wmglobe/src/sunpos.c

   1 /*     WMGlobe 0.5  -  All the Earth on a WMaker Icon
   2  *     copyright (C) 1998,99 Jerome Dumonteil <jerome.dumonteil@capway.com>
   3  * sunpos.c is taken from Xearth :
   4  */
   5 /*
   6  * sunpos.c
   7  * kirk johnson
   8  * july 1993
   9  *
  10  * code for calculating the position on the earth's surface for which
  11  * the sun is directly overhead (adapted from _practical astronomy
  12  * with your calculator, third edition_, peter duffett-smith,
  13  * cambridge university press, 1988.)
  14  *
  15  *
  16  * Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
  17  *
  18  * Parts of the source code (as marked) are:
  19  *   Copyright (C) 1989, 1990, 1991 by Jim Frost
  20  *   Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
  21  *
  22  * Permission to use, copy, modify and freely distribute xearth for
  23  * non-commercial and not-for-profit purposes is hereby granted
  24  * without fee, provided that both the above copyright notice and this
  25  * permission notice appear in all copies and in supporting
  26  * documentation.
  27  *
  28  * Unisys Corporation holds worldwide patent rights on the Lempel Zev
  29  * Welch (LZW) compression technique employed in the CompuServe GIF
  30  * image file format as well as in other formats. Unisys has made it
  31  * clear, however, that it does not require licensing or fees to be
  32  * paid for freely distributed, non-commercial applications (such as
  33  * xearth) that employ LZW/GIF technology. Those wishing further
  34  * information about licensing the LZW patent should contact Unisys
  35  * directly at (lzw_info@unisys.com) or by writing to
  36  *
  37  *   Unisys Corporation
  38  *   Welch Licensing Department
  39  *   M/S-C1SW19
  40  *   P.O. Box 500
  41  *   Blue Bell, PA 19424
  42  *
  43  * The author makes no representations about the suitability of this
  44  * software for any purpose. It is provided "as is" without express or
  45  * implied warranty.
  46  *
  47  * THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
  48  * INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
  49  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
  50  * OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
  51  * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
  52  * NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
  53  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  54  */
  55
  56 /*************************************************************************/
  57
  58 #include <math.h>
  59 #include <time.h>
  60
  61 #ifndef PI
  62 #define PI 3.141592653
  63 #endif
  64 #define TWOPI (2*PI)
  65
  66 /*
  67  * the epoch upon which these astronomical calculations are based is
  68  * 1990 january 0.0, 631065600 seconds since the beginning of the
  69  * "unix epoch" (00:00:00 GMT, Jan. 1, 1970)
  70  *
  71  * given a number of seconds since the start of the unix epoch,
  72  * DaysSinceEpoch() computes the number of days since the start of the
  73  * astronomical epoch (1990 january 0.0)
  74  */
  75
  76 #define EpochStart           (631065600)
  77 #define DaysSinceEpoch(secs) (((secs)-EpochStart)*(1.0/(24*3600)))
  78
  79 /*
  80  * assuming the apparent orbit of the sun about the earth is circular,
  81  * the rate at which the orbit progresses is given by RadsPerDay --
  82  * TWOPI radians per orbit divided by 365.242191 days per year:
  83  */
  84
  85 #define RadsPerDay (TWOPI/365.242191)
  86
  87 /*
  88  * details of sun's apparent orbit at epoch 1990.0 (after
  89  * duffett-smith, table 6, section 46)
  90  *
  91  * Epsilon_g    (ecliptic longitude at epoch 1990.0) 279.403303 degrees
  92  * OmegaBar_g   (ecliptic longitude of perigee)      282.768422 degrees
  93  * Eccentricity (eccentricity of orbit)                0.016713
  94  */
  95
  96 #define Epsilon_g    (279.403303*(TWOPI/360))
  97 #define OmegaBar_g   (282.768422*(TWOPI/360))
  98 #define Eccentricity (0.016713)
  99
 100 /*
 101  * MeanObliquity gives the mean obliquity of the earth's axis at epoch
 102  * 1990.0 (computed as 23.440592 degrees according to the method given
 103  * in duffett-smith, section 27)
 104  */
 105 #define MeanObliquity (23.440592*(TWOPI/360))
 106
 107 /*
 108  * solve Kepler's equation via Newton's method
 109  * (after duffett-smith, section 47)
 110  */
 111 static double solve_keplers_equation(double M)
 112 {
 113         double E;
 114         double delta;
 115
 116         E = M;
 117         while (1) {
 118                 delta = E - Eccentricity * sin(E) - M;
 119                 if (fabs(delta) <= 1e-10)
 120                         break;
 121                 E -= delta / (1 - Eccentricity * cos(E));
 122         }
 123
 124         return E;
 125 }
 126
 127
 128 /*
 129  * compute ecliptic longitude of sun (in radians)
 130  * (after duffett-smith, section 47)
 131  */
 132 static double sun_ecliptic_longitude(time_t ssue)
 133                                 /* seconds since unix epoch */
 134 {
 135         double D, N;
 136         double M_sun, E;
 137         double v;
 138
 139         D = DaysSinceEpoch(ssue);
 140
 141         N = RadsPerDay * D;
 142         N = fmod(N, TWOPI);
 143         if (N < 0)
 144                 N += TWOPI;
 145
 146         M_sun = N + Epsilon_g - OmegaBar_g;
 147         if (M_sun < 0)
 148                 M_sun += TWOPI;
 149
 150         E = solve_keplers_equation(M_sun);
 151         v = 2 * atan(sqrt((1 + Eccentricity) / (1 - Eccentricity)) * tan(E / 2));
 152
 153         return (v + OmegaBar_g);
 154 }
 155
 156
 157 /*
 158  * convert from ecliptic to equatorial coordinates
 159  * (after duffett-smith, section 27)
 160  */
 161 static void ecliptic_to_equatorial(double lambda, double beta, double *alpha, double *delta)
 162 /*
 163  *    double  lambda;            ecliptic longitude
 164  *    double  beta;               ecliptic latitude
 165  *    double *alpha;              (return) right ascension
 166  *    double *delta;           (return) declination
 167  */
 168 {
 169         double sin_e, cos_e;
 170
 171         sin_e = sin(MeanObliquity);
 172         cos_e = cos(MeanObliquity);
 173
 174         *alpha = atan2(sin(lambda) * cos_e - tan(beta) * sin_e, cos(lambda));
 175         *delta = asin(sin(beta) * cos_e + cos(beta) * sin_e * sin(lambda));
 176 }
 177
 178
 179 /*
 180  * computing julian dates (assuming gregorian calendar, thus this is
 181  * only valid for dates of 1582 oct 15 or later)
 182  * (after duffett-smith, section 4)
 183  */
 184 static double julian_date(int y, int m, int d)
 185 /*
 186  *     int y;                     year (e.g. 19xx)
 187  *     int m;                      month (jan=1, feb=2, ...)
 188  *     int d;                      day of month
 189  */
 190 {
 191         int A, B, C, D;
 192         double JD;
 193
 194         /* lazy test to ensure gregorian calendar */
 195 /*
 196  *   ASSERT(y >= 1583);
 197  */
 198
 199         if ((m == 1) || (m == 2)) {
 200                 y -= 1;
 201                 m += 12;
 202         }
 203         A = y / 100;
 204         B = 2 - A + (A / 4);
 205         C = (int) (365.25 * y);
 206         D = (int) (30.6001 * (m + 1));
 207
 208         JD = B + C + D + d + 1720994.5;
 209
 210         return JD;
 211 }
 212
 213
 214 /*
 215  * compute greenwich mean sidereal time (GST) corresponding to a given
 216  * number of seconds since the unix epoch
 217  * (after duffett-smith, section 12)
 218  */
 219 static double GST(time_t ssue)
 220    /*time_t ssue;          seconds since unix epoch */
 221 {
 222         double JD;
 223         double T, T0;
 224         double UT;
 225         struct tm *tm;
 226
 227         tm = gmtime(&ssue);
 228
 229         JD = julian_date(tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday);
 230         T = (JD - 2451545) / 36525;
 231
 232         T0 = ((T + 2.5862e-5) * T + 2400.051336) * T + 6.697374558;
 233
 234         T0 = fmod(T0, 24.0);
 235         if (T0 < 0)
 236                 T0 += 24;
 237
 238         UT = tm->tm_hour + (tm->tm_min + tm->tm_sec / 60.0) / 60.0;
 239
 240         T0 += UT * 1.002737909;
 241         T0 = fmod(T0, 24.0);
 242         if (T0 < 0)
 243                 T0 += 24;
 244
 245         return T0;
 246 }
 247
 248
 249 /*
 250  * given a particular time (expressed in seconds since the unix
 251  * epoch), compute position on the earth (lat, lon) such that sun is
 252  * directly overhead.
 253  */
 254 void GetSunPos(time_t ssue, double *lat, double *lon)
 255 /*     time_t  ssue;               seconds since unix epoch */
 256 /*     double *lat;                (return) latitude        */
 257 /*     double *lon;                (return) longitude       */
 258 {
 259         double lambda;
 260         double alpha, delta;
 261         double tmp;
 262
 263         lambda = sun_ecliptic_longitude(ssue);
 264         ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta);
 265
 266         tmp = alpha - (TWOPI / 24) * GST(ssue);
 267         if (tmp < -PI) {
 268                 do
 269                         tmp += TWOPI;
 270                 while (tmp < -PI);
 271         } else if (tmp > PI) {
 272                 do
 273                         tmp -= TWOPI;
 274                 while (tmp < -PI);
 275         }
 276         *lon = tmp;
 277         *lat = delta;
 278 }