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/*
* Sun clock - astronomical routines.
*/
#include "sunclock.h"
long jdate(struct tm *);
double jtime(struct tm *);
double kepler(double m, double ecc);
void sunpos(double jd, int apparent, double *ra, double *dec, double *rv, double *slong);
double gmst(double jd);
/* 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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