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astro.c

/*
 * Sun clock - astronomical routines.
 */

#include "sunclock.h"

/*  JDATE  --  Convert internal GMT date and time to Julian day
             and fraction.  */

long
jdate(t)
struct tm *t;
{
      long c, m, y;

      y = t->tm_year + 1900;
      m = t->tm_mon + 1;
      if (m > 2)
         m = m - 3;
      else {
         m = m + 9;
         y--;
      }
      c = y / 100L;              /* Compute century */
      y -= 100L * c;
      return t->tm_mday + (c * 146097L) / 4 + (y * 1461L) / 4 +
          (m * 153L + 2) / 5 + 1721119L;
}

/* JTIME --    Convert internal GMT  date  and  time  to  astronomical
             Julian  time  (i.e.   Julian  date  plus  day fraction,
             expressed as a double).      */

double
jtime(t)
struct tm *t;
{
      return (jdate(t) - 0.5) + 
         (((long) t->tm_sec) +
           60L * (t->tm_min + 60L * t->tm_hour)) / 86400.0;
}

/*  KEPLER  --    Solve the equation of Kepler.  */

double
kepler(m, ecc)
double m, ecc;
{
      double e, delta;
#define EPSILON 1E-6

      e = m = dtr(m);
      do {
         delta = e - ecc * sin(e) - m;
         e -= delta / (1 - ecc * cos(e));
      } while (abs(delta) > EPSILON);
      return e;
}

/*  SUNPOS  --    Calculate position of the Sun.      JD is the Julian  date
            of  the  instant for which the position is desired and
            APPARENT should be nonzero if  the  apparent  position
            (corrected  for  nutation  and aberration) is desired.
                The Sun's co-ordinates are returned  in  RA  and  DEC,
            both  specified  in degrees (divide RA by 15 to obtain
            hours).  The radius vector to the Sun in  astronomical
                units  is returned in RV and the Sun's longitude (true
            or apparent, as desired) is  returned  as  degrees  in
            SLONG.      */

void
sunpos(jd, apparent, ra, dec, rv, slong)
double jd;
int apparent;
double *ra, *dec, *rv, *slong;
{
      double t, t2, t3, l, m, e, ea, v, theta, omega,
             eps;

      /* Time, in Julian centuries of 36525 ephemeris days,
         measured from the epoch 1900 January 0.5 ET. */

      t = (jd - 2415020.0) / 36525.0;
      t2 = t * t;
      t3 = t2 * t;

      /* Geometric mean longitude of the Sun, referred to the
         mean equinox of the date. */

      l = fixangle(279.69668 + 36000.76892 * t + 0.0003025 * t2);

        /* Sun's mean anomaly. */

      m = fixangle(358.47583 + 35999.04975*t - 0.000150*t2 - 0.0000033*t3);

        /* Eccentricity of the Earth's orbit. */

      e = 0.01675104 - 0.0000418 * t - 0.000000126 * t2;

      /* Eccentric anomaly. */

      ea = kepler(m, e);

      /* True anomaly */

      v = fixangle(2 * rtd(atan(sqrt((1 + e) / (1 - e))  * tan(ea / 2))));

        /* Sun's true longitude. */

      theta = l + v - m;

      /* Obliquity of the ecliptic. */

      eps = 23.452294 - 0.0130125 * t - 0.00000164 * t2 + 0.000000503 * t3;

        /* Corrections for Sun's apparent longitude, if desired. */

      if (apparent) {
         omega = fixangle(259.18 - 1934.142 * t);
         theta = theta - 0.00569 - 0.00479 * sin(dtr(omega));
         eps += 0.00256 * cos(dtr(omega));
      }

        /* Return Sun's longitude and radius vector */

      *slong = theta;
      *rv = (1.0000002 * (1 - e * e)) / (1 + e * cos(dtr(v)));

      /* Determine solar co-ordinates. */

      *ra =
      fixangle(rtd(atan2(cos(dtr(eps)) * sin(dtr(theta)), cos(dtr(theta)))));
      *dec = rtd(asin(sin(dtr(eps)) * sin(dtr(theta))));
}

/*  GMST  --  Calculate Greenwich Mean Siderial Time for a given
            instant expressed as a Julian date and fraction.      */

double
gmst(jd)
double jd;
{
      double t, theta0;


      /* Time, in Julian centuries of 36525 ephemeris days,
         measured from the epoch 1900 January 0.5 ET. */

      t = ((floor(jd + 0.5) - 0.5) - 2415020.0) / 36525.0;

      theta0 = 6.6460656 + 2400.051262 * t + 0.00002581 * t * t;

      t = (jd + 0.5) - (floor(jd + 0.5));

      theta0 += (t * 24.0) * 1.002737908;

      theta0 = (theta0 - 24.0 * (floor(theta0 / 24.0)));

      return theta0;
}

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