279 lines
7.3 KiB
C
279 lines
7.3 KiB
C
/* WMGlobe 0.5 - All the Earth on a WMaker Icon
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* copyright (C) 1998,99 Jerome Dumonteil <jerome.dumonteil@capway.com>
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* sunpos.c is taken from Xearth :
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*/
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/*
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* sunpos.c
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* kirk johnson
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* july 1993
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*
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* code for calculating the position on the earth's surface for which
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* the sun is directly overhead (adapted from _practical astronomy
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* with your calculator, third edition_, peter duffett-smith,
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* cambridge university press, 1988.)
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*
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*
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* Copyright (C) 1989, 1990, 1993, 1994, 1995 Kirk Lauritz Johnson
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*
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* Parts of the source code (as marked) are:
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* Copyright (C) 1989, 1990, 1991 by Jim Frost
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* Copyright (C) 1992 by Jamie Zawinski <jwz@lucid.com>
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*
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* Permission to use, copy, modify and freely distribute xearth for
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* non-commercial and not-for-profit purposes is hereby granted
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* without fee, provided that both the above copyright notice and this
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* permission notice appear in all copies and in supporting
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* documentation.
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*
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* Unisys Corporation holds worldwide patent rights on the Lempel Zev
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* Welch (LZW) compression technique employed in the CompuServe GIF
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* image file format as well as in other formats. Unisys has made it
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* clear, however, that it does not require licensing or fees to be
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* paid for freely distributed, non-commercial applications (such as
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* xearth) that employ LZW/GIF technology. Those wishing further
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* information about licensing the LZW patent should contact Unisys
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* directly at (lzw_info@unisys.com) or by writing to
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*
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* Unisys Corporation
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* Welch Licensing Department
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* M/S-C1SW19
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* P.O. Box 500
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* Blue Bell, PA 19424
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*
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* The author makes no representations about the suitability of this
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* software for any purpose. It is provided "as is" without express or
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* implied warranty.
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*
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* THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE,
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* INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS,
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* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, INDIRECT
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* OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
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* LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
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* NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
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* CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*/
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/*************************************************************************/
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#include <math.h>
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#include <time.h>
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#ifndef PI
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#define PI 3.141592653
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#endif
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#define TWOPI (2*PI)
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/*
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* the epoch upon which these astronomical calculations are based is
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* 1990 january 0.0, 631065600 seconds since the beginning of the
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* "unix epoch" (00:00:00 GMT, Jan. 1, 1970)
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*
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* given a number of seconds since the start of the unix epoch,
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* DaysSinceEpoch() computes the number of days since the start of the
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* astronomical epoch (1990 january 0.0)
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*/
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#define EpochStart (631065600)
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#define DaysSinceEpoch(secs) (((secs)-EpochStart)*(1.0/(24*3600)))
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/*
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* assuming the apparent orbit of the sun about the earth is circular,
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* the rate at which the orbit progresses is given by RadsPerDay --
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* TWOPI radians per orbit divided by 365.242191 days per year:
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*/
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#define RadsPerDay (TWOPI/365.242191)
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/*
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* details of sun's apparent orbit at epoch 1990.0 (after
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* duffett-smith, table 6, section 46)
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*
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* Epsilon_g (ecliptic longitude at epoch 1990.0) 279.403303 degrees
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* OmegaBar_g (ecliptic longitude of perigee) 282.768422 degrees
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* Eccentricity (eccentricity of orbit) 0.016713
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*/
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#define Epsilon_g (279.403303*(TWOPI/360))
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#define OmegaBar_g (282.768422*(TWOPI/360))
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#define Eccentricity (0.016713)
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/*
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* MeanObliquity gives the mean obliquity of the earth's axis at epoch
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* 1990.0 (computed as 23.440592 degrees according to the method given
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* in duffett-smith, section 27)
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*/
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#define MeanObliquity (23.440592*(TWOPI/360))
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/*
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* solve Kepler's equation via Newton's method
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* (after duffett-smith, section 47)
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*/
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static double solve_keplers_equation(double M)
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{
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double E;
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double delta;
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E = M;
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while (1) {
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delta = E - Eccentricity * sin(E) - M;
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if (fabs(delta) <= 1e-10)
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break;
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E -= delta / (1 - Eccentricity * cos(E));
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}
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return E;
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}
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/*
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* compute ecliptic longitude of sun (in radians)
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* (after duffett-smith, section 47)
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*/
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static double sun_ecliptic_longitude(time_t ssue)
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/* seconds since unix epoch */
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{
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double D, N;
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double M_sun, E;
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double v;
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D = DaysSinceEpoch(ssue);
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N = RadsPerDay * D;
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N = fmod(N, TWOPI);
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if (N < 0)
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N += TWOPI;
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M_sun = N + Epsilon_g - OmegaBar_g;
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if (M_sun < 0)
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M_sun += TWOPI;
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E = solve_keplers_equation(M_sun);
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v = 2 * atan(sqrt((1 + Eccentricity) / (1 - Eccentricity)) * tan(E / 2));
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return (v + OmegaBar_g);
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}
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/*
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* convert from ecliptic to equatorial coordinates
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* (after duffett-smith, section 27)
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*/
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static void ecliptic_to_equatorial(double lambda, double beta, double *alpha, double *delta)
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/*
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* double lambda; ecliptic longitude
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* double beta; ecliptic latitude
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* double *alpha; (return) right ascension
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* double *delta; (return) declination
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*/
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{
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double sin_e, cos_e;
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sin_e = sin(MeanObliquity);
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cos_e = cos(MeanObliquity);
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*alpha = atan2(sin(lambda) * cos_e - tan(beta) * sin_e, cos(lambda));
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*delta = asin(sin(beta) * cos_e + cos(beta) * sin_e * sin(lambda));
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}
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/*
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* computing julian dates (assuming gregorian calendar, thus this is
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* only valid for dates of 1582 oct 15 or later)
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* (after duffett-smith, section 4)
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*/
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static double julian_date(int y, int m, int d)
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/*
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* int y; year (e.g. 19xx)
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* int m; month (jan=1, feb=2, ...)
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* int d; day of month
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*/
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{
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int A, B, C, D;
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double JD;
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/* lazy test to ensure gregorian calendar */
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/*
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* ASSERT(y >= 1583);
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*/
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if ((m == 1) || (m == 2)) {
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y -= 1;
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m += 12;
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}
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A = y / 100;
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B = 2 - A + (A / 4);
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C = (int) (365.25 * y);
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D = (int) (30.6001 * (m + 1));
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JD = B + C + D + d + 1720994.5;
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return JD;
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}
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/*
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* compute greenwich mean sidereal time (GST) corresponding to a given
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* number of seconds since the unix epoch
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* (after duffett-smith, section 12)
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*/
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static double GST(time_t ssue)
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/*time_t ssue; seconds since unix epoch */
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{
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double JD;
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double T, T0;
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double UT;
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struct tm *tm;
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tm = gmtime(&ssue);
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JD = julian_date(tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday);
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T = (JD - 2451545) / 36525;
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T0 = ((T + 2.5862e-5) * T + 2400.051336) * T + 6.697374558;
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T0 = fmod(T0, 24.0);
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if (T0 < 0)
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T0 += 24;
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UT = tm->tm_hour + (tm->tm_min + tm->tm_sec / 60.0) / 60.0;
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T0 += UT * 1.002737909;
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T0 = fmod(T0, 24.0);
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if (T0 < 0)
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T0 += 24;
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return T0;
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}
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/*
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* given a particular time (expressed in seconds since the unix
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* epoch), compute position on the earth (lat, lon) such that sun is
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* directly overhead.
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*/
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void GetSunPos(time_t ssue, double *lat, double *lon)
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/* time_t ssue; seconds since unix epoch */
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/* double *lat; (return) latitude */
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/* double *lon; (return) longitude */
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{
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double lambda;
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double alpha, delta;
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double tmp;
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lambda = sun_ecliptic_longitude(ssue);
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ecliptic_to_equatorial(lambda, 0.0, &alpha, &delta);
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tmp = alpha - (TWOPI / 24) * GST(ssue);
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if (tmp < -PI) {
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do
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tmp += TWOPI;
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while (tmp < -PI);
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} else if (tmp > PI) {
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do
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tmp -= TWOPI;
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while (tmp < -PI);
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}
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*lon = tmp;
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*lat = delta;
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}
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