha < 0.e0)
         *gha = *gha + 360.e0;
 }