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dockapps/wmglobe/src/sunpos.c
Doug Torrance 0075abd36f Remove trailing whitespace.
2018-01-08 16:08:44 -02:00

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/* WMGlobe 1.3 - All the Earth on a WMaker Icon
* copyright (C) 1998,99,2000,01 Jerome Dumonteil <jerome.dumonteil@linuxfr.org>
* sunpos.c is taken from Xearth 1.0 (and part of 1.1):
*/
/*
* 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.)
*
*
* 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.
*
* Unisys Corporation holds worldwide patent rights on the Lempel Zev
* Welch (LZW) compression technique employed in the CompuServe GIF
* image file format as well as in other formats. Unisys has made it
* clear, however, that it does not require licensing or fees to be
* paid for freely distributed, non-commercial applications (such as
* xearth) that employ LZW/GIF technology. Those wishing further
* information about licensing the LZW patent should contact Unisys
* directly at (lzw_info@unisys.com) or by writing to
*
* Unisys Corporation
* Welch Licensing Department
* M/S-C1SW19
* P.O. Box 500
* Blue Bell, PA 19424
*
* 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.
*/
/*************************************************************************/
#include <math.h>
#include <time.h>
#ifndef PI
#define PI 3.141592653
#endif
#define TWOPI (2*PI)
#define DegsToRads(x) ((x)*(TWOPI/360))
/*
* 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 --
* TWOPI radians per orbit divided by 365.242191 days per year:
*/
#define RadsPerDay (TWOPI/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 (DegsToRads(279.403303))
#define OmegaBar_g (DegsToRads(282.768422))
#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 (DegsToRads(23.440592))
/*
* Lunar parameters, epoch January 0, 1990.0 (from Xearth 1.1)
*/
#define MoonMeanLongitude DegsToRads(318.351648)
#define MoonMeanLongitudePerigee DegsToRads( 36.340410)
#define MoonMeanLongitudeNode DegsToRads(318.510107)
#define MoonInclination DegsToRads( 5.145396)
#define SideralMonth (27.3217)
/*
* Force an angular value into the range [-PI, +PI]
*/
#define Normalize(x) \
do { \
if ((x) < -PI) \
do (x) += TWOPI; while ((x) < -PI); \
else if ((x) > PI) \
do (x) -= TWOPI; while ((x) > PI); \
} while (0)
static double solve_keplers_equation(double M);
static double mean_sun(double D);
static double sun_ecliptic_longitude(time_t ssue);
static void ecliptic_to_equatorial(double lambda, double beta,
double *alpha, double *delta);
static double julian_date(int y, int m, int d);
static double GST(time_t ssue);
/*
* 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;
}
/*
* Calculate the position of the mean sun: where the sun would
* be if the earth's orbit were circular instead of ellipictal.
*/
static double mean_sun(double D)
/* double D; days since ephemeris epoch */
{
double N, M;
N = RadsPerDay * D;
N = fmod(N, TWOPI);
if (N < 0)
N += TWOPI;
M = N + Epsilon_g - OmegaBar_g;
if (M < 0)
M += TWOPI;
return M;
}
/*
* compute ecliptic longitude of sun (in radians)
* (after duffett-smith, section 47)
*/
static double sun_ecliptic_longitude(time_t ssue)
/* seconds since unix epoch */
{
double D, N;
double M_sun, E;
double v;
D = DaysSinceEpoch(ssue);
N = RadsPerDay * D;
M_sun = mean_sun(D);
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 */
/*
* ASSERT(y >= 1583);
*/
if ((m == 1) || (m == 2)) {
y -= 1;
m += 12;
}
A = y / 100;
B = 2 - A + (A / 4);
C = (int) (365.25 * y);
D = (int) (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.
*/
void sun_position(time_t ssue, double *lat, double *lon)
/* time_t ssue; seconds since unix epoch */
/* double *lat; (return) latitude in rad */
/* double *lon; (return) longitude in rad */
{
double lambda;
double alpha, delta;
double tmp;
lambda = sun_ecliptic_longitude(ssue);
ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta);
tmp = alpha - (TWOPI / 24) * GST(ssue);
Normalize(tmp);
*lon = tmp;
*lat = delta;
}
/*
* given a particular time (expressed in seconds since the unix
* epoch), compute position on the earth (lat, lon) such that the
* moon is directly overhead.
*
* Based on duffett-smith **2nd ed** section 61; combines some steps
* into single expressions to reduce the number of extra variables.
*/
void moon_position(time_t ssue, double *lat, double *lon)
/* time_t ssue; seconds since unix epoch */
/* double *lat; (return) latitude in ra */
/* double *lon; (return) longitude in ra */
{
double lambda, beta;
double D, L, Ms, Mm, N, Ev, Ae, Ec, alpha, delta;
D = DaysSinceEpoch(ssue);
lambda = sun_ecliptic_longitude(ssue);
Ms = mean_sun(D);
L = fmod(D / SideralMonth, 1.0) * TWOPI + MoonMeanLongitude;
Normalize(L);
Mm = L - DegsToRads(0.1114041 * D) - MoonMeanLongitudePerigee;
Normalize(Mm);
N = MoonMeanLongitudeNode - DegsToRads(0.0529539 * D);
Normalize(N);
Ev = DegsToRads(1.2739) * sin(2.0 * (L - lambda) - Mm);
Ae = DegsToRads(0.1858) * sin(Ms);
Mm += Ev - Ae - DegsToRads(0.37) * sin(Ms);
Ec = DegsToRads(6.2886) * sin(Mm);
L += Ev + Ec - Ae + DegsToRads(0.214) * sin(2.0 * Mm);
L += DegsToRads(0.6583) * sin(2.0 * (L - lambda));
N -= DegsToRads(0.16) * sin(Ms);
L -= N;
lambda = (fabs(cos(L)) < 1e-12) ?
(N + sin(L) * cos(MoonInclination) * PI / 2) :
(N + atan2(sin(L) * cos(MoonInclination), cos(L)));
Normalize(lambda);
beta = asin(sin(L) * sin(MoonInclination));
ecliptic_to_equatorial(lambda, beta, &alpha, &delta);
alpha -= (TWOPI / 24) * GST(ssue);
Normalize(alpha);
*lon = alpha;
*lat = delta;
}
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