source: Sophya/trunk/SophyaExt/XephemAstroLib/utc_gst.c@ 1895

#include "P_.h"
#include "astro.h"

static double gmst0 P_((double mjd));

/* given a modified julian date, mjd, and a universally coordinated time, utc,
 * return greenwich mean siderial time, *gst.
 * N.B. mjd must be at the beginning of the day.
 */
void
utc_gst (mjd, utc, gst)
double mjd;
double utc;
double *gst;
{
        static double lastmjd = -18981;
        static double t0;

        if (mjd != lastmjd) {
            t0 = gmst0(mjd);
            lastmjd = mjd;
        }
        *gst = (1.0/SIDRATE)*utc + t0;
        range (gst, 24.0);
}

/* given a modified julian date, mjd, and a greenwich mean siderial time, gst,
 * return universally coordinated time, *utc.
 * N.B. mjd must be at the beginning of the day.
 */
void
gst_utc (mjd, gst, utc)
double mjd;
double gst;
double *utc;
{
        static double lastmjd = -10000;
        static double t0;

        if (mjd != lastmjd) {
            t0 = gmst0 (mjd);
            lastmjd = mjd;
        }
        *utc = gst - t0;
        range (utc, 24.0);
        *utc *= SIDRATE;
}

/* gmst0() - return Greenwich Mean Sidereal Time at 0h UT; stern
 */
static double
gmst0 (mjd)
double mjd;     /* date at 0h UT in julian days since MJD0 */
{
        double T, x;

        T = ((int)(mjd - 0.5) + 0.5 - J2000)/36525.0;
        x = 24110.54841 +
                (8640184.812866 + (0.093104 - 6.2e-6 * T) * T) * T;
        x /= 3600.0;
        range(&x, 24.0);
        return (x);
}

#ifdef TEST_GMST

/* original routine by elwood; has a secular drift of 0.08s/cty */
static double
tnaught (mjd)
double mjd;     /* julian days since 1900 jan 0.5 */
{
        double dmjd;
        int m, y;
        double d;
        double t, t0;

        mjd_cal (mjd, &m, &d, &y);
        cal_mjd (1, 0., y, &dmjd);
        t = dmjd/36525;
        t0 = 6.57098e-2 * (mjd - dmjd) - 
             (24 - (6.6460656 + (5.1262e-2 + (t * 2.581e-5))*t) -
                   (2400 * (t - (((double)y - 1900)/100))));
        range(&t0, 24.0);
        return (t0);
}

#include <stdlib.h>
main(argc, argv)
  int argc;
  char *argv[];
{
        double mjd, gst;
        while (scanf("%lf", &mjd) == 1) {
                mjd -= MJD0;
                gst = tnaught(mjd);
                printf("%17.9f %10.7f %10.7f\n", mjd + MJD0, gst, gmst0(mjd));
        }
}
#endif

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: utc_gst.c,v $ $Date: 2001-10-22 12:08:28 $ $Revision: 1.2 $ $Name: not supported by cvs2svn $"};