/* * 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 * * 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 #include #include #include "sunpos.h" #include "fg_time.h" #include "../constants.h" #include "../Main/views.h" #include "../Math/fg_geodesy.h" #include "../Math/mat3.h" #include "../Math/polar.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; } /* update the cur_time_params structure with the current sun position */ void fgUpdateSunPos(struct fgCartesianPoint scenery_center) { struct fgTIME *t; struct VIEW *v; MAT3vec nup, nsun; double sun_gd_lat, sl_radius, temp; static int time_warp = 0; t = &cur_time_params; v = ¤t_view; time_warp += 0; /* increase this to make the world spin real fast */ fgSunPosition(t->cur_time + time_warp, &t->sun_lon, &sun_gd_lat); fgGeodToGeoc(sun_gd_lat, 0.0, &sl_radius, &t->sun_gc_lat); t->fg_sunpos = fgPolarToCart(t->sun_lon, t->sun_gc_lat, sl_radius); /* printf("Geodetic lat = %.5f Geocentric lat = %.5f\n", sun_gd_lat, t->sun_gc_lat); */ /* the sun position has to be translated just like everything else */ t->sun_vec[0] = t->fg_sunpos.x - scenery_center.x; t->sun_vec[1] = t->fg_sunpos.y - scenery_center.y; t->sun_vec[2] = t->fg_sunpos.z - scenery_center.z; /* make this a directional light source only */ t->sun_vec[3] = 0.0; /* calculate thesun's relative angle to local up */ MAT3_COPY_VEC(nup, v->local_up); nsun[0] = t->fg_sunpos.x; nsun[1] = t->fg_sunpos.y; nsun[2] = t->fg_sunpos.z; MAT3_NORMALIZE_VEC(nup, temp); MAT3_NORMALIZE_VEC(nsun, temp); t->sun_angle = acos(MAT3_DOT_PRODUCT(nup, nsun)); printf("SUN ANGLE relative to current location = %.3f rads.\n", t->sun_angle); } /* $Log$ /* Revision 1.11 1997/10/28 21:07:21 curt /* Changed GLUT/ -> Main/ /* * Revision 1.10 1997/09/13 02:00:09 curt * Mostly working on stars and generating sidereal time for accurate star * placement. * * Revision 1.9 1997/09/05 14:17:31 curt * More tweaking with stars. * * Revision 1.8 1997/09/05 01:36:04 curt * Working on getting stars right. * * Revision 1.7 1997/09/04 02:17:40 curt * Shufflin' stuff. * * Revision 1.6 1997/08/27 03:30:37 curt * Changed naming scheme of basic shared structures. * * Revision 1.5 1997/08/22 21:34:41 curt * Doing a bit of reorganizing and house cleaning. * * Revision 1.4 1997/08/19 23:55:09 curt * Worked on better simulating real lighting. * * Revision 1.3 1997/08/13 20:23:49 curt * The interface to sunpos now updates a global structure rather than returning * current sun position. * * Revision 1.2 1997/08/06 00:24:32 curt * Working on correct real time sun lighting. * * Revision 1.1 1997/08/01 15:27:56 curt * Initial revision. * */