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flightgear/Time/sunpos.c
curt 28fad1d296 Prepended "fg" on the name of all global structures that didn't have it yet. 
i.e. "struct WEATHER {}" became "struct fgWEATHER {}"
1997-12-10 22:37:34 +00:00

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/*
* 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 <time.h>
#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 fgLIGHT *l;
struct fgTIME *t;
struct fgVIEW *v;
MAT3vec nup, nsun;
double sun_gd_lat, sl_radius, temp;
static int time_warp = 0;
l = &cur_light_params;
t = &cur_time_params;
v = &current_view;
time_warp += 0; /* increase this to make the world spin real fast */
fgSunPosition(t->cur_time + time_warp, &l->sun_lon, &sun_gd_lat);
fgGeodToGeoc(sun_gd_lat, 0.0, &sl_radius, &l->sun_gc_lat);
l->fg_sunpos = fgPolarToCart(l->sun_lon, l->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 */
l->sun_vec_inv[0] = l->fg_sunpos.x - scenery_center.x;
l->sun_vec_inv[1] = l->fg_sunpos.y - scenery_center.y;
l->sun_vec_inv[2] = l->fg_sunpos.z - scenery_center.z;
MAT3_SCALE_VEC(l->sun_vec, l->sun_vec_inv, -1.0);
/* make these are directional light sources only */
l->sun_vec[3] = 0.0;
l->sun_vec_inv[3] = 0.0;
/* calculate thesun's relative angle to local up */
MAT3_COPY_VEC(nup, v->local_up);
nsun[0] = l->fg_sunpos.x;
nsun[1] = l->fg_sunpos.y;
nsun[2] = l->fg_sunpos.z;
MAT3_NORMALIZE_VEC(nup, temp);
MAT3_NORMALIZE_VEC(nsun, temp);
l->sun_angle = acos(MAT3_DOT_PRODUCT(nup, nsun));
printf("SUN ANGLE relative to current location = %.3f rads.\n",
l->sun_angle);
}
/* $Log$
/* Revision 1.15 1997/12/10 22:37:55 curt
/* Prepended "fg" on the name of all global structures that didn't have it yet.
/* i.e. "struct WEATHER {}" became "struct fgWEATHER {}"
/*
* Revision 1.14 1997/12/09 04:25:39 curt
* Working on adding a global lighting params structure.
*
* Revision 1.13 1997/11/25 19:25:42 curt
* Changes to integrate Durk's moon/sun code updates + clean up.
*
* Revision 1.12 1997/11/15 18:15:39 curt
* Reverse direction of sun vector, so object normals can be more normal.
*
* 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.
*
*/
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