source: Sophya/trunk/SophyaExt/XephemAstroLib/utc_gst.c@ 2974

#include "astro.h"

static double gmst0 (double mj);

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

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

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

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

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

        T = ((int)(mj - 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 (mj)
double mj;      /* julian days since 1900 jan 0.5 */
{
        double dmj;
        int m, y;
        double d;
        double t, t0;

        mjd_cal (mj, &m, &d, &y);
        cal_mjd (1, 0., y, &dmj);
        t = dmj/36525;
        t0 = 6.57098e-2 * (mj - dmj) - 
             (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 mj, gst;
        while (scanf("%lf", &mj) == 1) {
                mj -= MJD0;
                gst = tnaught(mj);
                printf("%17.9f %10.7f %10.7f\n", mj + MJD0, gst, gmst0(mj));
        }
}
#endif

/* For RCS Only -- Do Not Edit */
static char *rcsid[2] = {(char *)rcsid, "@(#) $RCSfile: utc_gst.c,v $ $Date: 2005-08-21 10:02:40 $ $Revision: 1.5 $ $Name: not supported by cvs2svn $"};