Initial revision.

diff --git a/Time/sunpos.c b/Time/sunpos.c
new file mode 100644 (file)
index 0000000..c022b4b
--- /dev/null
+++ b/Time/sunpos.c
@@ -0,0 +1,260 @@
+/*
+ * sunpos.c
+ * kirk johnson
+ * july 1993
+ *
+ * code for calculating the position on the earth's surface for which
+ * the sun is directly overhead (adapted from _practical astronomy
+ * with your calculator, third edition_, peter duffett-smith,
+ * cambridge university press, 1988.)
+ *
+ * RCS $Id$
+ *
+ * Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
+ *
+ * Parts of the source code (as marked) are:
+ *   Copyright (C) 1989, 1990, 1991 by Jim Frost
+ *   Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
+ *
+ * Permission to use, copy, modify and freely distribute xearth for
+ * non-commercial and not-for-profit purposes is hereby granted
+ * without fee, provided that both the above copyright notice and this
+ * permission notice appear in all copies and in supporting
+ * documentation.
+ *
+ * The author makes no representations about the suitability of this
+ * software for any purpose. It is provided "as is" without express or
+ * implied warranty.
+ *
+ * THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
+ * OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+ * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
+ * NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * $Id$
+ * (Log is kept at end of this file)
+ */
+
+
+#include <math.h>
+#include <stdio.h>
+#include <sys/time.h>
+
+#include "sunpos.h"
+#include "../constants.h"
+#undef E
+
+
+/*
+ * the epoch upon which these astronomical calculations are based is
+ * 1990 january 0.0, 631065600 seconds since the beginning of the
+ * "unix epoch" (00:00:00 GMT, Jan. 1, 1970)
+ *
+ * given a number of seconds since the start of the unix epoch,
+ * DaysSinceEpoch() computes the number of days since the start of the
+ * astronomical epoch (1990 january 0.0)
+ */
+
+#define EpochStart           (631065600)
+#define DaysSinceEpoch(secs) (((secs)-EpochStart)*(1.0/(24*3600)))
+
+/*
+ * assuming the apparent orbit of the sun about the earth is circular,
+ * the rate at which the orbit progresses is given by RadsPerDay --
+ * FG_2PI radians per orbit divided by 365.242191 days per year:
+ */
+
+#define RadsPerDay (FG_2PI/365.242191)
+
+/*
+ * details of sun's apparent orbit at epoch 1990.0 (after
+ * duffett-smith, table 6, section 46)
+ *
+ * Epsilon_g    (ecliptic longitude at epoch 1990.0) 279.403303 degrees
+ * OmegaBar_g   (ecliptic longitude of perigee)      282.768422 degrees
+ * Eccentricity (eccentricity of orbit)                0.016713
+ */
+
+#define Epsilon_g    (279.403303*(FG_2PI/360))
+#define OmegaBar_g   (282.768422*(FG_2PI/360))
+#define Eccentricity (0.016713)
+
+/*
+ * MeanObliquity gives the mean obliquity of the earth's axis at epoch
+ * 1990.0 (computed as 23.440592 degrees according to the method given
+ * in duffett-smith, section 27)
+ */
+#define MeanObliquity (23.440592*(FG_2PI/360))
+
+static double solve_keplers_equation(double);
+static double sun_ecliptic_longitude(time_t);
+static void   ecliptic_to_equatorial(double, double, double *, double *);
+static double julian_date(int, int, int);
+static double GST(time_t);
+
+/*
+ * solve Kepler's equation via Newton's method
+ * (after duffett-smith, section 47)
+ */
+static double solve_keplers_equation(double M) {
+    double E;
+    double delta;
+
+    E = M;
+    while (1) {
+       delta = E - Eccentricity*sin(E) - M;
+       if (fabs(delta) <= 1e-10) break;
+       E -= delta / (1 - Eccentricity*cos(E));
+    }
+
+    return E;
+}
+
+
+/* compute ecliptic longitude of sun (in radians) (after
+ * duffett-smith, section 47) */
+
+static double sun_ecliptic_longitude(time_t ssue) {
+    /* time_t ssue;              seconds since unix epoch */
+    double D, N;
+    double M_sun, E;
+    double v;
+
+    D = DaysSinceEpoch(ssue);
+
+    N = RadsPerDay * D;
+    N = fmod(N, FG_2PI);
+    if (N < 0) N += FG_2PI;
+
+    M_sun = N + Epsilon_g - OmegaBar_g;
+    if (M_sun < 0) M_sun += FG_2PI;
+
+    E = solve_keplers_equation(M_sun);
+    v = 2 * atan(sqrt((1+Eccentricity)/(1-Eccentricity)) * tan(E/2));
+
+    return (v + OmegaBar_g);
+}
+
+
+/* convert from ecliptic to equatorial coordinates (after
+ * duffett-smith, section 27) */
+
+static void ecliptic_to_equatorial(double lambda, double beta, 
+                                  double *alpha, double *delta) {
+    /* double  lambda;            ecliptic longitude       */
+    /* double  beta;              ecliptic latitude        */
+    /* double *alpha;             (return) right ascension */
+    /* double *delta;             (return) declination     */
+
+    double sin_e, cos_e;
+
+    sin_e = sin(MeanObliquity);
+    cos_e = cos(MeanObliquity);
+
+    *alpha = atan2(sin(lambda)*cos_e - tan(beta)*sin_e, cos(lambda));
+    *delta = asin(sin(beta)*cos_e + cos(beta)*sin_e*sin(lambda));
+}
+
+
+/* computing julian dates (assuming gregorian calendar, thus this is
+ * only valid for dates of 1582 oct 15 or later) (after duffett-smith,
+ * section 4) */
+
+static double julian_date(int y, int m, int d) {
+    /* int y;                    year (e.g. 19xx)          */
+    /* int m;                    month (jan=1, feb=2, ...) */
+    /* int d;                    day of month              */
+
+    int    A, B, C, D;
+    double JD;
+
+    /* lazy test to ensure gregorian calendar */
+    if (y < 1583) {
+       printf("WHOOPS! Julian dates only valid for 1582 oct 15 or later\n");
+    }
+
+    if ((m == 1) || (m == 2)) {
+       y -= 1;
+       m += 12;
+    }
+
+    A = y / 100;
+    B = 2 - A + (A / 4);
+    C = 365.25 * y;
+    D = 30.6001 * (m + 1);
+
+    JD = B + C + D + d + 1720994.5;
+
+    return JD;
+}
+
+
+/* compute greenwich mean sidereal time (GST) corresponding to a given
+ * number of seconds since the unix epoch (after duffett-smith,
+ * section 12) */
+static double GST(time_t ssue) {
+    /* time_t ssue;           seconds since unix epoch */
+
+    double     JD;
+    double     T, T0;
+    double     UT;
+    struct tm *tm;
+
+    tm = gmtime(&ssue);
+
+    JD = julian_date(tm->tm_year+1900, tm->tm_mon+1, tm->tm_mday);
+    T  = (JD - 2451545) / 36525;
+
+    T0 = ((T + 2.5862e-5) * T + 2400.051336) * T + 6.697374558;
+
+    T0 = fmod(T0, 24.0);
+    if (T0 < 0) T0 += 24;
+
+    UT = tm->tm_hour + (tm->tm_min + tm->tm_sec / 60.0) / 60.0;
+
+    T0 += UT * 1.002737909;
+    T0 = fmod(T0, 24.0);
+    if (T0 < 0) T0 += 24;
+
+    return T0;
+}
+
+
+/* given a particular time (expressed in seconds since the unix
+ * epoch), compute position on the earth (lat, lon) such that sun is
+ * directly overhead.  (lat, lon are reported in radians */
+
+void fgSunPosition(time_t ssue, double *lon, double *lat) {
+    /* time_t  ssue;           seconds since unix epoch */
+    /* double *lat;            (return) latitude        */
+    /* double *lon;            (return) longitude       */
+
+    double lambda;
+    double alpha, delta;
+    double tmp;
+
+    lambda = sun_ecliptic_longitude(ssue);
+    ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta);
+
+    tmp = alpha - (FG_2PI/24)*GST(ssue);
+    if (tmp < -FG_PI) {
+       do tmp += FG_2PI;
+       while (tmp < -FG_PI);
+    } else if (tmp > FG_PI) {
+       do tmp -= FG_2PI;
+       while (tmp < -FG_PI);
+    }
+
+    *lon = tmp;
+    *lat = delta;
+}
+
+
+/* $Log$
+/* Revision 1.1  1997/08/01 15:27:56  curt
+/* Initial revision.
+/*
+ */

diff --git a/Time/sunpos.h b/Time/sunpos.h
new file mode 100644 (file)
index 0000000..d153042
--- /dev/null
+++ b/Time/sunpos.h
@@ -0,0 +1,53 @@
+/*
+ * sunpos.h
+ * kirk johnson
+ * july 1993
+ *
+ * code for calculating the position on the earth's surface for which
+ * the sun is directly overhead (adapted from _practical astronomy
+ * with your calculator, third edition_, peter duffett-smith,
+ * cambridge university press, 1988.)
+ *
+ * RCS $Id$
+ *
+ * Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
+ *
+ * Parts of the source code (as marked) are:
+ *   Copyright (C) 1989, 1990, 1991 by Jim Frost
+ *   Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
+ *
+ * Permission to use, copy, modify and freely distribute xearth for
+ * non-commercial and not-for-profit purposes is hereby granted
+ * without fee, provided that both the above copyright notice and this
+ * permission notice appear in all copies and in supporting
+ * documentation.
+ *
+ * The author makes no representations about the suitability of this
+ * software for any purpose. It is provided "as is" without express or
+ * implied warranty.
+ *
+ * THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
+ * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
+ * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
+ * OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+ * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
+ * NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+
+#ifndef SUNPOS_H
+#define SUNPOS_H
+
+
+#include <sys/time.h>
+
+
+/* given a particular time (expressed in seconds since the unix
+ * epoch), compute position on the earth (lat, lon) such that sun is
+ * directly overhead.  (lat, lon are reported in radians */
+
+void fgSunPosition(time_t ssue, double *lon, double *lat);
+
+
+#endif /* SUNPOS_H */